Pulmonary perfusion in HFV: a potential hazard?

Capnograms recorded during HFV do not indicate ETCO2 but the washout difference (WOD). A method by which the ETCO2 can be accurately measured during HFV using a special device and a low flow sidestream capnograph is described. The importance of continuous, instantaneous ETCO2 monitoring is stressed. Capnograms recorded by this method were found to exhibit regular, periodical falls in WOD as soon as the heart rate (HR) approached the ventilator rate (VR) or its second or third harmonic. Simultaneous rises in PAP, RVP and CVP were noted. Using an oscillator model and two frequencies representing the HR and VR, innumerable patterns could be produced resulting from the interference of one frequency with the other. Different combinations of frequencies produced, when expressed in terms related to alveolar capillary flow, zero or near-zero zones which varied in duration from extremely prolonged to minimal or entirely absent. Analysis of the capnographic phenomena previously observed, by means of the patterns produced by the oscillator model, enabled such disturbances to be interpreted and avoided, clinically, by adjustment of the relative HR/VR. The advantages of capnographic monitoring in the prevention of such sequelae during HFV is emphasised.