The Upper Body Hypoxemia (UBH), also known as "Harlequin syndrome" or "differential hypoxemia", is a major complication that can occur during mechanical circulatory support (“MCS”, such as extracorporeal circulation, ECLS, ECMO or Impella support) leading to brain and coronary hypoxemia with consequently very high risk of stroke and myocardial infarction. This complication typically occurs in MCS established through a femoral artery cannulation: in this case, the blood flow of the MCS does not reach the supraortic vessels due to the competition with the flow coming from rest cardiac activity. In fact, the heart with rest activity will still pump deoxygenated blood originating from the impaired lungs in the ascending aorta [1-2]. This is due to the short structure of the current arterial cannulas, which extremity ends at the level of iliac artery [3-4]. This well-known complication is observed in about 10% of all cases of ECLS/ECMO, independently from the admission diagnosis [5].

References: 

1. Hoeper MM, Tudorache I, Kühn C, Marsch G, Hartung D, Wiesner O, Boenisch O, Haverich A, Hinrichs J. Extracorporeal membrane oxygenation watershed. Circulation. 2014 Sep 2;130(10):864-5. 

2. Pasrija C, Bedeir K, Jeudy J, Kon ZN. Harlequin Syndrome during Venoarterial Extracorporeal Membrane Oxygenation. Radiol Cardiothorac Imaging. 2019 Jun 20;1(2):e190031. doi: 10.1148/ryct.2019190031. PMID: 33778505; PMCID: PMC7970096. 

3. Stevens MC, Callaghan FM, Forrest P, Bannon PG, Grieve SM. A computational framework for adjusting flow during peripheral extracorporeal membrane oxygenation to reduce differential hypoxia. J Biomech. 2018 Oct 5;79:39-44. doi: 10.1016/j.jbiomech.2018.07.037. 

4. Geier A, Kunert A, Albrecht G, Liebold A, Hoenicka M. Influence of Cannulation Site on Carotid Perfusion During Extracorporeal Membrane Oxygenation in a Compliant Human Aortic Model. Ann Biomed Eng. 2017 Oct;45(10):2281-2297. doi: 10.1007/s10439-017-1875-8. 

5. Rupprecht L, Lunz D, Philipp A, Lubnow M, Schmid C. Pitfalls in percutaneous ECMO cannulation. Heart Lung Vessel. 2015;7(4):320-6.