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Problems of extrcorporeal circulation are discussed in relation to newer methods and techniques applied in emergency.
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A donor heart failed to adequately sustain hemodynamic function after cardiac transplantation. The cause of the donor heart dysfunction was unknown, and there were no definite risk factors identified to suggest the heart would not recover. A Symbion Acute Ventricular Assist Device System was used to support both ventricles. The heart gradually recovered and the system was explanted after 1 week of support. The patient recuperated and has been discharged from the hospital.
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Leakage flow through the disk clearance of a magnetically suspended centrifugal blood pump is essential for a good washout. An analytical approach, based on the theory of lubrication, is used to predict the leakage volume flow rate, nondimensional radial velocity, nondimensional mean pressure distribution, and comparative shear stress distribution for different disk clearance geometry under varying rotational speeds. The results showed that nondimensional mean pressure distribution and nondimensional radial velocity distribution along the clearance are independent of rotational speed. The flow through the gap depends on a nondimensional parameter S that denotes the ratio of centrifugal forces to the head generating capability of the impeller. It was found that an impeller having a lower S has less possibility of flow reversals in the gap, and gap with maximum height at the outside radius also is more susceptible to flow reversals at the impeller surface. The comparative shear stress within the gap reveals that, in general, the scalar stress is below 500 N/m2.
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Transesophageal echocardiography (TEE) is an easy, noninvasive and reproducible method for evaluation of left ventricular function, coronary flow patterns, and intracardiac blood flow patterns during cardial assist. TEE facilitated better management of the patients with LVAD and gave us the various information: (a) Observing the drawing catheter tip by TEE, it could be settled just above the mitral orifice to obtain the maximum bypass flow; in addition, dislocation of the catheter tip could be easily diagnosed. (b) The intracardiac thrombus could be clearly detected by TEE; it was removed without complications, weaning from left ventricular assist device (LVAD). (c) The left ventricular wall motion was periodically observed. The end systolic LV diameter (Ds) was decreasing and fractional shortening (%FS) was increasing as the natural heart recovery. Additionally, the preejection period (PEP) and the ejection time (ET) were measured. The ratio of PEP/ET was decreasing gradually. Ds did not alter with preload change during on-off testing unlike the other parameters. Ds seems to be the most reliable parameter for the natural heart recovery within certain amounts of bypass flow. (d) The flow velocity in the left anterior descending coronary artery could be measured by transesophageal Doppler method; it was clinically shown that counterpulsation produced a larger amount of coronary blood flow than copulsation.
A total of eight patients, including three infants, received left or biventricular assist using centrifugal pump (CFP) following open heart surgery. Three infants, aged 9-11 months and with complex cardiac lesions, were supported by left heart bypass (LHB) using pediatric type CFP for 63 h, 64 h, and 13 days. All were weaned from LHB, but long-term survival was not obtained, mainly due to complications. In five adult patients, LHB alone was used in three, and biventricular support in two for 33-240 h with three survivals. The factors related to unsuccessful recovery were delayed start of support and multiorgan failure.
In this study we have employed a single channel, pulsed ultrasonic Doppler velocimeter to measure instantaneous velocity distributions within the pumping chamber of a ventricular assist device. Instantaneous velocities have been decomposed into periodic mean and turbulent fluctuating components from which estimates of Reynolds stresses within the chamber and mean shear stresses along the wall of the chamber have been obtained. A review of the complete data set indicates a maximum value of the mean wall shear stress of 25 dynes/cm2 and a maximum Reynolds stress of 212 dynes/cm2. These values are lower than those measured distal to aortic valve prostheses in vitro and are well below levels known to damage blood components. Core flow patterns, wall washing patterns and flow stagnation points are also revealed.
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The hemodynamic effects of phasic variations in intrathoracic pressure (ITP) timed to the cardiac cycle were predicted by a mathematical model and were compared with data from canine experimental studies. The model was used to predict the hemodynamic effects of changing the onset of the ITP rise relative to the start of cardiac systole, as well as the hemodynamic effects of changes in the duration and amplitude of the ITP rise. The predictions of the model were compared with hemodynamic data from seven anesthetized dogs. Cardiac function was depressed with large doses of verapamil and propranolol, and the hearts were atrioventricular sequentially paced at a rate of 72 beats/min. Phasic ITP variations were generated by a perithoracic vest and were electronically timed to the cardiac cycle. The model predicted, and the experimental data confirmed, that phasic intrathoracic pressure variations generated by vest inflation, timed to the cardiac cycle, can augment both peak and mean aortic flow. The following predictions of the model were also confirmed by the experimental data: 1) Maximum flow augmentation occurs when the onset of the ITP rise is simultaneous with the onset of left ventricular isovolumic contraction, and the ITP rise has a duration of 400 msec. 2) The magnitude of the flow augmentation is a function of the amplitude of the ITP rise. The experimental data showed that there was little further flow augmentation when the ITP rise was greater than 30-40 mm Hg. 3) The magnitude of flow augmentation was inversely proportional to the peak left ventricular elastance (Emax). The best fit between the measured and predicted flow augmentations was obtained for an assumed Emax of 0.5 mm Hg/ml, while Emax measurements in three dogs, using a volume conductance catheter and transient vena caval occlusion, yielded values of 0.4-1.6 mm Hg/ml. Thus, both the mathematical model and canine experiments showed that relatively low-amplitude ITP variations, rising synchronously with the onset of cardiac systole and having an optimal duration, assist the failing heart by augmentation of aortic flow. The degree of cardiac assistance decreases if the ITP variations do not rise synchronously with the onset of systole, or if their duration is not optimal. Thus, properly applied ITP variations may be used as an efficient, noninvasive method to temporarily assist the failing heart.
Invasive mechanical circulatory support is the approach of choice in patients with acute myocardial failure resistant to conventional therapy. Today survival rates between 24% for postcardiotomy cardiac failure and 40% for bridge to heart transplantation can be achieved. However, a broad armamentarium of devices for mechanical circulatory support is necessary to provide optimum assistance for the failing heart in the various clinical situations. The available devices and their potential indications and limitations are reviewed.