Tuesday May 30, 2006
Use of Capnography in Assessment of CPR Adequacy
Myocardial blood flow is determined by the difference between aortic diastolic and right atrial pressures. Because both aorta and atrium experience the same intrathoracic pressure change during cardiopulmonary resuscitation (CPR), myocardial blood flow is very poor during cardiac resuscitation. Even high compression forces that may generate acceptable systemic and pulmonary artery pressures yield only small coronary perfusion pressures.
The arterial blood gas values during CPR manifest complex abnormalities. The reduction in cardiac output, and thus tissue perfusion, promotes anaerobic metabolism and lactic acidosis. However, arterial blood samples reflect either a normal or low PCO2 during CPR, while venous blood gases manifest both a respiratory and metabolic acidosis.
When perfusion is absent in the presence of ventilation, the primary influence on arterial acid-base status is alveolar ventilation. Venous acidosis develops as tissue beds drain CO2 and lactate is produced by anaerobic metabolism. The PCO2 in pulmonary veins increases due to reduced pulmonary blood flow and a resulting decrease in CO2 excretion.
With effective CPR or return of spontaneous circulation, pulmonary blood flow is improved and arterial pH decreases as more of the venous acid load (CO2 and lactate) reaches the arterial side. Aerobic and anaerobic metabolism produce carbon dioxide that is transported in venous blood to the lung and eliminated from the lung by minute ventilation. End-tidal CO2 is a measure of the partial pressure of carbon dioxide at the airway opening at the end of expiration.
During cardiac arrest, the abrupt decrease in cardiac output results in reduction of carbon dioxide transport from the tissues to lung and, hence, decreased carbon dioxide. More recently, capnography has been used to determine the adequacy of cardiopulmonary resuscitation.
There are 2 good sources to understand capnography:
1. capnography.com , educational site on subject from Bhavani-Shankar Kodali MD, Department of Anesthesia, Brigham and Women's Hospital, Harvard Medical School, Boston, USA
2. See concise INTERPRETATION OF CAPNOGRAPHIC WAVEFORM
(from biotel.ws website)
