Practical Matters: Abnormally low capnography readings may not be due to hyperventilation

Using capnography or a capnometer to determine end-tidal CO2 (ETCO2) is a simple, practical, and noninvasive way to monitor patient ventilation during anesthesia. ETCO2 is a reasonable reflection of arterial CO2 when cardiovascular function is normal and no pulmonary disease is present. However, when physiologic dead space (area of the lung where there is no perfusion and, thus, no gas exchange) increases, there will be an increase in the arterial to ETCO2 difference. A variety of circumstances produce low ETCO2 concentrations in patients, but not all are purely a result of changes in ventilatory status. In some circumstances, evaluating the capnographic waveform—vs. only evaluating the numerical reading from a capnometer—can help you diagnose the cause of the low ETCO2 concentration.

POOR CARDIAC OUTPUT

Most important, when reduced cardiac output leads to poor pulmonary perfusion, ETCO2 will be substantially lower than arterial CO2 and, thus, will be an inaccurate reflection of true ventilation. Commonly these patients also exhibit systemic hypotension because of the lowered cardiac output, so concomitant blood pressure measug becomes physiologic dead space, thus the patient's ETCO2 concentration is low, often 0 to 5 mm hg. And indeed, capnography can be used to monitor the effectiveness of cardiac massage in patients with cardiac arrest since measurable ETCO2 (> 10 mm hg) indicates some pulmonary perfusion, though it is important to remember that ventilation must be controlled in order to hold that variable constant such that ETCO2 is a reflection of circulatory status. And keep in mind that using high doses of epinephrine during resuscitative efforts will affect pulmonary perfusion and increase v-q mismatch. Under these conditions, ETCO2 may still drop even though coronary perfusion pressure increases. However, using capnography to evaluate trends in patients with low cardiac output will be of questionable value and can lead to a decision to change ventilation in the wrong direction.

OTHER MISLEADING SITUATIONS

Additionally, low capnography readings in anesthetized patients may occur during panting or when tidal volume is extremely low, with a low proportion of alveolar ventilation to dead space ventilation. High oxygen flow rates used with Bain coaxial circuits in combination with small patient tidal volume will give low capnography readings. Again, these situations will erroneously lead one to believe a patient is hyperventilated if conclusions are drawn based on ETCO2 concentrations alone.

Kris Kruse-Elliott, DVM, PhD, DACVA

Department of Surgical Sciences

School of Veterinary Medicine

University of Wisconsin-Madison

Madison, WI 53706