Session 1: Understanding the Oxygen Pressure Field Using the Krogh Cylinder Model

CO2 Pressure Field & Cardiac Index

The carbon dioxide pressure field normally has small gradients due to the high solubility of CO₂. The formula for calculating the cardiac index using the venoarterial CO₂ gradient comes from Johnson & Weil, 1991.
The formation of an anoxic lethal corner is not the only mechanism that can result in tissue acidosis. The trapping and retention of carbon dioxide in the lethal corner area also occurs and can be described using the Krogh cylinder model. This phenomenon is particularly deadly because it is almost totally unknown to clinicians caring for critically ill cardiopulmonary patients and it can occur without any deviation of the arterial blood gas from normal values or a change in hemodynamic parameters.
There is a carbon dioxide pressure field but it does not normally have the large range of values like those seen in the oxygen pressure field . Carbon dioxide is 23 times more soluble in water than oxygen, so it requires only 1/23rd the gradient needed by oxygen for removal from the Krogh cylinder. Carbon dioxide has its highest concentration in the distal corner of the Krogh cylinder and its lowest concentration at the capillary’s proximal end, nearest the capillary wall; the opposite configuration of the oxygen pressure field. In this illustration, the arterial blood pCO₂ is 40 mmHg and the arterial-end tissue pCO₂ is 42 mmHg. The venous blood pCO₂ is 45 mmHg and the venous-end tissue pCO₂ is 47 mmHg. As long as the intracapillary velocity of the blood remains normal (~200μ/sec) and the diameter of the Krogh cylinder does not change, then adequate amounts of carbon dioxide will be removed from the tissues. The evidence of this is a normal venoarterial carbon dioxide gradient (p[v-a]CO₂) which is about 5 to 7 mmHg. Since the p[v-a]CO₂ is in part dependent on the intracapillary velocity of the blood, the cardiac index can be estimated from the p[v-a]CO₂. In adults, dividing the constant “12.9” by the p[v-a]CO₂ gives an estimate of the cardiac index with R=0.76 and R² = 0.58. In the younger the patient, the constant is larger. This is because of the higher metabolic rate in younger patients. In infants the constant is estimated at 18.