Pressure–flow relationship of cannulae for extracorporeal membrane oxygenation

Abstract

Dear Editor, We read with interest the literature1,2 by Broman et al. published in the Journal. The authors analyzed the relationship between flow rate and pressure gradient of venous and arterial cannulae for extracorporeal membrane oxygenation (ECMO) and found a poor correlation between in vitro results and pressure–flow diagrams provided by the manufacturers. However, in clinical practice, the achievable blood flow might be significantly affected by patient factors, such as hematocrit, intravascular volume, and cannulae position in the vena cava. For this reason, we performed a retrospective analysis of circuit pressures during veno-venous (V-V) ECMO, which are daily measured and recorded in our electronic chart by perfusionists. We included acute respiratory distress syndrome (ARDS) patients admitted to our ECMO Unit at ASST Monza in the last 5 years (from June 2014 to May 2019). At our Institution, we usually perform percutaneous femoro-femoral cannulation, while a femoro-jugular approach is used in a minority of patients. In adult patients, multi-staged 55-cm HLS cannulae (Getinge Group, Rastatt, Germany) are inserted for drainage (21-25 Fr) into a femoral vein. For femoral reinfusion, we use Medtronic Bio-Medicus (Medtronic Europe Sárl, Tolochenaz, Switzerland) cannulae (21-25 Fr), whereas for jugular reinfusion we use 23-cm arterial Maquet cannulae (17-21 Fr). Seventy-eight consecutive patients were included in the study, for a total of 1420 ECMO days. Mean hemoglobin level was 10.4 ± 2.5 g/ dL. Pressure–flow diagrams for different cannulae are depicted in Figure 1. As expected, drainage pressure drop (see Panel A) showed a weak correlation with blood flow, as it is significantly affected by cannula position and patient blood volume. Contrarily, pressure gradient of reinfusion cannulae (see Panel B) showed a good correlation with blood flow and was progressively reduced with increasing cannula diameter, according to Poiseuille’s Law. The in vivo pressure gradients in our patients were moderately higher than those observed in vitro by Broman et al. A minor part of this difference may be explained by the difference in hematocrit between our patients and their experimental setting (~27%). In line with Broman’s findings, we observed a significant difference with the gradients reported by the manufacturers. We could not find any adjustment or correction factor, which can be easily applied to correct the gradients reported by manufacturers. Clinicians dealing with ECMO should be familiar with their ECMO equipment and understand the complex in vivo relation between pressure and flow properties. Our findings may help clinicians in the selection of extracorporeal cannulae of Pressure–flow relationship of cannulae for extracorporeal membrane oxygenation