Reply to the comment by Dr. Hasibeder et al.

Excerpt

We thank Dr. Hasibeder et al. [1] for their interest in and comments on our study [2]. We fully agree with their statement that in skeletal muscle the number of perfused capillaries (what they call the capillary surface area) is directly related to skeletal muscle metabolism. Therefore it is difficult to know how many well perfused capillaries are necessary for an adequate oxygen supply. Nevertheless, according to the Fick equation, the determinants of tissue PCO₂ are arterial PCO₂, tissue blood flow, and tissue CO₂ production, the latter being directly related to tissue oxygen consumption. In healthy volunteers the gradient between sublingual PCO₂ and arterial PCO₂ (PslCO₂ gap) does not exceed 6 mmHg [3]. In physiological conditions any increase in tissue metabolism (and therefore in tissue CO₂ production) is accompanied by a proportional increase in tissue blood flow, resulting in an increase in tissue CO₂ washout, without CO₂ accumulation in the tissues. Therefore any increase in tissue CO₂ reflects a decreased ability of the microcirculation to remove CO₂ from the tissues. In other words, an increase in the PslCO₂ gap reflects an inadequacy between local oxygen transport and oxygen demand. Therefore in our study in septic patients the combination of high PslCO₂ values and low percentages of well perfused capillaries in the sublingual area compared to healthy volunteers [4] strongly argues for an inadequacy between local oxygen transport and oxygen demand and suggests that the decrease in the number of capillaries in the sublingual area of the septic patients was not due to low muscle metabolism but to primary microcirculatory failure. …