The physiological basis of clinical decision-making in venoarterial extracorporeal life support
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REVIEW ARTICLE
The physiological basis of clinical decision-making in venoarterial extracorporeal life support Paul Ramesh Thangaraj 1,2 Received: 21 June 2020 / Revised: 15 September 2020 / Accepted: 17 September 2020 # Indian Association of Cardiovascular-Thoracic Surgeons 2020
Abstract Venoarterial (VA) extracorporeal life support (ECLS) or extracorporeal membrane oxygenation (ECMO) as it is commonly known is used in many clinical situations to support both the pumping action of the heart and the gas exchange function of the lungs. This review hopes to refresh, in the mind of the reader, aspects of basic physiological principles that have relevance in VA ECLS therapy. The dynamics of the interaction of the machine with the patient and vice versa plays an important role in clinical outcome. An understanding of the variation from normal physiology imposed both by the machine and the disease process will help make enlightened decisions in the use of this challenging therapy. The key physiological changes during initiation, maintenance, and weaning are discussed as well as certain specific clinical scenarios. Keywords Venoarterial ECMO . Physiology . Clinical decision-making in ECPR . Sepsis . ECMO
Basic physiological principles The heart and lungs are an integrated organ system, the main function of which is to extract oxygen from the atmosphere and deliver it to tissues and remove carbon dioxide and transport it back to the atmosphere. The ventricles themselves have a relationship of both a series and a parallel connection with each other. The attachment of a venoarterial circuit is in effect an extracorporeal parallel circuit with the native circulation. The major components of the heart for it to function as a pump are the circulating volume available to the ventricles (preload), the ability of the ventricles to relax and accommodate blood without undergoing a steep increase in chamber pressure (compliance), and the ability of the ventricles to contract and expel at least 60–80% of its contents (contractility) against the resistance offered by the vascular system (afterload). Each ventricle depends on the function of the other ventricle to work optimally—the phenomenon is called ventricular interdependence. There are both a series and a parallel connection between the ventricles. The series * Paul Ramesh Thangaraj [email protected] 1
Department of Cardiothoracic Surgery, Apollo Hospitals, Chennai, India
2
Department of Mechanical Engineering, IIT-Madras, Chennai, India
connection highlights the fact that the preload of either ventricle depends on the contractility and afterload of the other while the parallel connection is due to a shared ventricular wall, shared circumferential fibers, and the common pericardial envelope. Venous return (preload), stroke volume (a measure of contractility), and wall tension (developed as a consequence of afterload) are key determinants of flow through the circulatory system.
Flow ∝ ΔPressure × Radius4 / Length × Viscosity The pressure gradient that drives systemic flo
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