Connect the nasal hood to the hoses and adjust the flow of oxygen to a flow rate that would approximate the patient’s minute respiratory volume. The minute respiratory volume is the amount of new air a person breathes in a minute. It is calculated by multiplying the patient’s tidal volume (the volume of air in a normal breath) by the patient’s respiratory rate per minute.
A patient’s minute respiratory volume will vary with body size and age. An adult may have a greater tidal volume than a child, however, a child has a greater respiratory rate than an adult therefore the minute respiratory volumes are not that different between a child and an adult (Table 3).
|Infant||75-125 ml||30/min||2250-3750 ml/min|
|Child||200-250 ml||20-24/min||4000-6000 ml/min|
|Adult||400-450 ml||12-18/min||4800-8100 ml/min|
It is most important that gas is flowing before placing the nasal hood over the patient’s nose. It is nothing more disconcerting to an anxious patient than the inability to breathe. Unless the gas is flowing, the patient cannot breathe. One hundred percent oxygen is administered. The nasal hood is gently placed and fitted over the patient’s nose so there are no leaks of gas into the surrounding environment (Figure 1,2).
Attention is now drawn to the reservoir bag. If there is sufficient flow of gas to the patient, the bag will be inflated half of its full volume and inflate and deflate in synchronization with the patient’s inhalation and exhalation pattern. If too little gas is flowing, the bag will collapse. If too much gas is flowing, the bag will overinflate. Once the proper flow rate is established it remains constant throughout the procedure.
Once the correct flow rate is determined the patient’s optimum nitrous oxide/oxygen concentration is titrated. The titration process begins by decreasing the oxygen flow and increasing the nitrous oxide flow to obtain a concentration of 20% nitrous oxide and 80% oxygen.
This concentration remains at this level for three minutes. There should be minimal communication between the patient and dentist and/or auxiliary. Talking should be minimized so nasal breathing is maximized and the nitrous oxide/oxygen level remains constant during this period. At the end of three minutes, the dentist inquires of the patient what symptoms, if any, do they feel. Leading questions should be avoided, i.e., Do you feel tingling? Lightheaded? Relaxed? Asking such questions when the anxious patient is not feeling the effects of the nitrous oxide will just make the patient more anxious, for fear the gas is not working properly. The patient is asked to describe what they feel. If they respond there is no difference, the nitrous oxide/oxygen levels are adjusted to 30% nitrous oxide and 70% oxygen. The process is repeated and after three minutes the patient is once again asked to describe what they feel. If they respond in the negative, the nitrous oxide/oxygen concentration is increased to 40% nitrous oxide and 60% oxygen. This process may be repeated until the patient expresses positive symptoms. However, the concentration of nitrous oxide/oxygen may not exceed 70% nitrous oxide and 30% oxygen due to the fail-safe limitations set on the delivery unit.