To the Editor:
The ELSO Interim Guidelines for venoarterial extracorporeal membrane oxygenation (VA ECMO)1 is timely. I would like to raise a few points on weaning recommendations.
First, the parameters detailed in Figure 11 and that in the text appear to be discrepant (VTI > 0.15 m/s vs. > 0.12 m/s; and CVP < 18 mmHg vs. ≤ 10 mmHg). These differences are clinically significant and deserve clarification.
Second, the stated goal of VA ECMO is “to maintain systemic oxygen delivery at least three times oxygen consumption (DO2:VO2 ratio is > 3)”. The logical extension is that weaning should only be considered if the patient could be expected to generate adequate DO2 without VA ECMO support. However, the Guidelines do not discuss DO2 during weaning. Indeed, target hemoglobin level, a determinant of DO2 was not discussed. Is the aim of maintaining DO2:VO2 ratio > 3 no longer relevant during or after weaning? Assuming VO2 of 110–125 ml/min/m2 in cardiogenic shock2 and hypothetical hemoglobin of 100 g/L, the cardiac index (CI) of 2.75 L/min/m2 may be needed to achieve the stated goal of DO2:VO2 ratio > 3 (i.e. DO2 of 375 ml/min/m2 for VO2 of 125 ml/min/m2), which is significantly higher than the recommended CI of > 2.2 L/min/m2 (produce DO2 326 ml/min/m2). In addition, the authors suggested that “successful weaning is expected if…LVOT VTI of 0.12 m/s…”. With normal LVOT diameter of about 2 cm,3 the stroke volume would be approximately 38 ml. In the absence of marked tachycardia and typical body surface area of 1.8 m2, the CI would be < 2.2 L/min/m2, which is again inconsistent with sufficient DO2.
Third, the heart as a generator of hydraulic energy should be considered during weaning. Mechanical circulatory support (MCS) is deployed when the heart fails to impart sufficient energy into the circulation (i.e. generate sufficient cardiac power). This is the basis for the use of cardiac power output (CPO) in guiding the deployment of MCS in cardiogenic shock.4 The corollary is that the heart must have sufficient CPO reserve for successful liberation from MCS. The CPO derived from the Guidelines for weaning (MAP > 60 mmHg, CVP < 10 mmHg and CI > 2.2 L/min/m2) falls into the range where MCS is indicated.5 Could the CPO level be simultaneously low enough for instituting MCS and yet high enough for successful weaning from MCS?
Fourth, the weaning criteria and Figure 11 are conspicuous by the absence of any discussion on the right heart and post-decannulation management. Right heart failure is a major cause of weaning failure. Are the proposed hemodynamic criteria sufficiently sensitive to identify the risk of right heart failure? The patient’s condition post-decannulation can be precarious and requires careful management and monitoring, with criteria for ‘re-cannulation’. Our practice of VA ECMO weaning incorporates assessment of DO2, CPO, and right heart function.
Finally, in Figure 11, the authors imply two routes out from temporary ventricular assist device (VAD) – palliation or bridging to durable left VAD (LVAD). The basis for this recommendation is not clear and contrasts with our experience from the UK, where good clinical outcomes have been achieved in patients bridged to heart transplantation from temporary VAD (i.e. not via durable LVAD).6
References
1. Lorusso R, Shekar K, MacLaren G, et al.: ELSO interim guidelines for venoarterial extracorporeal membrane oxygenation in adult cardiac patients. ASAIO J. 67: 827–844, 2021.
2. Chioléro R, Flatt JP, Revelly JP, Jéquier E: Effects of catecholamines on oxygen consumption and oxygen delivery in critically ill patients. Chest. 100: 1676–1684, 1991.
3. Kou S, Caballero L, Dulgheru R, et al.: Echocardiographic reference ranges for normal cardiac chamber size: results from the NORRE study. Eur Heart J Cardiovasc Imaging. 15: 680–690, 2014.
4. Lim HS: Cardiac power output revisited. Circ Heart Fail. 13: e007393, 2020.
5. Tehrani BN, Truesdell AG, Sherwood MW, et al.: Standardized team-based care for cardiogenic shock. J Am Coll Cardiol. 73: 1659–1669, 2019.
6. Rushton S, Parameshwar J, Lim S, et al.: The introduction of a super-urgent heart allocation scheme in the UK: a 2-year review. J Heart Lung Transplant. 39: 1109–1117, 2020.
