Letter by McLachlan et al Regarding Article, “Cough Cardiopulmonary Resuscitation Revisited”
To the Editor:
We read with interest the article by Girsky and Criley1 that shows that forceful, repetitive cough can maintain consciousness in patients with induced ventricular fibrillation at the time of an electrophysiological study. We examined the second ECG and pressure waveform recording in the study1; this recording represents cough-induced blood pressure augmentation/support during ventricular fibrillation. If each individual pressure wave corresponded to a cough, the number of coughs needed during the 5-second recording period would be 12 (ie, each lasting 0.42 seconds). Because humans cannot physically cough at these high rates, it is more likely that the cough-induced pressure changes may have interacted with a passive flow circuit, independently of cough rate. The hemodynamic effect of coughing is attributed to increased intrathoracic pressure, which leads to an emptying of low-resistance pulmonary veins into the left side of the heart, generating (albeit passively) a column of blood through an opened aortic valve.2 For this to take place, a sufficiently deep breath must precede each such “hemodynamically useful” cough. Whether this theory is correct likely depends on consideration of forward blood flow.
Indeed, the mechanism for cough-assisted cardiopulmonary resuscitation has been suggested to be the forward flow of blood. On the other hand, some investigators have suggested that a rise in brachial artery pressure and a corresponding generated blood flow may represent the transmission of intrathoracic pressure through the arterial system, with a static blood column moving to and fro rather than via forward motion only.2 Cough-generated aortic pressure is sufficient to maintain consciousness but is limited by the oxygen content of the static blood volume; this might explain why patients are only able to maintain consciousness for up to 90 seconds.2 Cohen and associates2 could find no evidence for maintaining forward blood flow during asystole when assessed by 2-dimensional and Doppler echocardiography. Although there is evidence for forward blood flow in assisted coughing in conscious patients with significant bradycardia,3 coughing did not improve coronary perfusion pressure or flow velocity, despite marked increases in arterial diastolic pressure.3
In summary, we find the pioneering work of Girsky and Criley1 very interesting, although at this point, cough-assisted blood pressure augmentation via significant arterial forward blood flow in ventricular fibrillation remains elusive.