Monitoring techniques
The solution to the aforementioned problems lies in several areas: (1) Use drugs that minimally depress respiration; (2) Avoid combinations of different drugs that affect several areas at once, and (3) Monitor the patient to assure that adequate arterial oxygen levels are maintained and that CO2 levels do not increase.
Hypoventilation is characterized by a reduction in arterial oxygen tension and an elevation of arterial CO2 tension. With the advent of pulse oximetry, it is now possible to easily monitor oxygen saturation of hemoglobin. Equipment also exists to monitor end-tidal CO2 tension. Although elevated CO2 tension is generally regarded as the earliest sign of hypoventilation, the equipment is extremely expensive.
Pulse oximetry shows the oxygen saturation of hemoglobin. In addition, most machines also display pulse rate. Knowing the saturation of hemoglobin, one can approximate the arterial oxygen tension, assuming a normal pH of the blood. Although 90% saturation provides reasonable assurance of adequate arterial oxygen tension, 95% is preferred. One should not overestimate the value of this information. Although normal hemoglobin saturation reasonably assures adequate oxygenation, this does not rule out an elevated CO2 tension. If the clinician supplements the patient with enriched oxygen concentrations (as is common when administering nitrous oxide) that may sustain hemoglobin saturation despite hypoventilation. Well oxygenated patients may hypoventilate to the point of significant hypercapnia. Therefore, when using pulse oximetry, oxygen supplementation should be reserved for those cases in which adequate arterial oxygen cannot be sustained, despite verbal commands. This should be a rare event when using light to moderate sedation for ASA I and II patients. By allowing the patient to breathe room air, the oximeter will function more effectively as an early warning of hypoventilation. Although oximetry is not equivalent to capnography, it is valuable in alerting the dentist that ventilation is depressed.
The importance of monitoring a patient's respiration can not be overemphasized. If that patient is awake and responding to verbal commands, we can usually assume our patients are safe. If they are unconscious (asleep?), we must have more concern. Several studies of medical and dental anesthesia have shown inadequate ventilation to be the most common cause of death or brain damage. Cheney and Jastak, after reviewing malpractice cases, arrived at the conclusion that airway management represents the most common etiologic factor in brain damage and death in general anesthesia problems. It is the cases of the idiosyncratic reaction, overdose, or the patient on other drugs that were not reported to us that give us the most concern. It is possible for these patients to become unconscious. As practitioners we must be prepared to monitor and assist the respiration of such a patient if it should become necessary. To know if assistance is needed, we need to know the status of the oxygenation of the patient's tissues.
The Reservoir Bag
In the 1960's, watching the reservoir bag was used on several popular TV programs as a means of determining when it was time to discontinue surgery and give condolences to the next of kin. At least one manufacturer suggested that the presence of a reservoir bag on our nitrous oxide machines could have a negative psychologic effect on some patients. Why do we need one unless it is to monitor the passage of the patient to the great beyond? On the other hand, there have been several studies that show one should not depend on movement of the bag as an accurate indication of the adequacy of respiratory exchange. Fortunately, the lay person's medical knowledge has improved over the years and everyone can now recognize a "flat line" as the determining factor.
Pulse oximeter
The advent of an affordable pulse oximeter has made our life much easier and the patient's life more secure. By passing two different frequencies of light through various tissues, and reading the absorption of the two frequencies and evaluating these differences, this device can determine the percentage of oxygen saturation of the arterial blood with great accuracy. In addition to O2 saturation, most equipment also shows pulse rate and some shows a pletysmograph of the pulse wave. The use of such monitoring has made general anesthesia much safer and consequently has decreased the frequency of tragic outcomes. Although not strictly necessary for sedation, the security the equipment provides would be sorely missed if I no longer had a such a device to monitor my sedation patients.
However, there is at least one possible caveat to their use: If a patient is given supplemental oxygen, their hemoglobin saturation will approach 100%. In patients with severe respiratory complications, oxygen saturation could be normal even though exchange rates were inadequate to cleanse the blood of CO2. This could lead to high CO2 levels and resulting low pH of the blood. As was pointed out, however, the patient will be damaged more by a lack of O2 than by high CO2 levels. This potential problem can be circumvented by limiting sedation to patients with no significant respiratory problems.
Capnography
Equipment now exists that constantly senses a patient's expired gas. It then gives a reading of the concentration of CO2 in these gasses. This information can be invaluable when monitoring patients with respiratory problems or those undergoing general anesthesia but it is not necessary for the sedated patient. Typically, this equipment indicates end tidal carbon dioxide (ETCO2) concentrations. ETCO2 is the last portion of an expired breath. The concentration of CO2 in the expired gas at the very end of an expiration closely approximates the concentration of CO2 in the alveoli and that of the venous blood. Many machines also show graphically the shape of the breath. This can be of interest as it quickly shows a patient who may be taking very long shallow breaths or rapid shallow breaths that could lead to respiratory insufficiency. The shape of the breath also can be an aid in diagnosing some respiratory pathology, including chronic obstructive lung disease and asthma. These machines have alarms that respond to apnea as well as high and low ETCO2 levels, alerting the practitioner when preset parameters are broached.
It is interesting to take a normal, healthy, nonsedated patient and monitor them with the pulse oximeter and capnograpy. Have this patient hold their breath for a minute and watch the reading. The capnograph immediately shows apnea, and the alarm sounds when the apnea has exceeded the preset limit, usually about 20 seconds. At one minute, the pulse oximeter usually starts to show a drop in oxygen saturation, although typically it will still be reading in excess of 95% saturation. Once the patient starts breathing, the ETCO2 will read high until the build up of CO2 has been removed; the oxygen saturation will continue to fall for 30 seconds to a minute. In our trials, however, we were never able to set off the low O2 saturation alarms. The capnograph is a much more sensitive indicator of respiratory depression/cessation. If our patient is on supplemental oxygen before they hold their breath, the pulse oximeter will normally be reading 99 to 100% saturation and will remain at that level until the person starts breathing again, even though their CO2 levels will have climbed significantly. It is possible to counter the warnings of a pulse oximeter by having a patient on supplemental oxygen.
Cardiovascular Concerns
For our sedation patients, pulse rate and blood pressure should be monitored. In my study and in practice I have a constant pulse rate displayed by the pulse oximeter. In addition, I feel a preoperative blood pressure should be recorded and updated at least every 15 minutes, provided the patient remains awake. Should the patient become unconscious, I would assume they are under general anesthesia or have had some medical problem and I would monitor blood pressure at least every 5 minutes until I have a conscious patient.