What are the major reasons for a high PvCO2 ?
The PvCO 2 is essentially the summation of the arterial PCO 2 (PaCO 2 ) entering the capillaries and the CO 2 added by cellular metabolism.
At the usual blood flow rate at rest, this results in a brachial PvCO 2 of 46 mm Hg, assuming a normal input arterial PaCO 2 of 40 mm Hg. Accordingly, the reasons for a high PvCO 2 are due to either a high PaCO 2 (i.e., respiratory suppression/failure), which raises the lowest possible value for PvCO 2 , and/or a slow blood flow rate to muscle (e.g., poor perfusion states such as evolving shock).
The reason that diminished tissue perfusion raises PvCO 2 is outlined as follows:
1. When capillary blood flow falls, tissue oxygen consumption remains constant.
2. Constant oxygen consumption leads to constant CO 2 production (as related by the respiratory quotient; i.e., 0.7 to 1.0 mmol of CO 2 will be formed per mmol of O 2 consumed).
3. Despite constant CO 2 production, in the context of low flow, there is relatively more CO 2 added to each liter of blood volume; consequently, PvCO 2 rises.
