Letter Regarding: Continuous Versus Pulsatile Flow in 24-Hour Vascularized Composite Allograft Machine Perfusion in Swine: A Pilot Study.

We read, with interest, the article “Continuous versus Pulsatile Flow in 24-Hour Vascularized Composite Allograft Machine Perfusion in Swine: A Pilot Study”. 1 Tawa P. Goutard M. Andrews A.R. et al. Continuous versus pulsatile flow in 24-hour vascularized composite allograft machine perfusion in swine: a pilot study. J Surg Res. 2023; 283: 1145-1153https://doi.org/10.1016/j.jss.2022.11.003 Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar The authors present a pilot study in a swine partial hind limb model, investigating the outcomes of ex vivo subnormothermic perfusion with a modified Steen solution for 24 h with pulsatile flow (n = 3) compared to continuous flow (n = 3) at subnormothermic temperatures. They conclude that pulsatile perfusion may be more physiologic and beneficial in the preservation of VCAs based on improved endothelial function and decreased ischemic injury. In machine perfusion of organs, there is no consensus on the impact of the flow type on graft outcomes and the investigators should be commended on the effort to address this question; however, there are weaknesses in the study design that reduce the strength of the conclusions. 1- The investigators used a roller pump (07,522-20 Drive MFLEX L/S, Cole-palmer, Vernon Hills, Illinois) to generate a continuous flow. Roller pumps generate pulsatility with periodic oscillations, dependent on the pump head. To effectively study the impact of continuous flow, a pump delivering continuous nonpulsatile flow rather than a roller pump. 2- The physiology of a partial hind limb model is more similar to a musculocutaneous flap than a limb. The absence of closed fascial compartments prevents development of compartment syndrome with increasing flap edema during machine perfusion. Therefore, the model is not as stringent as true limb models, as this primary end point of perfusion would not be met with weight gain. 3- Weight gain is one of the earliest indicators of microscopic skeletal muscle injury during machine perfusion. 2 Meyers A. Pandey S. Kopparthy V. et al. Weight gain is an early indicator of injury in ex vivo normothermic limb perfusion (EVNLP). Artif Organs. 2023; 47: 290-301https://doi.org/10.1111/aor.14442 Crossref PubMed Scopus (0) Google Scholar In our previous studies, there was a positive correlation between the percentage of weight gained and supraphysiologic compartment pressures, increases in vascular resistance, and perfusion failure. 2 Meyers A. Pandey S. Kopparthy V. et al. Weight gain is an early indicator of injury in ex vivo normothermic limb perfusion (EVNLP). Artif Organs. 2023; 47: 290-301https://doi.org/10.1111/aor.14442 Crossref PubMed Scopus (0) Google Scholar ,3 Fahradyan V. Said S.A.D. Ordenana C. et al. Extended ex vivo normothermic perfusion for preservation of vascularized composite allografts. Artif Organs. 2020; 44: 846-855 Crossref PubMed Scopus (11) Google Scholar Both experimental groups in this study experienced significant weight gain (14.48% in continuous versus 12.43% in pulsatile flow group) 1 Tawa P. Goutard M. Andrews A.R. et al. Continuous versus pulsatile flow in 24-hour vascularized composite allograft machine perfusion in swine: a pilot study. J Surg Res. 2023; 283: 1145-1153https://doi.org/10.1016/j.jss.2022.11.003 Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar confirming the presence of skeletal muscle injury. This degree of weight gain indicates suboptimal preservation of the limb during machine perfusion. 4- Edema and weight gain during ex vivo perfusion determine elevated vascular resistance and a reduction in blood flow. 3 Fahradyan V. Said S.A.D. Ordenana C. et al. Extended ex vivo normothermic perfusion for preservation of vascularized composite allografts. Artif Organs. 2020; 44: 846-855 Crossref PubMed Scopus (11) Google Scholar The study reported the pulsatile group exhibited higher vascular resistance as compared to the control group. Despite the greater vascular resistance, the authors concluded that the pulsatile group exhibited better endothelial function due to superior vasoconductivity. Vascular conductivity or ease with which blood flows through the vessels is measured as blood flow divided by the change in pressure (blood flow/Δpressure), which is a reciprocal index of vascular resistance. 4 Joyce W. White D.W. Raven P.B. Wang T. Weighing the evidence for using vascular conductance, not resistance, in comparative cardiovascular physiology. J Exp Biol. 2019; 222: jeb197426 Crossref PubMed Scopus (11) Google Scholar Improved vascular conductivity cannot coexist with increased resistance as reported. 5- The article focuses on the Nitric Oxide/Endothelin-1 ratio (NO/ET-1) as evidence of superior vasoconductivity. NO is a powerful vasodilator that regulates vascular tone and blood flow to help maintain vascular homeostasis. 5 Chen K. Pittman R.N. Popel A.S. Nitric oxide in the vasculature: where does it come from and where does it go? A quantitative perspective. Antioxid Redox Signal. 2008; 10: 1185-1198 Crossref PubMed Scopus (181) Google Scholar However, NO synthesis is upregulated in prolonged states of vasoconstriction, such as those caused by ischemia or hypoxia, as a compensatory mechanism to restore blood flow and oxygen delivery to tissues. 5 Chen K. Pittman R.N. Popel A.S. Nitric oxide in the vasculature: where does it come from and where does it go? A quantitative perspective. Antioxid Redox Signal. 2008; 10: 1185-1198 Crossref PubMed Scopus (181) Google Scholar ,6 Ahmad A. Dempsey S.K. Daneva Z. et al. Role of nitric oxide in the cardiovascular and renal systems. Int J Mol Sci. 2018; 19: 1281-1293 Crossref PubMed Scopus (106) Google Scholar Therefore, increased NO/ET-1 does not necessarily mean successful perfusion. 6- Significance of lactate as a marker of increased anaerobic metabolism in machine perfusion is controversial. In the absence of a liver, lactate levels continue to increase in the perfusate. 7 Werner N.L. Alghanem F. Rakestraw S.L. et al. Ex situ perfusion of human limb allografts for 24 hours. Transplantation. 2017; 101: e68-e74 Crossref PubMed Scopus (44) Google Scholar Lactate levels more than 5 mmol/L during ex vivo heart perfusion have been considered an indicator of poor post-transplant outcomes. 3 Fahradyan V. Said S.A.D. Ordenana C. et al. Extended ex vivo normothermic perfusion for preservation of vascularized composite allografts. Artif Organs. 2020; 44: 846-855 Crossref PubMed Scopus (11) Google Scholar ,7 Werner N.L. Alghanem F. Rakestraw S.L. et al. Ex situ perfusion of human limb allografts for 24 hours. Transplantation. 2017; 101: e68-e74 Crossref PubMed Scopus (44) Google Scholar , 8 Schraufnagel D.P. Steffen R.J. Vargo P.R. et al. Devices for ex vivo heart and lung perfusion. Expert Rev Med Devices. 2018; 15: 183-191 Crossref PubMed Scopus (7) Google Scholar , 9 Messer S. Ardehali A. Tsui S. Normothermic donor heart perfusion: current clinical experience and the future. Transpl Int. 2015; 28: 634-642 Crossref PubMed Scopus (97) Google Scholar Although average numeric lactate levels were not reported in this paper, the graphical representation shows that lactate levels in both groups were close to and higher than this threshold. 7- As a preliminary study with three flaps in each group, this study is underpowered. Based on global experience, the influence of flow patterns, if any, is subtle, and larger experimental groups are needed to detect true differences. In conclusion, the study aims to investigate the impact of pulsatile flow versus continuous flow in machine perfusion. The significant weight gain in both experimental groups, along with increased vascular resistance in the pulsatile group, points more toward suboptimal perfusion conditions in both groups. Further studies are needed to understand the impact of flow patterns on outcomes of machine perfusion in VCA preservation.