Simultaneous monitoring of noninvasive hemodynamic profile and capnography for tissue perfusion evaluation

To study the simultaneous variations of end-tidal CO₂ pressure (PetCO₂) and aortic blood flow (ABF) during modifications of tissue perfusion, continuous noninvasive hemodynamic monitoring and continuous recording of PetCO₂ were performed on 30 patients under general anesthesia and artificial mechanical ventilation. The 30 patients underwent orthopedic surgery on one of the lower limbs using a hemostatic tourniquet. Deflation of the pneumatic tourniquet resulted in a rise of ABF up to 39% (P<0.001), a rise of PetCO₂ up to 17% (P<0.001), and a drop of total vascular systemic resistance (TVSR) of 59% (P<0.001). In all cases, the gradient of Paco₂-PetCO₂ showed mean variations of 1.2±0.5 mmHg. According to these results, the observed variations can not be explained by an alteration of the Ventilation/Perfusion (V_o/Q) ratio alone. It may be suggested that tissue hypoperfusion produced by a tourniquet generates CO₂ and other metabolic products accumulation in tissues, which are removed during reperfusion. This would be expected to produce parallel increases in ABF and PetCO₂. If the results are confirmed with further studies, rapid variations of PetCO₂ during anesthesia may provide a noninvasive means of assessing the quality of global tissue perfusion.