Give ’Em the Real Story

While working on ventilators with some foreign students and even some US service reps, I have heard most of them make the following statement: “Blood pressure is the only sensitive indicator of the cardiovascular system while under anesthesia.” To correct their misunderstanding, I had to reach way back in my memory bank to come up with the following explanation.

Monitoring bioelectric events is difficult in the operating room because of its electrically noisy environment and frequent use of the electrosurgical units. Despite these difficulties, however, the electrocardiogram is often monitored to identify the heart rate and occasionally to detect arrhythmias. Although not every ventricular excitation signaled by an R wave means that a forceful mechanical beat occurred, a change in R-wave rate is an alerting signal to the anesthesiologist.

Perhaps most routinely monitored is blood pressure. Although blood pressure can be measured noninvasively with the auscultatory method, the operating room is usually noisy and the Korotkoff sounds are often difficult to hear, especially if the blood pressure is low. More frequently employed the oscillometric method, in which the amplitude of cuff pressure oscillations is used to identify systolic, diastolic, and mean pressure as well as heart rate. Heart rate can be determined accurately in this way. Moreover, the oscillometric method performs well in situations of low blood pressure. Blood pressure is not a sensitive indicator of the status of the cardiovascular system, however, because it reflects the product of cardiac output and peripheral resistance. Peripheral resistance can decrease and cardiac output can increase, and the blood pressure will give no evidence of these events.

Another indicator is respiration. It is assessed by the number of breaths per minute (as shown by the breathing bag) and the color of the lips, gums, or nail beds. Although there may be adequate delivery of oxygen to the airway by the anesthesia machine, the uptake (mL per minute). Its measurement requires an airway flow meter and a rapidly responding gaseous oxygen sensor. The uptake per breath is the volume per breath multiplied by the difference between inspired and expired oxygen concentrations. The oxygen uptake is assessed indirectly by the oxygen saturation measured by an oximeter. An oximeter indicates percent saturation, though, not content (mL O2/100 mL blood). Nonetheless, a reduction in saturation is another alerting signal for the anesthesiologist to investigate.

The anesthesiologist must consider many indicators, such as end-expired carbon dioxide assessment of arterial pCO2, another sensitive indicator of the function of the cardio-respiratory system, and pO2 and pCO2. These indicators are needed to ensure vigilance—key to a successful anesthetic procedure.

As an experienced pilot and having talked with several anesthesiologists, I have found that giving anesthesia is a lot like flying an airplane: The pilot responds to all the cues that permit him or her to make a timely and successful flight. So the next time someone tells you that one thing in a medical device is the most important and sensitive, give ’em the real story.

Scottie R. DeWolfe, CE, BMET, is the sole proprietor of B-METS Corp, Dothan, Ala.