VIII. The Effects of Nitrous Oxide Concentration

The dosage of N2O is a function of the concentration,expressed as a percentage of N2O administered together with oxygen. When the expression of a percentage of N2O is given, for example, 30% N2O, it is implied that at the same time 70% oxygen is also being administered. This can cause some confusion, as some N2O machines are calibrated in percentage of oxygen, while others are calibrated in percentage of N2O.

Changing the percentage of N2O that is administered to the patient changes the percentages of N2O in the alveoli of the lungs. This, in turn, disrupts the balance across the respiratory membrane, and ultimately, changes the amount of nitrous oxide dissolved in the circulating blood. Therefore, changing the percent of N2O given to the patient has the effect of changing the dosage of the drug.

Percentages less than 10% generally have so little effect on most normal patients that it can be considered ineffective. Usually, concentrations greater than 20% are necessary before the patient is aware of much change, and from about 25 to 50% all of the desired conscious sedation effects are seen. Certainly there are exceptions, and some patients require higher percentages to obtain the desired effect.

Verbal reinforcement is very important in administering N2O because pain tolerance can be significantly altered by providing the patient with positive verbal reinforcement. Alternatively, if a patient is unaware of the benefit of N2O or has no particular expectations from the drug's use, pain threshold and tolerance may not improve or can even decrease. It is unusual to have to exceed 50% concentrations to obtain all of the sedation and analgesia needed. In fact, if it is necessary to exceed 50%, it is felt the wrong form of sedation has been selected, and consideration should be given to utilizing an additional or a more potent form of sedation, such as intravenous sedation or general anesthesia. Exceeding 50% N2O may be considered anesthetic or near- anesthetic concentrations which may cause the patient to become unresponsive, produce amnesia and perhaps enter the "excitement stage" of general anesthesia.

It is usually accepted that the longer the period of N2O administration, the deeper the analgesic effects; but recent data suggests a peak affect may be around 30 minutes and then there follows a gradual decrease in analgesia. This will not usually be a problem since sedation continues; relaxation has already taken place and most often, the "traumatic" segment of the dental procedure will be over.

NORMAL DISTRIBUTION CURVE

The following information was reported by Malamed5, and was compiled from 3500 N2O administrations at the USC School of Dentistry. It shows the percentage of patients obtaining "ideal" sedation with the various percentage of N2O.

percentage of N2O.....% of patients achieving ideal sedation

10.......................................................<1
15.........................................................1
20.........................................................4
25.........................................................7
30.......................................................21
35.......................................................25
40.......................................................24
45........................................................9
50........................................................5
55........................................................1
60........................................................2
65......................................................<1
70......................................................<1

As one might expect, a majority (70%) of patients achieved ideal sedation between 30% and 40% N2O. Less than 4% required concentrations over 50%. It should also be noted that N2O failed to achieve ideal sedation in about 3% of patients due to poor patient selection, extreme apprehension, being mouth breathers, having claustrophobia, or for whatever reason.

The effects of N2O are also somewhat governed by the duration of administration; the longer the administration of N2O, the more pronounced the effects, especially sedation, may become. After 30 minutes or so, the concentration of N2O can often be reduced, yet the same effects will remain.

The above concentrations and their effects are those to be expected at or near sea level. At higher altitudes a higher concentration of N2O may be required.

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