Veno-arterial extracorporeal membrane oxygenation: Special reference for use in 'post-cardiotomy cardiogenic shock' - A review with an Indian perspective

The ultimate goals of cardiovascular physiology are to ensure adequate end-organ perfusion to satisfy the local metabolic demand, to maintain homeostasis and achieve 'milieu intérieur'. Cardiogenic shock is a state of pump failure which results in tissue hypoperfusion and its associated complications. There are a wide variety of causes which lead to this deranged physiology, and one such important and common scenario is the post-cardiotomy state which is encountered in cardiac surgical units. Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) is an important modality of managing post-cardiotomy cardiogenic shock with variable outcomes which would otherwise be universally fatal. VA-ECMO is considered as a double-edged sword with the advantages of luxurious perfusion while providing an avenue for the failing heart to recover, but with the problems of anticoagulation, inflammatory and adverse systemic effects. Optimal outcomes after VA-ECMO are heavily reliant on a multitude of factors and require a multi-disciplinary team to handle them. This article aims to provide an insight into the pathophysiology of VA-ECMO, cannulation techniques, commonly encountered problems, monitoring, weaning strategies and ethical considerations along with a literature review of current evidence-based practices.

Vascular Thrombosis During Support With Continuous Flow Ventricular Assist Devices: Correlation With Computerized Flow Simulations

Continuous flow pumps are increasingly used to treat severe heart failure. These pumps alter flow physiology by lowering pulsatility in the arterial circulation. In patients with peripheral stenosis, continuous flow pumps may lead to thrombosis of peripheral vessels, possibly predisposing to vascular thrombosis in areas of non-flow-limiting stenosis. The authors performed a computerized flow modeling simulation to analyze the effects of altered hemodynamics in a stenotic area. Drawing on previous clinical experience, we modeled a stenotic area in the common carotid artery. Computerized flow modeling revealed blood stagnation zones with low shear stress and velocity adjacent to the stenotic area during nonpulsatile flow. Such stagnation was not present during pulsatile flow. These results indicate a mechanism by which altered physiologic flow may accelerate occlusion of arterial conduits in patients with preexisting stenosis. This finding may be important for patients with continuous flow devices who have peripheral vascular disease; therefore, further study is warranted.

Influence of Flow Design on Microcirculation in Conditions of Undulation Pump-Left Ventricle Assist Device Testing

To study the microvessels in bulbar conjunctiva, we conducted an experiment in goat with a pneumatically driven left heart bypass pump, which was replaced with an undulation pump-left ventricle assist device for 9 days. Three flow patterns were tested: complete pulsatile, continuous, and percentage of pulsatile. We studied the morphology of arterioles and venules of the bulbar conjunctiva using photograph records. The setting up of continuous flow caused global vasoconstriction (significant in venules-P < 0.05). During the pumping in the pulsatile and percentage of pulsatile modes, no significant changes of microvessel morphology were observed. The findings described could point to some disturbances in the microcirculatory bed in conditions of continuous flow.

Chronic nonpulsatile blood flow is compatible with normal end-organ function: implications for LVAD development

Evolving blood pump technology has produced user-friendly continuous-flow left ventricular assist devices, but uncertainty exists about the safety of chronic nonpulsatile circulation. Recent experimental and clinical evidence suggest that pulse pressure is not required from a blood pump. End-organ function is well maintained with nonpulsatile systems, although pulse pressure may accelerate recovery from cardiogenic shock. Form follows function, so the effects of reduced pulse pressure on the arterial wall are not surprising. The ability to alter aortic wall morphology by reducing pulse pressure may have important implications for the future treatment of arterial pathology. Both centrifugal and axial-flow pumps can be miniaturized and are reliable and silent. Doubts about the feasibility of long-term circulation with reduced pulse pressure are disappearing.

Physiology of continuous blood flow in recipients of rotary cardiac assist devices

The beating heart and the resultant pulse wave have been a symbol of life for centuries. The development history of roller pumps for cardiopulmonary bypass shows that the human body tolerates non-pulsatile blood flow, at least for short-term support. Over the last few years, many types of rotary blood pumps have been developed for clinical use in patients requiring mid- to long-term support. Although early clinical experiences in patients with long-term support have been promising, the matter of whether pulsatile flow is needed or not remains controversial. Therefore, this review summarizes the observed clinical consequences of continuous blood flow in patients supported by rotary blood pumps and relates these consequences to underlying experimental studies.

Hemodynamics of chronic nonpulsatile flow: implications for LVAD development

Both experimental and clinical evidence suggest that pulse pressure is not required from a blood pump. End-organ function is well maintained with nonpulsatile systems, though pulse pressure may accelerate recovery from cardiogenic shock. Form follows function, so the effects of reduced pulse pressure on the arterial wall are not surprising. The ability to alter aortic wall morphology by reducing pulse pressure may have important implications for the future treatment of arterial pathology. Both centrifugal and axial-flow pumps can be miniaturized and are silent. Their reliability and user-friendly status may soon allow implantation at an earlier stage of cardiac deterioration. Doubts about the feasibility of long-term pulseless circulation are disappearing.

The roles of vascular smooth muscle cells in the aortic wall thinness under prolonged continuous flow left heart bypass

Aortic wall thinness was one of the most characteristic changes observed in experimental animals under prolonged continuous flow left heart bypass. The goal of this study was to determine the roles of smooth muscle cells in the vascular remodeling process in cases demonstrating aortic wall thinness under prolonged continuous flow left heart bypass. The aortic samples from three goats in which continuous flow left heart bypass was performed were subjected to histological and immunohistochemical analyses. After 4 weeks of observation, the pulse pressure in the goats under the continuous flow left heart bypass was clearly lower than that in the normal healthy goats. The aortic walls of these goats became thinner, an effect caused by the dilation of their internal diameter. These aortic smooth muscle cells maintained contractile formation due to the fact that they contained abundant alpha-smooth muscle actin (SMA) and smooth muscle myosin heavy chain (SMMS). These cells also synthesized redundant matrix metalloproteinase-2 and -9, and the ratio of the SMMS-positive to the SMA-positive area was significantly lower (0.76) than that observed in the control goat (1.00; P < 0.05). The smooth muscle cells demonstrated synthetic-dedifferentiated formation, which is one of the phenotypes of smooth muscle cell function. In conclusion, aortic wall thinness under prolonged continuous flow left heart bypass is caused by over-synthesis of matrix metalloproteinase in smooth muscle cells, and this refers the vascular remodeling process of the extracellular matrix.

Morphological changes of the arterial systems in the kidney under prolonged continuous flow left heart bypass

We investigated morphological changes of the arterial systems in the kidneys under prolonged continuous flow left heart bypass. Twelve goats were subjected to 2 weeks of pulsatile left heart bypass followed by 4 weeks of continuous flow left heart bypass (group CF). After autopsy, the kidneys underwent pathological evaluation. Six normal healthy goats were used as controls. The media of the afferent arterioles of group CF were frequently thickened by an increase in the number of the mature smooth muscle cells (SMCs). The juxtaglomerular areas (JGA) were expanded because of an increase in the number and size of SMCs and/or SMC-like cells. Furthermore, the percentage of anti-proliferating cell nuclear antigen antibody-positive cells in the JGA of group CF (9.9 +/- 1.9%) was significantly higher (p = 0.025) than that of the control group (4.6 +/- 3.4%), indicating active proliferation in group CF. We concluded that prolonged continuous flow left heart bypass causes proliferation of SMCs and/or SMC-like cells in the afferent arterioles and their perivascular tissue.

Change in vasoconstrictive function during prolonged nonpulsatile left heart bypass

We investigated changes in vasoconstrictive function accompanying prolonged nonpulsatile left heart bypass (NLHB). After 2-week pulsatile left heart bypass (PLHB) in 11 goats, NLHB was conducted for another 4 weeks (Group N) in 6 goats. In the other 5 goats, PLHB was continued for another 4 weeks (Group P). Systemic vascular resistance at rest (rSVR) was measured on the last days of the second and sixth postoperative week (W2 and W6, respectively). Subsequently, phenylephrine was injected, and the maximum values (SVRmax) and the maximum increasing change in SVR (DeltaSVR) were measured. No significant difference was observed in rSVR between groups at W2 or W6. The SVRmax and the DeltaSVR at W2 were consistent in both groups. However, at W6, the SVRmax and the DeltaSVR of Group N were significantly lower than those of Group P. In conclusion, prolonged NLHB caused a significant decrease in the SVR response to phenylephrine, indicating a dimunition of vasoconstrictive function.