XI. Indications and Contraindications for the Use of Nitrous Oxide

When choosing a treatment modality, including the selection of drugs or the technique of administration, the choice must always be on the basis of choosing the proper drug for the needs of the patient, and NOT the adaptation of the patient to a method of treatment.

Once it is determined that a patient would benefit from sedation, the best form of sedation should be determined. Nitrous oxide is only one of a growing list of agents available to the dentist today for the control of anxiety in the dental office. While perhaps the safest of all presently available agents, it is relatively weak, and therefore not indicated for the very apprehensive patient.

While N2O is an effective sedative agent, the analgesia produced at the sedative level is insufficient for pain relief in most dental procedures. Therefore, nitrous oxide should NOT be considered a form of pain control. Except for a few procedures that produce only very minor discomfort, it is still necessary to utilize local anesthesia for adequate pain control with N2O conscious sedation.

The selection of N2O conscious sedation may be made for the mildly fearful or apprehensive patient, for the fearful child, to reduce the awareness of time and fatigue in long procedures, for reducing dental stress for cardiac or other patients in which stress needs to be avoided. Nitrous oxide is valuable in reducing the gag reflex for those patients in whom gagging is a problem. This is probably a combination of removing anxiety and the fact that it is impossible to gag when breathing only through the nose. Many patients ask for nitrous oxide as a method of making the dental experience one that is actually pleasant and no longer to be avoided.

Nitrous oxide may also be selected as an adjunct to other forms of sedation, and it combines very well with many of the other sedative drugs. It must be remembered, however, that the risk of complications is greatly increased when other sedative drugs are combined with N2O, and additional training and experience is necessary before the dentist should use combinations. In this course, we are considering the use of N2O and O2 only.


Contraindications for the use of N2O

There are very few absolute contraindications for the use of N2O. In general, the more medically compromised the patient, the less able he or she may be to withstand the additional stress of a dental procedure. The risk of stress is often greater than the risk associated with N2O, and a patient receiving N2O may, in fact, be under less risk than if N2O were not being used.

However, there are times and conditions when N2O should not be used. Patients with chronic obstructive pulmonary disease have both a reduced ability to move gases into and out of the lungs because of reversible bronchospasm and irreversible bronchial obstruction, and hypoxemia and hypercarbia resulting from chronic hypoventilation or poor gas exchange across the respiratory membranes. Therefore, it is more difficult to administer N2O, and more importantly, is more difficult to get it back out of the patients, thus losing the great safety factor mentioned earlier. In addition to reducing the rate of intake and removal of nitrous oxide, patients with severe emphysema lose their carbon dioxide induced stimulus to breath, and may rely mainly on reduced oxygen content of the blood to drive this stimulus. When high concentrations of O2 are administered, as is always the case with N2O - O2 conscious sedation, the patient may actually lose much or all of the respiratory drive. Fortunately, this potential for apnea is more theoretical and rarely becomes a clinical problem.1

Since N2O is not irritating to the tracheobronchial tree, ASTHMA IS NOT A CONTRAINDICATION TO THE USE OF N2O, providing the patient is not having an"attack". In fact, there is benefit in administering nitrous oxide since in many asthmatics, the primary precipitant appears to be emotional stress, especially in children. No dental procedure should be attempted if the patient is having respiratory difficulty due to asthma.

Any nasal obstruction will severely restrict the patient's ability to breathe through the nose. Since nitrous oxide must be administered via a nasal mask, this becomes a relative contraindication depending on the severity of the obstruction. Patients who are "mouth breathers", either due to nasal restriction or simply from habit, do not do well attempting to breathe through the nasal mask and often can not exchange well enough nasally to be comfortable. Any upper respiratory infection that causes blockage of the nasal air passages will, of course, restrict the ability to administer N2O. In addition, coughing prevents the dentist from accurately regulating the N2O and O2 when the patient takes in large quantities of room air by mouth during coughing. In addition, coughing by the patient sprays large amounts of N2O (not to mention microorganisms) into the operatory to levels that are above recommended amounts.

Multiple sclerosis is a disease characterized by nerve demyelination especially in the central nervous system. Symptoms of weakness, incoordination, paresthesia, speech disturbances and other neurological disturbances are common findings. Since chronic exposure to N2O may produce similar neuropathies, it would seem unwise to expose patients with multiple sclerosis repeatedly to N2O. An isolated or infrequent use probably would not be contraindicated; one author1,in fact, recommends inhalation sedation for patients with multiple sclerosis.

Nitrous oxide readily enters fetal circulation, and because of the possible toxicity of N2O to cells undergoing mitosis, pregnant patients probably should not receive nitrous oxide electively, especially in the early weeks of pregnancy. However, necessary emergency dental care should not be denied a pregnant patient, and if it is determined that N2O is necessary to reduce stress, it may be used following consultation with the obstetrician. Non-emergency N2O use should be postponed until after delivery whenever possible.

Dealing with psychiatric patients can be a challenge. Many times, the medication taken by the patient will provide a sedated individual. It is best to consult the psychiatrist before administering N2O. Remember that psychotropic drugs usually alter cerebral cortex function. However, many psychiatric patients can not handle dental stress easily. If N2O, is chosen, these patients should be titrated carefully, and need very close monitoring since their reaction may be somewhat unpredictable.

Since much of the patient monitoring with N2O conscious sedation is done verbally, being unable to communicate with the patient becomes a contraindication to the use of N2O. This would include severely mentally retarded patients, very young patients, language barriers or any condition that prevents easy exchange of thoughts between doctor and patient.

The use of marijuana or hallucinogenic drugs are contraindications for N2O. Marijuana apparently increases the pleasure of a pleasant situation, but it also can increase the dysphoria of a stressful or unpleasant situation. Because nitrous oxide appears to increase the effects of marijuana, a moderate "stressful" dental experience may be magnified into a very traumatic and unpredictable reaction. Following heavy use of marijuana, the drug may remain in the circulation for seven or more days due to its very long half life. Hallucinogenic drugs act in a similar manner only with a great deal more intensity. Reactions may be very frightening for the patient and even more so for the dentist.

Nitrous oxide has been shown to increase intracranial pressure in patients with certain injuries and intracranial disorders, therefore, N2O should not be administered to these patients.

Since N2O is a relatively weak agent, it should not be used as the sole agent in a very apprehensive patient, or as a substitute for anesthesia.

Because nitrous oxide has a solubility coefficient that is 35 times more than nitrogen, it can quickly displace nitrogen in any closed cavity, dramatically increasing the pressure within. Therefore, conditions such as; blocked eustachian tube, blocked bowel, acute blocked sinusitis, pneumothorax, that allows a rapid pressure increase in a closed body cavity, leading to pain, contraindicates the use of N2O.

Exposure to N2O causes a depression of bone marrow activity resulting in a reduction in the production of erythrocytes and leukocytes. However, since normal marrow contains a "store" of mature cells sufficient to supply several days' needs, and the marrow recovers to return to production within 3 to 4 days, no hematologic change is seen following an isolated anesthetic or conscious sedation exposure to nitrous oxide. However, a second exposure within this period of time will extend inhibition of synthesis, which may exceed the safety factor of stored cells. Since repeated exposures at close intervals may produce leukopenia.2, frequent exposure to nitrous oxide (less than 1 week between administrations) should be avoided.

Because N2O seems to reduce chemotaxis (the motility of leukocytes) toward foreign proteins such as bacteria, as well as to reduce leukocyte action against tumor cells, patients with compromised immune systems should avoid nitrous oxide unless urgently required. This would include patients with AIDS (acquired immune deficiency syndrome), or those taking immunosuppressive drugs.

Finally, N2O should never be used on an unwilling patient. If, after explaining the benefits, the patient for whatever reason does not want N2O, it should not be used.

Relative contraindications to N2O

Absolute contraindications to N2O

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