Introduction

Inhalation sedation is the safest form of sedation, due principally to the nature of nitrous oxide, which is almost universally used in this technique. The term ‘inhalation sedation’ describes the induction of a state of conscious sedation by administering sub‐anaesthetic concentrations of gaseous anaesthetic agents. Its most common application is in children’s dentistry, where it has been used successfully for many decades, but its use in adult dentistry is increasing. The favourable pharmacological properties of nitrous oxide make it the agent of choice for most inhalation sedation techniques.

Since its discovery in the eighteenth century, nitrous oxide has been the basic constituent of gaseous general anaesthesia, although it was not until the 1960s that it was more widely used in inhalation sedation. Harold Langa of the United States introduced the concept of ‘relative analgesia’, a specific type of inhalation sedation. This sedation uses variable mixtures of nitrous oxide and oxygen to induce a state of psycho‐pharmacological sedation.

Relative analgesia has now become the standard technique for inhalation sedation in dentistry. Other methods of inhalation sedation do exist, such as the use of fixed concentrations of nitrous oxide and oxygen (Entonox®) but these are not commonly used in dentistry.

Inhalation Sedation in Dentistry

The aims of inhalation sedation are to alleviate fear by producing anxiolysis, to reduce pain by inducing analgesia, and to improve patient co‐operation so that dental treatment can be performed. Inhalation sedation embodies a triad of elements:

1. The administration of low to moderate titrated concentrations of nitrous oxide in oxygen to patients who remain conscious.
2. The use of a specifically designed machine with a number of safety features, including the ability to deliver a minimum of 30% oxygen and a fail‐safe device that cuts off the delivery of nitrous oxide if the oxygen supply fails.
3. The use of semi‐hypnotic suggestion to reassure and encourage the patient throughout the period of sedation and treatment.

The success of inhalation sedation relies on a balanced combination of pharmacology and behaviour management. Nitrous oxide (N₂O) will produce a degree of pharmacological sedation on its own but this is unpredictable and should be supplemented and reinforced with psychological reassurance. The pharmacological properties of nitrous oxide produce physiological changes that enhance the patient’s susceptibility to suggestion. The use of semi‐hypnotic suggestion to positively reinforce feelings of relaxation and well‐being, will increase the extent of the anxiolysis and co‐operation. In contrast to intravenous sedation, which produces pharmacological sedation regardless of any element of suggestion, inhalation sedation induces a state of psycho‐pharmacological sedation.

Planes of Analgesia

The clinical effects of sedation with nitrous oxide can be divided into three broad categories. These form part of the stages of anaesthesia (Figure 6.1).

The first stage of anaesthesia, the analgesic stage, is subdivided into three ‘planes of analgesia’:

In general terms, most clinically useful sedation is produced in Plane I and sometimes in Plane II, although some patients find the dissociation effects disorientating. It is these planes that are encompassed by the definition of relative analgesia (inhalation sedation). Plane III is a transition zone between the state of conscious sedation and true general anaesthesia and thus it is termed total analgesia rather than relative analgesia. There is considerable overlap between the planes and a large variation in susceptibility of individual patients to the effects of nitrous oxide. While one person may be adequately sedated with 10% nitrous oxide, another individual may require in excess of 50% nitrous oxide to achieve the same degree of sedation.

Each plane of analgesia is accompanied by specific clinical signs:

Plane I (N₂O Concentrations of 5–25%)

• relaxation and a general sense of well‐being
• paraesthesia, a tingling feeling in the fingers, toes and cheeks
• a feeling of suffusing warmth is common
• alert and readily responds to questioning
• slight reduction in spontaneous movements
• decreased reaction to painful stimuli
• pulse, blood pressure, respiration rate, reflexes and pupil reactions will all be normal.

As the nitrous oxide concentration is increased to the 20–55% range there will be a gradual transition from Plane 1 to Plane II.

Plane II (N₂O Concentrations of 20–55%)

• marked relaxation and sleepiness
• a feeling of detachment from the environment
• senses will be altered
• possible dreaming
• widespread paraesthesia, moderate analgesia
• reduction in the gag reflex
• delayed response to questioning
• vital signs and the laryngeal reflexes should be unaffected.

When the nitrous oxide concentration goes above 50%, there will normally be a transition into Plane III.

Plane III (N₂O Concentrations above 50%)

• marked sleepiness and a ‘glazed’ appearance
• complete analgesia
• nausea and dizziness are common
• patient may vomit
• unresponsive to questioning
• may lose consciousness and enter Stage 2 of general anaesthesia.

If any of these signs occur, the nitrous oxide level should be reduced. There is usually a gradual transition between planes and not all patients show all of the clinical signs. However, the planes of analgesia are a useful guide to what to expect when sedating a patient with nitrous oxide. Specific signs such as nausea, dizziness and a glazed appearance provide a warning that the level of sedation is too high and the percentage of nitrous oxide should be reduced. However, there is considerable variation in individual response and it should be remembered that the success of the technique is probably more dependent on the operator’s ability to infuse hypnotic suggestion, than it is to the effect of nitrous oxide.

Indications and Contraindications for Inhalation Sedation

Indications

• Management of dental anxiety (children and adults)
• Management of needle phobia
• Management of gag reflex
• Management of medically compromised patients.

Inhalation sedation is particularly useful for anxious children. Children must be able to understand the purpose and mechanisms (in appropriate terminology) of inhalation sedation, so the minimum age for treating children under inhalation sedation is approximately three years. This is usually the lowest age at which the child has an appropriate degree of understanding to enable sufficient co‐operation for treatment. Older children scheduled for orthodontic extractions may also benefit from inhalation sedation. Such children may not be particularly frightened of routine treatment but multiple extractions of permanent teeth or surgical procedures, such as the exposure of canines, can be somewhat traumatic. Sedation can help to make the procedure more acceptable and the time pass more quickly.

Another key indication for inhalation sedation is the treatment of adults who have a general (as opposed to dental) phobia of needles or injections. Such individuals find it impossible to accept venepuncture and venous cannulation. They can benefit considerably from inhalation sedation, either as the sole form of sedation or as an adjunct to intravenous cannulation. In these individuals, the inhalation sedation is used to induce a level of sedation sufficient to enable venous cannulation. Once the cannula is successfully located, the delivery of nitrous oxide is terminated and the intravenous sedative can be administered.

Inhalation sedation is also used for a number of special categories of patients who are at risk from the respiratory depressive effects of intravenous agents. These include patients with sickle cell anaemia or asthma, who benefit from the guaranteed level of oxygenation (at least 30% and usually significantly more) used in inhalation sedation. For the few patients with a proven allergy to intravenous sedatives, the only alternative sedation technique may be inhalation sedation.

Contraindications

Many of the contraindications to inhalation sedation are relative or temporary and include:

• upper respiratory tract infections
• large tonsils or adenoids
• serious respiratory disease
• mouth breathers
• very young children
• moderate to severe learning difficulties
• severe psychiatric disorders
• pregnant women
• patients taking Methotrexate (due to the anti‐folate effect of nitrous oxide)
• patient who have had vitreoretinal surgery in the last 3 months
• upper anterior apicectomy.

Very few of the indications and contraindications for inhalation sedation are absolute. In many cases it is necessary to carefully balance the risk of giving the patient sedation against the risk of general anaesthesia, which is often the only option for some anxious dental patients. Each patient should be individually assessed, although only those who fit the above selection criteria and who meet the general standards discussed in Chapter 3, should be treated in dental practice. There may be others, however, who can be referred for treatment under inhalation sedation in a hospital setting, where any complications can be dealt with more easily.

Advantages and Disadvantages of Inhalation Sedation

Advantages

• Non‐invasive technique with no requirement for venepuncture/cannulation
• Nitrous oxide is relatively inert so that there are no metabolic demands
• The low solubility of nitrous oxide ensures a rapid onset and recovery
• The level of sedation can easily be altered or discontinued
• Little effect on the cardiovascular and respiratory systems
• Some analgesia produced.

Disadvantages

• The drug is administered continuously via a nose mask close to the operative site
• The mask may be objectionable to the patient
• The level of sedation relies heavily on psychological reassurance
• The technique requires a certain level of compliance in terms of breathing through the nose
• It is not suitable for very young children and patients with severe behavioural problems.

Patient Preparation for Inhalation Sedation

Assessment and treatment planning for patients for inhalation sedation should follow the format described earlier in Chapter 3. Inhalation sedation should be seen as part of an overall behaviour management strategy and the aim of the assessment appointment should be to select those patients who need some form of extra support to help them through treatment. When assessing children for inhalation sedation it is important to involve both the child and the parent.

The type and extent of dental treatment needed should be taken into account when considering sedation. Although most routine operative dentistry can be performed under inhalation sedation, the nature of the treatment must be matched against the age of the patient and their predicted level of co‐operation. One or two extractions in a four‐year‐old could, quite reasonably, be performed under inhalation sedation. However, if the same patient required the extraction of multiple grossly carious teeth it might be kinder to refer the patient for a short general anaesthetic. Similarly, a 13‐year‐old could willingly accept the extraction of four premolars under inhalation sedation, but if they required the exposure of a deeply buried canine, general anaesthesia may be preferable.

Assessment of the medical status of a patient scheduled for inhalation sedation is identical to that described in Chapter 3. Particular attention should be paid to respiratory