Fans of FOAMEd will doubtless welcome Critical Care’s latest educational venture into cardiovascular physiology which includes “Volume and its relationship to cardiac output and venous return” by Sheldon Magder. Critical Care 2016;20:271 DOI: 10.1186/s13054-016-1438-7 Published: 10 September 2016
I was delighted by the very last sentence of this well-written review, and recently tweeted it; “… clinical responses to treatments can only be in the realm of the physiologically possible.” On Sept 13 I initiated a little twitter banter with @rupert_pearse, one of the UK’s leading researchers in fluid challenge. He responded that “It would help a lot if more people knew about this fairly simple bit of physiology!” I was also delighted that my efforts to educate clinicians on the clinical consequences of the asymmetry of the capillary barrier due to the EGX, and the resultant steady-state Starling principle, have been noticed by the near-legendary Paul Marik and Rinaldo Bellomo in “A rational approach to fluid therapy in sepsis.” British Journal of Anaesthesia, 116 (3): 339–49 (2016) doi: 10.1093/bja/aev349. Advance Access Publication Date: 27 October 2015.
So it is with trepidation and a heavy heart that I venture to argue against the teaching that “the amount of blood pumped out of the heart (cardiac output) is equivalent to venous return (volume entering the right atrium).” Better yet, I will let Benjamin Reddi explain why students find cardiovascular physiology so confusing and sometimes misleading: “The conventional explanation of the circulation … assigns fictitious roles to vaguely defined variables. The main offender is ‘venous return’.” Other offenders that trouble me include mean circulatory filling pressure (MCFP), which only exists when there is no circulatory flow, and mean systemic filling pressure (MSFP) which is… well, I’m not sure what it is. Marik and Bellomo tell us that venous return is propotionate to the difference between MCFP and right atrial pressure, while Magder claims that; “the right heart is fed by the large compliant venules and small veins. The pressure in this region is called mean systemic filling pressure (MSFP) instead of MCFP, for it is only related to the volume in the systemic veins.” Professor Brengelmann tells us that both accounts are wrong; “Neither steady-state nor dynamic venous return is properly described as driven by mean circulatory pressure in proportion to the back pressure from Pra.” Brengelmann affirms that venous return at steady state “equilibrates with cardiac output at a level set by variables such as total system volume, contractility, and elastic state of the vasculature.” In response to stresses that disturb the steady-state, he tells us that venous return “changes dynamically as volumes redistribute among the organ vasculatures, conduit vessels, and heart.” THE CLASSICAL GUYTON VIEW THAT MEAN SYSTEMIC PRESSURE, RIGHT ATRIAL PRESSURE, AND VENOUS RESISTANCE GOVERN VENOUS RETURN IS NOT CORRECT.
“The finished system (Fig. 5) contains nothing mysterious, no abstract, unmeasurable variables. By systematically working through it, students can very easily work out for themselves the effects of exercise, hemorrhage, and other perturbations.”
In the clear absence of expert concordance over the interpretation of Arthur Guyton’s classic experiments on the determination of cardiac output, the interested clinician can reasonably choose for himself which he has faith in. As the inventor of RSE&GM paradigm for rational fluid prescription, it will come as no surprise I choose the one that is easiest to understand and most likely to help me make a rational therapy decision. Step forward Roger Carpenter and Benjamin Reddi who teach students at Cambridge University and University of South Australia, Adelaide. In their Invited Commentary of 2005 they even include a section on how to teach their Venous Excess approach to medical students in easily-digestible steps. It seems that this paper has only been cited 15 times, and it has not achieved the FOAMEd classic status I believe it deserves. Give it a read and let me know what you think. And if you were a student of Reddi or Carpenter in Cambridge or Adelaide, tell us how well you think you were served by the Venous Excess approach!