Procedures in Conscious Sedation
Many patients regard all dental treatment, and especially surgical procedures, as stressful and potentially painful. Reactions range from ‘normal’ apprehension, through various degrees of anxiety to irrational fear or even phobia. The adverse physiological effects of these psychological responses can increase the risk of treatment and should be controlled. This is particularly important for patients suffering from medical conditions which are made worse by fear. Internationally, conscious sedation is increasingly recognised to be an integral element of the control of pain and anxiety and an important aspect of the modern practice of dentistry (General Dental Council, 2002; Intercollegiate Advisory Committee for Sedation in Dentistry (IACSD), 2015).
Conscious sedation has been defined as:
A technique in which the use of a drug or drugs produces a state of depression of the central nervous system enabling treatment to be carried out, but during which verbal contact is maintained throughout the period of sedation. The drugs and techniques used to provide conscious sedation should carry a margin of safety wide enough to render loss of consciousness unlikely. The level of consciousness must be such that the patient remains conscious, retains protective reflexes, and is able to understand and respond to verbal commands.
In the UK, the most commonly used dental conscious sedation techniques (titrated intravenous midazolam or titrated inhaled nitrous oxide/oxygen) have an excellent safety record. For many patients, conscious sedation combined with effective local anaesthesia has been a very acceptable alternative to general anaesthesia. Ensuring that patients understand the benefits and risks of local anaesthesia, conscious sedation and general anaesthesia is an important part of the consent process. Despite the safety, efficacy and cost benefits of using conscious sedation techniques there are still indications for general anaesthesia for some dental/surgical procedures and certain patient groups.
Typical signs and symptoms of anxiety are:
|Clenched fists/sweaty hands||Fainting|
|Distracted appearance||Dry mouth|
|Not sitting back fully in the dental chair||Need to visit lavatory
|Holding handbag/tissue tightly|
|Very quiet or voluble|
This chapter provides an introduction to conscious sedation techniques for dental procedures: patient assessment and treatment planning, essential pharmacology, sedation equipment, clinical sedation techniques and the avoidance/management of sedation‐related complications. Before administering any form of conscious sedation the dental team must have received appropriate training in accordance with contemporary professional guidance.
Patient Assessment and Treatment Planning
A satisfactory first visit is crucial to the success of subsequent treatment under conscious sedation. There is a great deal of information to be acquired from the patient. At the same time, it should never be forgotten that the patient is also assessing the dental team. The first meeting should ideally be outside the surgery environment and in the nature of an informal ‘chat’. The following topics should be explored:
What Is the Problem?
|It is often helpful to get the patient to complete a questionnaire asking the nature of their fears. Suitable questionnaires for adults include the Modified Dental Anxiety Scale (Corah, Gale and Illig, 1978; Humphris, Morrison and Lindsay, 1995) (Figure 10.1) and for children the Modified Venham Scale of Anxiety (Venham, 1979) (Figure 10.2). This breaks the ice, and other questions may be included that will steer the conversation in the right direction. Remember that for some patients even discussing dentistry or meeting a dentist can frightening.||Understanding the precise causes of an individual’s anxiety makes it easier to prescribe the most appropriate dental care and anxiety management regimen. The latter may include pharmacological and/or non‐pharmacological techniques (e.g. cognitive behavioural therapy, hypnosis, acupuncture).|
|A detailed medical history must be obtained. From the sedation point of view, special note should be made of respiratory and cardiovascular problems, liver and kidney disease. Prescribed medication may alert the operator to undisclosed medical conditions and also raise the question of drug interactions. Some medicines potentiate the effect of sedation drugs. It may sometimes be necessary to discuss the patient’s medical history with their general medical practitioner or hospital consultant. A baseline recording of arterial blood pressure, heart rate and arterial oxygen saturation should be obtained and the result recorded in the clinical notes.||Baseline readings, taken before any drugs are administered, are essential in order to be able to compare the ‘normal’ and intraoperative values for an individual patient.|
|It is important to appreciate that in many cases it is helpful to prescribe sedation. In other cases the sedation technique may require modification.|
Examples Where Sedation Is Almost Certainly Beneficial
|Angina may be provoked by anxiety or stress during the dental procedure. This may cause a tachycardia and increase the work of the heart.||Apart from the other usual anxiety management measures employed, the use of sedation protects patients from these effects and significantly reduces the likelihood of angina‐related symptoms.|
|Most hypertensive patients are taking medication, and some may have a normal arterial blood pressure in the assessment clinic; however, many still have higher than normal levels. The anxiety of treatment can cause an increase in heart rate and an elevation of blood pressure.||Sedation modifies these responses and protects the patient.|
|It is particularly important to ascertain if attacks are known to be provoked by stress.||Sedation reduces the physiological response to stress and so reduces the risk of an attack.|
|Midazolam is particularly useful in reducing the likelihood of fits when the patient is poorly controlled.||Midazolam has anti‐seizure properties and may be used in the management of status epilepticus.|
|In patients with uncontrolled movements, intravenous sedation will often suppress or at least reduce abnormal motor activity.||As midazolam wears off, the uncontrolled movements return to their usual intensity.|
Examples of Conditions Where the Technique Might Require Modification
|Controlled heart failure|
|Patients might be distressed when supine, and liver perfusion (and therefore drug metabolism) is likely to be reduced.||Asking the patient if they will be comfortable fully reclined and adjusting the dental chair and operating position appropriately may be all that is necessary. It is also useful to enquire how many pillows the patient has at night.|
|Chronic anaemia (diagnosed and managed)|
|Be aware of the potential effects of falling oxygen saturation levels and respond promptly.||Undiagnosed and poorly managed anaemia may lead to incorrect pulse oximetry readings. Management of a low oxygen saturation involves the administration of supplemental oxygen and possibly other measures (see ‘Management of Sedation‐Related Complications’).|
|Chronic airways disease(s)|
|Interpretation of oxygen saturation levels in smokers and patients with chronic obstructive pulmonary disease (COPD) may be difficult. The possibility that the patient’s respiratory drive is oxygen (rather than carbon dioxide) dependent must be considered.||Administering oxygen to a patient whose respiratory drive is oxygen dependent may lead to apnoea.|
|Ensure that the patient is managed appropriately. If possible, have a chairside measure of blood sugar at the beginning of treatment to avoid any later difficulties in assessing levels of consciousness.||Avoiding unnecessary starvation prior to sedation and ensuring that the patient’s escort is fully informed about the nature of the patient’s diabetes and the effects of sedation reduces the probability of intraoperative and postoperative problems.|
Examples Where Caution Is Required
Referral should be considered for the following:
Severe Cardiorespiratory Disease
The patient may be breathless at rest or after minimal exertion.
If there is active liver disease or known impairment of function, drug metabolism may be ineffective and sedation abnormally prolonged.
Severe Psychological Illness
Refer if the patient is using anti‐psychotic drugs or ‘major tranquillisers’.
Refer if the patient is opioid and/or benzodiazepine dependent or a frequent recreational drug user. Sedation in cannabis users is notoriously difficult to manage. Failure to achieve an adequate level or length of sedation is common.
Check for high levels of alcohol intake or known alcoholism. Patient who present for treatment who have clearly recently consumed alcohol should not be sedated.
Having collected this information it is possible to summarise the operative and/or sedation risk according to the scale of physical fitness devised by the American Society of Anesthesiologists (ASA; Kluger et al., 2002) (see Box 9.1).
Patients classified as ASA I or II are generally considered suitable for treatment in general dental practice or other primary dental care setting.
Patients falling into categories III and IV should be referred to a specialist centre such as a teaching hospital or specialist sedation clinic.
Some patients oscillate between ASA II and ASA III according to the severity of their disease and other factors such as the season of the year or a change in medication. Examples of this type of fluctuating condition include poorly controlled asthma, diabetes mellitus and epilepsy. It may be preferable to refer such patients or wait until their condition becomes more stable (ASA II) before providing treatment under sedation.
If a patient suffers from two relevant illnesses, or appears to be ASA II but with the use of multiple drugs it is probably sensible to consider the patient to be ASA III. The ASA scale is a useful ‘shorthand’ method of recording a patient’s medical status but it requires common sense and careful application in order to avoid creating either unnecessary concern or false confidence.
When assessing the medical status of an elderly patient, it must be remembered that some physiological functions decline naturally with age and even the apparently healthy patient with no declared medical problems cannot be treated exactly like a young fit adult. Elderly patients with one controlled illness (e.g. angina) may be suitable for treatment in a primary care setting but the presence of two known conditions (bearing in mind that other disease processes may be present but undiagnosed) should indicate referral.
Dental History (Dental Sedation Teachers’ Group/Society for the Advancement of Anaesthesia in Dentistry, 2001)
The patient’s experiences at the dentist over the years are important. The following questions may yield valuable information which will assist during treatment planning.
Useful Dental History Questions
|When did dental anxiety start?||The answer may reveal that dental anxiety or phobia was triggered by a ‘bad experience’ (often in late childhood).|
|What provoked the fear?||Many patients cannot identify a specific episode. Some patients even find trying to recall previous visits distressing.|
|Are there any specific triggers?||Although some patients cite the administration of local anaesthetic, the dental drill and ‘instruments in the mouth’, many say that they are scared of everything about dentistry – and all dentists!|
|When did the patient last visit a dentist?||This date may give an indication of the patient’s interest in their dental health and also provides a convenient starting point for more detailed questions about previous treatment.|
|Has the patient had treatment under general anaesthetic or conscious sedation in the past?||Many patients find this difficult to answer – they know that they were ‘asleep’ but whether this was general anaesthesia or due to the amnesic effects of intravenous midazolam is often unclear.|
|If sedation, what technique was used?||Knowing which sedation techniques have been successful in the past may help to decide which is most likely to be effective under the present circumstances. But be aware that a patient’s memory and understanding of sedation may be limited, for example, they may remember having a mask placed on the face but was this general anaesthetic or inhalation sedation?|
|Was this treatment successful?||Success is a relative term – it is important to establish if the intended dental treatment was completed and not only whether the patient found the experience pleasant or not. If treatment was unsuccessful, why? What happened? Was the patient referred elsewhere?|
|What concerns the patient most about their teeth?||In order to encourage attendance it is sometimes better to carry out cosmetic dentistry (for example, restoration of an unsightly upper incisor) before addressing more serious (but less visible) problems with posterior teeth.|
|Are there any current symptoms (particularly pain)?||Pain must be addressed but, wherever possible, it is better to avoid extractions at the first visit.|
Remember that non‐anxious patients may also be better managed under sedation if the proposed dental procedure is potentially threatening and/or prolonged.
The patient’s domestic circumstances are very important. An escort will be required for all intravenous sedation appointments (but not usually for adults treated using inhalation sedation). In addition, having responsibility for children or elderly relatives may make it difficult for the patient to attend or to be able to be recovered safely at home. Be aware that, in their eagerness to be accepted for treatment under sedation, some patients may be untruthful about their domestic circumstances. For example, they may confirm that an escort will be available but fail to say that this is a taxi driver who will simply abandon them at their front door.
Although some patients will allow a full intraoral examination, the dentist may have to be content with a visual examination at this stage. Many patients fear the dental probe and so this should only be used when absolutely necessary, and then with extreme caution. For a very few patients, intraoral radiographs may also be threatening or cause gagging, and so have to be carried out under sedation.
Discussion and Treatment Planning
Selection of the most appropriate method of pain and anxiety control requires careful consideration of a number of interlinking factors including the proposed dental treatment, the patient’s health and degree of anxiety, the operator’s training and experience and the environment in which the treatment is to be carried out. No matter how fashionable, it is not possible to design a ‘care pathway’ or ‘protocol’ which incorporates all the relevant factors. The correct and most successful approach involves a commitment by the whole team (dentist/sedationist/nursing staff) to carefully consider a range of options and choose the best for an individual patient. A ‘one size fits all’ approach to pain and anxiety management is not appropriate.
Once a preliminary dental treatment plan has been formulated, the following treatment options may then be considered and discussed with the patient:
|Local anaesthetic (LA) alone||Appropriate for most non‐anxious individuals and also some phobic patients who are not anxious about either LA or the proposed procedure, for example, scaling.|
|LA with inhalational sedation (nitrous oxide/oxygen)||The option of choice for children and mildly anxious adults undergoing relatively non‐threatening procedures, for example, fillings.|
|LA with intravenous sedation (midazolam)||Best for more severely anxious adults and/or those undergoing surgery or other prolonged and/or threatening procedures, for example, surgical removal of third molar teeth. (Note that IV midazolam is increasingly being used for very anxious younger patients for whom inhalational sedation is unlikely to be effective).|
|LA with oral or intranasal sedation (midazolam) (IACSD, 2015)||Helpful for needle‐phobic patients and patients with a disability which makes the use of intravenous sedation difficult or impossible. (But note that practitioners using oral or intranasal midazolam must have training in and experience of administering IV midazolam).|
|LA with ‘alternative’ or ‘advanced’ sedation techniques (IACSD, 2015)||For the very small number of patients for whom none of the above techniques is effective. Requires additional training and experience.|
|General anaesthesia (GA)||Sometimes the only option – indications include very severe dental phobia, a large amount of treatment required, the age of the patient, severe disability or proven allergy to all LA drugs.|
The simplest technique which will enable treatment to be carried out is generally considered to be the most appropriate. However, it is entirely inappropriate to subject patients to a rigid (often protocol‐driven) cascade of management options which only permits the dentist to offer more appropriate sedation techniques when all simpler modalities have failed. This is unnecessarily distressing for patients (and the dental team) and often only serves to increase anxiety and make subsequent management more difficult. An example would be a severely anxious needle‐phobic patient requiring extensive dental treatment who would clearly benefit from the prescription of intranasal midazolam followed by intravenous sedation being forced to endure the distress and indignity of having to first demonstrate that inhalational sedation is inappropriate.
Written consent is required for both the dental procedure and the administration of all forms of conscious sedation. Consent for dentistry under conscious sedation must, under all but emergency circumstances, be obtained at the assessment appointment rather than when the patient attends for treatment. If extractions or advanced procedures are required, these must be agreed on a tooth‐by‐tooth basis; however, this is not usually practical for routine restorative dentistry involving multiple fillings, scaling and polishing.
Finally, patients must be given written and verbal pre‐ and postoperative instructions and be given the opportunity to ask questions. Some sedationists (particularly anaesthetists) prefer that patients are starved in preparation for treatment under conscious sedation. However, there is no compelling evidence that this is necessary or even desirable when conscious sedation is administered according to current guidelines.
Patient Instruction Prior to Sedation for Dental Treatment
|For your safety, please read and follow these instructions carefully|
|Before sedation – on the day of treatment:|
||Missing essential medication may put the patient at risk.|
||There is no convincing evidence that abstaining from either liquids or solids increases safety when sedation is administered as described here and it is possible that anxiety may be increased by changing the patient’s normal routine. For some patients, for example diabetics, starvation represents a real hazard.|
||A fit adult escort is required for IV sedation in order to care for the patient on the journey home and then until the patient is fully recovered. Discharging a patient without an appropriate escort is likely to lead to disciplinary action.|
|After sedation – until the following day:|
||Patients recovering from sedation are often unaware that their judgement and memory are impaired. This may lead to accidents or inappropriate behaviour.|
Physiological Control and Monitoring
In order to fully understand the principles of safe sedation practice, it is necessary to review certain aspects of physiology, in particular those relating to the respiratory and cardiovascular systems. A knowledge of the anatomy of the upper airway assists in airway management. Familiarity with the pattern of veins in the cubital fossa and on the dorsum of the hand is essential for the administration of intravenous sedation.
The major function of the respiratory system is to ensure continuous effective gas exchange so that oxygen enters the bloodstream and carbon dioxide is removed.
Quiet breathing is characterised by the rhythmic expansion and relaxation of the lungs and thorax. The diaphragm is the most important muscle of respiration but the intercostal muscles contribute to the increase in the volume of the thorax during inspiration. The accessory muscles of inspiration are not used during quiet breathing. Expiration is normally a passive process resulting from the elastic recoil of the lungs. Active expiration, primarily involving the muscles of the anterior abdominal wall and the intercostal muscles, is seen during exercise and hyperventilation.
The size of the thorax and lungs determines the lung capacities whereas lung volumes are determined by inspiratory and expiratory effort (Figure 10.3).
Tidal volume (TV) is the volume of gas inhaled during a normal inspiration. A fit adult patient at rest normally has a tidal volume of approximately 500 ml. Minute volume (MV) is the product of the tidal volume and the respiratory rate. A normal adult at rest breathes approximately 12 times each minute. Thus, the minute volume for an adult is usually about 6 litres. These figures provide the sedationist with a physiological basis for estimating the initial fresh gas flow required when using inhalational sedation techniques.
The dead space volume refers to the portion of the airways which is not available for the exchange of gases. Dead space increases with age and a reduction in cardiac output. The term ‘alveolar ventilation’ is used to describe the volume of gas entering the alveoli each minute and taking part in gas exchange. It is important to recognise that a patient who has very shallow breathing (where the tidal volume is less than the dead space volume) is effectively not breathing at all. Hypoventilation is common following the administration of central nervous system (CNS) depressant drugs such as benzodiazepines.
Pulmonary gas exchange occurs at the alveolar capillary membrane, where only two or three cells separate alveolar gas from the bloodstream. Oxygen and carbon dioxide cross the alveolar membrane by diffusion. Most of the oxygen is transported to the periphery of the body in combination with haemoglobin. Oxygen combines loosely and reversibly with haemoglobin. Each molecule of haemoglobin can combine with four atoms of oxygen, but the association of each atom alters the affinity of the haemoglobin molecule for subsequent oxygen atoms. This results in the characteristic sigmoid shape of the oxygen dissociation curve (Figure 10.4).
The oxygen dissociation curve shows the oxygen saturation of haemoglobin on the y‐axis and the partial pressure of oxygen (oxygen tension) on the x‐axis. The plateau at the top of the curve results from the saturation of the binding sites with oxygen. This provides a potential reserve of oxygen when the partial pressure of oxygen falls. The steep vertical section of the curve allows for optimum loading and unloading of oxygen.
During sedation a pulse oximeter is used to estimate the patient’s arterial oxygen saturation (the y‐axis on the oxygen dissociation curve). However, the unremitting hunger of all the cells of the body for oxygen is only satisfied by a continuous supply and adequate partial pressure of oxygen. The shape of the dissociation curve determines the precise relationship of the axes and thus the relationship between the displayed arterial oxygen saturation (SaO2) and the quantity of oxygen available for cellular respiration. Careful consideration of the curve and the underlying biochemistry demonstrates the significance of the recommendation that the SaO2 must be maintained above 90% throughout sedation and the immediate recovery period.
Carbon dioxide is carried in the blood in solution, in the form of bicarbonate, and attached to protein as carbamino compounds. Carbon dioxide is much more soluble than oxygen and so the quantity of carbon dioxide carried in solution is significant. Most of the carbon dioxide carried in the blood is present in the form of bicarbonate.
Respiration is an automatic process under the control of the brain’s respiratory centre (Figure 10.5). The respiratory centre receives a large number of inputs, including those from the central and peripheral chemoreceptors, lung mechanoreceptors and the higher centres of the central nervous system (CNS). Changes in the rate and depth of breathing are produced by control of the firing rate in the nerves supplying the muscles of respiration.
At rest, at least 60% of the respiratory drive is derived from the central chemoreceptors in the medulla. The central chemoreceptors respond to changes in the pH (H+ ion concentration) of cerebrospinal fluid (CSF). When the level of carbon dioxide in the blood rises, carbon dioxide diffuses into the CSF from the cerebral blood vessels, liberating H+ ions which stimulate the chemoreceptors. Thus, the carbon dioxide level in blood regulates ventilation by its effect on the pH of the CSF. Under normal circumstances the body maintains the pH of CSF within very narrow limits.
The initial response to a rise in carbon dioxide is an increase in tidal volume followed by an increase in respiratory rate. That is, the patient first takes deeper breaths and then breathes more rapidly. Certain sedatives agents (particularly benzodiazepines and opioids) reduce the respiratory drive and cause a reduction in chemoreceptor sensitivity. They reduce the rate and depth of breathing (causing carbon dioxide levels to rise and oxygen levels to fall) and diminish the normal ventilatory response to these changes. This is why a pulse oximeter is considered to be an essential monitor during intravenous sedation and high dosage oral sedation using benzodiazepines.
The main purpose of the circulatory system is to deliver a continuous supply of oxygen and nutrients to the cells of the body and to remove the waste products of cellular metabolism (carbon dioxide and water).
The heart receives a sympathetic and a parasympathetic nerve supply. Sympathetic stimulation increases the heart rate and also the force of contraction of the myocardial muscle. An increase in sympathetic drive is part of the body’s normal response to fear and anxiety. Parasympathetic stimulation reduces the heart rate. The sympathetic nervous system is almost entirely responsible for the control of the vascular system (with the exception of the coronary, cerebral, pulmonary and renal circulations).
The average adult has a blood volume of 5–6 litres and a resting cardiac output of 5.5 litres per minute. Cardiac output is usually described as being the ‘product’ of heart rate and stroke volume.
Heart rate (normally 60–80 beats per minute) is generated by the activity of the sino‐atrial node but this rate is modified by autonomic tone, higher responses to pain and anxiety, baroreceptor mechanisms, chemoreceptor responses to hypoxia and hypercarbia and circulating hormones (particularly catecholamines).
Autonomic tone depends on the balance between sympathetic and parasympathetic nervous systems. At rest, the heart beats at a rate which is mostly dependent upon vagal (parasympathetic) tone. Input from higher centres, for example in response to anxiety and pain, increases sympathetic tone and hence heart rate.
Specialised stretch receptors (baroreceptors) located in the heart and major blood vessels provide a negative feedback mechanism for the control of systemic arterial pressure. A fall in arterial blood pressure is associated with a decrease in the firing rate in the baroreceptor nerve supply. This results in a reflex increase in the heart rate and vice versa. The amount of blood ejected by the heart (cardiac output) balanced against the resistance to blood flow offered by the peripheral circulation (peripheral resistance) determines the pressure generated in the major blood vessels (Figure 10.6).
Blood vessel size relates to arteriolar tone, which is controlled by the sympathetic nervous system and circulating catecholamines. Increased sympathetic activity results in vasoconstriction and decreased activity results in vasodilatation.
Differences Between Adults and Children
Children, especially very young children, should not be thought of as small adults. There are a number of important anatomical and physiological differences which distinguish ‘paediatric’ and adult patients. The following list summarises the differences which may be relevant to the use of sedation for paediatric patients:
- Children have a higher metabolic rate than adults and this leads to increased oxygen consumption and increased carbon dioxide production. The younger the child, the higher is the metabolic rate.
- The head and tongue are relatively large. The neck is shorter and the larynx located higher and more anteriorly. The trachea is proportionately narrower compared with adults. Children tend to breathe through the nose rather than through the mouth.
- Tidal volume is usually smaller than in adults but the respiratory rate is increased. The respiratory rate for 5–12‐year‐old children is normally between 15 and 20 breaths per minute. This means that the minute volume (the product of the tidal volume and the respiratory rate) of children and adults is much more similar than might be expected from a simple comparison of size. An initial fresh gas flow of 6 litres per minute is therefore a reasonable starting point for the administration of inhalational sedation for both adults and children. The inspiratory phase of breathing tends to be more diaphragmatic as the ribs are horizontal, reducing the lateral expansion of the chest.
- Children between 5 and 12 years of age have a higher heart rate (80–120 beats per minute) than adults although arterial blood pressure is lower (typically 90–110 mmHg, systolic). Haemoglobin levels are increased. The superficial veins are smaller than in adults and may have more fatty tissue covering them. This may make venepuncture difficult. The brachial pulse is often more easily palpated than the radial or even the carotid pulse. Arterial oxygen saturation measurements are similar for adults and children.
In addition to any electromechanical devices (e.g. pulse oximeter), the sedationist and nurse must be constantly aware of the patient’s respiration (rate and depth), the presence of airway obstruction, depth of sedation and skin colour. Periodic estimation of systemic arterial blood pressure and an electrocardiogram (ECG) may be advisable for some unfit patients.
Respiratory rate is quite variable (12–20 breaths per minute in adults), but this is nearly always reduced during sedation and so must be monitored closely. The depth of breathing is also reduced. Apnoea may occur with an overdosage of (or idiosyncratic response to) midazolam. Such side effects are potentially life threatening if recognition and management are not swift. Some degree of respiratory depression is probably present in all sedated patients, but serious problems are most likely to occur immediately following induction.
Pulse oximetry (Figure 10.7) measures the patient’s arterial oxygen saturation and pulse rate using a probe, which is attached to the finger or ear lobe. The pulse oximeter detects changes in the patient’s oxygen supply, oxygen uptake by the lungs and the delivery of oxygen to the tissues via the circulation. Thus it is an excellent monitor of both respiratory and cardiovascular function. However, correct functioning can be affected by metallic nail varnish or fake nails and excessive light falling on the probe. Oxygen saturations below 90% must be investigated and the cause immediately corrected. Asking the patient to take several deep breaths resolves the majority of cases of midazolam‐induced respiratory depression. If this fails, intermittent positive pressure ventilation (IPPV) must be started and the administration of flumazenil considered.
Bradycardia or tachycardia during sedation must be investigated. The former may be due to hypoxia or vagal stimulation; the latter is often the result of painful stimuli. Most pulse monitors have audible alarms, which can be set to give an audible and visible warning if the heart rate falls or rises beyond clinically acceptable levels. For adult patients who are ASA I or II, the bradycardia and tachycardia alarm limits are normally 50 beats/min and 150 beats/min respectively.
Measurement of Systemic Arterial Blood Pressure Using a Manual Sphygmomanometer and Stethoscope (Figure 10.8)
|The sphygmomanometer cuff is applied around the upper arm, leaving a clearance of 5 cm (2 inches) between the lower end of the cuff and cubital fossa.||Correct placement of the cuff is important in order to obtain an accurate measurement. Placing the cuff too low makes placement of the stethoscope difficult.|
|The brachial pulse (on the medial side of the cubital fossa) is located and the cuff inflated until the radial pulse (on the thumb side of the wrist) disappears.||This ensures that the cuff has been inflated to a pressure which is above the patient’s systolic blood pressure.|
|The diaphragm of the stethoscope is then placed over the brachial artery and the pressure in the cuff is slowly lowered. In order to maintain a steady fall in pressure, the release valve must be opened more and more as the pressure falls. As the pressure descends, no sounds are heard until the systolic blood pressure is reached, when tapping sounds, corresponding to the heart rate, are heard in the stethoscope. The pressure at which these sounds first appear is noted. This gives the systolic blood pressure.||The systolic blood pressure corresponds to the maximum pressure in the aorta and large arteries. It is frequently raised in anxious patients.|
|As the cuff pressure continues to be reduced, the sounds become louder and louder, but at the diastolic pressure they suddenly change their quality and become muffled. A little lower down they finally disappear. The point at which the sounds become muffled is taken as the diastolic blood pressure.||The diastolic blood pressure corresponds to the minimum pressure in the aorta and large arteries. Like the systolic BP it may also be raised in anxiety but a high diastolic BP often indicates underlying cardiovascular disease.|
‘Normal’ blood pressure is 120/80 (mmHg) or 16/10 (kPa). However, small variations are commonplace and the systolic blood pressure is often raised in anxious subjects. Although moderate hypertension which is controlled is not a contraindication to sedation, patients with a diastolic blood pressure above 100 mmHg should be investigated before sedation is given.
Automatic sphygmomanometers are available and are very easy to use (Figure 10.9). However, cheaper models tend to be technique sensitive and unreliable readings are often obtained. If in doubt, the blood pressure should be re‐taken using a manual sphygmomanometer and stethoscope.
Inhalation Sedation Using Nitrous Oxide and Oxygen (Crawford, 1990; Roberts, 1990a, b; Shaw et al., 1996)
Pharmacology of Nitrous Oxide (Table 10.1)
Table 10.1 Properties of nitrous oxide.
|Ease of titration||Easy|
|Induction and recovery rate||Rapid|
|Ease of breathing||Non‐pungent|
|Potency (MAC)||Weak (105%)|
|Blood gas solubility||Low – 0.47|
|Speed of change in sedation level||Rapid|
|Systemic toxicity||Yes – prolonged use|
Nitrous oxide is a colourless and virtually odourless anaesthetic gas. The gas has a blood/gas solubility coefficient of 0.47 and a minimum alveolar concentration (MAC) of 105%. The blood/gas solubility coefficient determines the rate at which the gas concentration in the lungs equilibrates with that being administered which, in turn, relates to the speed of induction and of recovery. Nitrous oxide is poorly soluble in blood and so induction and recovery are rapid.
The MAC value is related to the potency of the gas and determines the concentration needed to induce sedation. Nitrous oxide is not very potent, which means that it is a very safe gas for dental sedation. It is compressed at 800 lb per square inch (43.5 bar) to a liquid and supplied in cylinders which are coloured blue.
In sufficient concentrations (in excess of 100%), the drug will induce light surgical anaesthesia, but only at the expense of adequate oxygenation. In lesser concentrations, it has excellent analgesic and sedative properties. There are very few cardiovascular or respiratory effects and no direct depression of myocardial function or reduction in ventilation. The drug has a central analgesic and anaesthetic effect (the exact mechanism is not clear) and is excreted unchanged via the lungs very rapidly after its administration is discontinued.
Nitrous oxide has excellent anxiolytic, sedative and analgesic properties, with little or no depression of myocardial function or ventilation. Induction and recovery are rapid and it has a wide margin of safety. Inhalational sedation may also be useful for venepuncture in some needle‐phobic patients.
The variation between individual patients is such that, whilst one person may be adequately sedated with 20% nitrous oxide, another individual may require in excess of 50%. A titration technique of administration is employed in order to avoid the risk of over‐sedation.
Because of the relatively poor solubility of nitrous oxide in blood and body tissues, there is rapid outflow of nitrous oxide across the alveolar membrane when the incoming gas flow is stopped. This may dilute the percentage of alveolar oxygen available for uptake by up to 50%. This phenomenon is called diffusion hypoxia and is prevented by giving 100% oxygen for at least 2 minutes at the end of the procedure.
Advantages of Inhalational Sedation
|No ‘needles’.||Many anxious patients are needle phobic. However, LA is usually required – but only when the patient is sedated.|
|Level of sedation is easily altered.||The level of sedation may be increased or decreased in line with the patient’s anxiety relating to specific items of treatment, for example, LA, drilling, scaling. Note that patients differ widely – some perceive LA to be most the most unpleasant part of treatment whereas for others, scaling is most testing.|
|Minimal impairment of reflexes.||Pharyngeal and laryngeal reflexes are preserved, making accidental inhalation of fluids or debris very unlikely.|
|Rapid induction and recovery.||This facilitates efficient scheduling of patients, rapid discharge and, for the patient, a prompt return to a normal routine.|
|Some analgesia.||But rarely sufficient for significant dentistry to be carried out with effective LA.|
|An escort is not mandatory for fit adult patients.||Nitrous oxide has a high MAC and it is not metabolised, so full recovery is usual in less than 30 minutes (but each patient must be assessed individually).|
Disadvantages of Inhalational Sedation
|Sedation depends also on good psychological support.||Nitrous oxide facilitates psychological suggestion. Inhalational sedation often fails if the sedationist relies on nitrous oxide alone.|
|The mask may make oral access difficult.||Particularly when treating upper anterior teeth.|
|Variable postoperative amnesia.||May be an advantage as the patient is able to recall that treatment was not as terrifying as anticipated.|
|Nitrous oxide pollution.||Prolonged exposure may damage the health of the dental team but this is unlikely with effective active scavenging.|
Contraindications to Inhalational Sedation
|Nasal obstruction, e.g. cold, polyps, deviated septum||Inability to inhale the gas renders the technique useless but nasal blockage may not be permanent.|
|Cyanosis at rest||A patient who is ‘blue’ at rest must be referred for specialist care.|
|Poor cooperation||The patient must be able to follow simple instructions and able to tolerate the nasal mask and breathing system.|
|First trimester (12 weeks) of pregnancy||There is some evidence that nitrous oxide can affect the foetus during the first trimester of pregnancy.|
|Fear of masks||Commonly associated with a previous unpleasant gas induction for GA. Some patients also dislike the smell of the ‘rubber’ mask.|
Nitrous Oxide Pollution and Waste Gas Scavenging
Long‐term exposure to nitrous oxide may result in an increased incidence of liver, renal and neurological disease and there is evidence of bone marrow toxicity and interference with vitamin B12 synthesis, which may lead to signs and symptoms similar to those of pernicious anaemia. For this reason, the Health and Safety Executive specifies a maximum level of 100 ppm of nitrous oxide time‐weighted over 8 hours (Skelly, 1992). In order to achieve this level and so keep nitrous oxide pollution to a minimum, scavenging must be employed (Figure 10.10).
Nitrous oxide is supplied in a blue cylinder containing both a gas and a liquid phase; oxygen comes as compressed gas in a black cylinder with a white collar. Most portable inhalational sedation machines are designed to operate with two nitrous oxide and two oxygen cylinders. One cylinder of each gas is ‘IN USE’ whilst the other is held in reserve and designated ‘FULL’. Only the ‘IN USE’ cylinders should be turned on. A Pin Index System ensures that the nitrous oxide and oxygen gas cylinders cannot be accidentally interchanged.
Nitrous oxide and oxygen pressure gauges give an indication of the contents of each cylinder. However, whereas the oxygen gauge falls in a linear manner, the nitrous oxide gauge starts to fall only when the liquid phase is exhausted and pressure in the gas phase is reducing.
The popular MDM RA machine ‘head’ has flow meters for nitrous oxide and oxygen, a control valve for regulating the total gas flow and a mixture dial for adjusting the percentage of oxygen and nitrous oxide. All modern inhalational sedation machines are incapable of delivering a gas mixture containing less than 30% oxygen and also have a failsafe mechanism which shuts off the nitrous oxide if oxygen ceases to flow.
The mixed gases emerge at the common gas outlet to which the breathing system is connected. The reservoir bag is useful for adjusting the total gas flow to an individual patient’s minute volume and also for monitoring respiration during treatment. Reservoir bags are made of rubber and are liable to perish, especially in the area of the bag mount (at the neck of the bag) and also down the ‘seams’.
Although designs vary, all modern inhalational sedation breathing systems comprise an inspiratory limb, a nasal mask and an expiratory limb. Systems for use with ‘active’ scavenging differ from those for use with ‘passive’ removal of waste gases. Active scavenging is achieved by connecting the expiratory limb of the breathing system to a low power suction device, whereas passive scavenging often involves simply placing the open end of the expiratory tube as far away as possible, preferably outside the operating environment.
Nasal masks are available in a variety of styles and sizes. Older style breathing systems must be cold sterilised, but some of the newer materials are suitable for autoclaving. Modern nasal masks have both fresh gas and scavenging connectors (Figure 10.13).