Open-Heart Surgery ......

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Open-Heart Surgery ......

Post by Dr Sarhan on Thu Jan 17, 2008 9:14 am

The advent of open-heart surgery has provided a wealth of data and observations regarding cardiovascular physiology. Furthermore, our expanding ability to correct cardiovascular defects has been a stimulus to better understand how the system works. Delineation of the essential mechanical features of the heart has allowed construction of heart replacement pumps that: (1) automatically provide normal circulation during heart bypass, with the flow rate remaining under control of normal physiological mechanisms; (2) allow research, in animals, by isolating the peripheral vascular versus cardiac effect on circulation rate from responses to various drug, humeral, and neural stimuli; and (3) provide a mechanical model having the ten unique characteristics of the cardiovascular system, for teaching and the study of circulatory phonomena.
THE MECHANICAL HEART
REPLACEMENT PUMPS

The pumps have the four characteristics in common with the heart that allow them automatically, without pump regulation, to reproduce circulation under control of normal physiologic mechanisms: (1) The pumps fill passively and don't suck at their inlets. (2) The output of the pumps is pulsatile. (3) The pumps have atria that allow uninterrupted inflow to the intermittent outflow pumps. (4) The capacity of the pump ventricles is greater than any anticipated diastolic filling.
Many designs of pumps with the above characteristics could be made. One of the six variations that the author has used is shown in Figure 4. The pump consists of a flat atrial-ventricular silicone rubber tube with a reinforcing flat cotton cover (Fig. 5). The flat configuration, which prevents rebound to a round cross-section after being compressed, is responsible for the passive filling characteristic. The tube is mechanically compressed, sequentially, by four plates activated by cams and lifters (Fig. 6). Two narrow plates act as inlet and outlet valves to the ventricular portion of the tube. Two wide plates act as atrial and ventricular impellers (Fig. 4). The atrial impeller compression and sequence are critical in preventing interruption of venous inflow when the ventricles are being emptied. Therefore, atrial compression is incomplete, leaving the atria with a 3/16" channel at maximum compression. The slope of the atrial cams is gentle, to prevent any sharp rise in pump atrial pressure that might interrupt venous flow. The cam's timing is such that the "compression let go" occurs just before the inlet valve closes.
CLINICAL SURGERY USING THE
HEART-SIMULATING PUMP

Cardiac surgery using both right and left heart bypass pumps has one significant advantage over conventional cardiopulmonary bypass: No oxygenator is used, as the lungs are left in the perfusion circuit and are provided with normal circulation. With normal pulmonary blood flow during the operation, post-operative pulmonary insufficiency is less of a problem than when the lungs are bypassed using an oxygenator.
By connecting the unique pumps in parallel to the heart's ventricles and with the pumps at heart level, cardiac function can be temporarily interrupted during surgical procedures without any interruption in normal circulation. When both the heart and bypass pumps are functioning simultaneously, the combined output remains the same as when the heart alone is pumping. Because both fill passively, the combined output remains regulated by the mean cardiovascular pressure and inlet impedance. Any amount that goes to the pumps doesn't go to the heart, and vice versa. If the heart function is then interrupted, by induced fibrillation or diversion of all the venous flow from one or both ventricles, the flow automatically goes to the pumps and circulation is uninterrupted and continues at the same rate. Subsequently, when the heart is defibrillated, or when the flow to the heart is no longer interrupted, the heart output takes over part of the circulation, with the total circulation rate still remaining the same. Weaning off of bypass is done simply by sequentially elevating the pumps a few centimeters at a time, thereby diverting more and more of the venous flow to the heart. Myocardial competence to take over the entire circulation becomes evident if the circulation rate and blood pressure remain unchanged after the incremental elevations are made. By appropriate elevation of the pumps above heart level, booster pumping can be done to maintain any desired atrial pressure in the heart during a recovery period.
The advent of open-heart surgery has provided a wealth of data and observations regarding cardiovascular physiology. Furthermore, our expanding ability to correct cardiovascular defects has been a stimulus to better understand how the system works. Delineation of the essential mechanical features of the heart has allowed construction of heart replacement pumps that: (1) automatically provide normal circulation during heart bypass, with the flow rate remaining under control of normal physiological mechanisms; (2) allow research, in animals, by isolating the peripheral vascular versus cardiac effect on circulation rate from responses to various drug, humeral, and neural stimuli; and (3) provide a mechanical model having the ten unique characteristics of the cardiovascular system, for teaching and the study of circulatory phonomena.
THE MECHANICAL HEART
REPLACEMENT PUMPS

The pumps have the four characteristics in common with the heart that allow them automatically, without pump regulation, to reproduce circulation under control of normal physiologic mechanisms: (1) The pumps fill passively and don't suck at their inlets. (2) The output of the pumps is pulsatile. (3) The pumps have atria that allow uninterrupted inflow to the intermittent outflow pumps. (4) The capacity of the pump ventricles is greater than any anticipated diastolic filling.
Many designs of pumps with the above characteristics could be made. One of the six variations that the author has used is shown in Figure 4. The pump consists of a flat atrial-ventricular silicone rubber tube with a reinforcing flat cotton cover (Fig. 5). The flat configuration, which prevents rebound to a round cross-section after being compressed, is responsible for the passive filling characteristic. The tube is mechanically compressed, sequentially, by four plates activated by cams and lifters (Fig. 6). Two narrow plates act as inlet and outlet valves to the ventricular portion of the tube. Two wide plates act as atrial and ventricular impellers (Fig. 4). The atrial impeller compression and sequence are critical in preventing interruption of venous inflow when the ventricles are being emptied. Therefore, atrial compression is incomplete, leaving the atria with a 3/16" channel at maximum compression. The slope of the atrial cams is gentle, to prevent any sharp rise in pump atrial pressure that might interrupt venous flow. The cam's timing is such that the "compression let go" occurs just before the inlet valve closes.
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Re: Open-Heart Surgery ......

Post by Dr Sarhan on Thu Jan 17, 2008 9:14 am

SPECIFIC APPLICATIONS OF THE
MECHANICAL PUMPS

Coronary Bypass Surgery:
Cardiac bypass during coronary surgery is used to provide a non-moving target with decompressed ventricles and the security that adequate circulation is being maintained during manipulation of the heart. Coronary artery surgery is not an open heart procedure, as the coronary arteries are on the surface of the heart. Therefore, closed bypass is applicable. Four cannulae are used. One pump is connected from the left atrium to the aorta, and the other from the right atrium to the pulmonary artery (Fig. 7).
It is important to start the left-sided bypass before the right. If the right one is started and the heart happens to fibrillate before the left bypass is started, blood will be pumped into the pulmonary circuit while no blood is leaving it. The result would be acute pulmonary engorgement ("liver lungs") with a fatal outcome. Likewise, the left-sided bypass should be discontinued last.
The heart is electrically fibrillated while the coronary-graft anastomoses are made. Then the heart can be defibrillated before the aorta-graft anastomoses are made. If, during the coronary-graft anastomoses, greater decompression of the ventricles is desired, the atrial cannulae can be advanced into the ventricles to drain them as well as the atria. Figure 8 shows arterial blood pressures staying relatively constant before and during a coronary bypass procedure, using right and left heart bypass.
During cardiac bypass with the two pumps, with the lungs functioning and without the use of an oxygenator, the anesthesiologist maintains ventilation and support of circulation just as he would in other non-cardiac procedures. Circulation rate reacts to blood loss, fluid infusion, and vasoactive drugs, just as when the heart is functioning.
Pulmonary Stenosis:
While it might appear that pulmonary stenosis could be repaired by using a right-sided bypass alone, one laboratory experience has demonstrated this to be too hazardous for clinical application. During the course of a right heart bypass, with the left ventricle functioning, the heart unexpectedly fibrillated. With the left ventricle suddenly not pumping, and before the right heart bypass pump could be turned off and the heart defibrillated, the lungs became irreparably overloaded with blood. The resulting "liver lungs" were not reversible, causing the death of the animal. Therefore, for safety, total heart bypass is always used, even when a right heart bypass alone could allow adequate access for correction of the lesion.
Use of a Passive Filling Pump with a Bubble Oxygenator:
During open-heart surgery, two pump bypass is precluded because of the difficulty in bypassing the left atrium, with its many pulmonary veins. Therefore, cardiopulmonary bypass with an oxygenator is necessary whenever the cardiac chambers need to be opened.
The passive filling, pulsatile output, continuous inflow pump has advantages over other types of pumps when used in a pump-oxygenator circuit. It can automatically produce normal circulation rate with a normal pulse wave, without any control adjustments, and without the hazard of oxygenator blood level fluctuations or air emboli.
If the inlet of the pump is placed at the priming level in the oxygenator (Fig. 9), because the pump is non-sucking, the blood will never fall below that level. Any blood that runs into the oxygenator that would tend to raise the level is automatically pumped back into the patient. The oxygenator is positioned at such a level that normal venous pressure is maintained by gravity drainage during bypass.
With this pump-oxygenator setup, circulation rate is determined by the patient's mean cardiovascular pressure and inlet impedance, just as it does in the intact body. During the bypass, circulation rate is modified by using vasoactive drugs, blood and fluid replacement to change the mean cardiovascular pressure, not by pump alteration.
This technique beginning partial bypass in parallel with the heart results in normal circulation rate, as both the heart and the extra-corporeal system fill passively. When complete bypass is produced, by occluding the vena cava around the caval catheters, the circulation rate remains the same. Terminating bypass is very simple. The occluding tapes around the vena caval tubes are released and the venous drainage tubing is occluded in increments. As more and more blood is diverted to the heart, the venous pressure will indicate whether or not further decrease in extra-corporeal pumping will be tolerated. In this way, the pump-oxygenator can be used as an automatic booster device during the myocardial reovery period, following the cardiac repair procedure. Figure 10 shows the blood pressure before, during, and after bypass, including the parallel bypass at the end of the procedure, in a patient with wide-open aortic insufficiency. The wide, abnormal pulse wave of aortic insufficiency is followed with a normal pulse wave while on bypass and after valve replacement. Figure 11 shows typical pulse waves during bypass and Figure 12 shows superimposed waves during circulation produced by parallel pumping of the heart and extra-corporeal pump.
Figure 13 illustrates a safety feature of the passive filling pump used with an oxygenator. Bypass was just underway, during a mitral valve replacement, when the inferior vena cava cannula slipped back into the right atrium. The inferior vena cava being temporarily occluded by the circumferential tape, caused the venous drainage to the pump to drop to one liter per minute. The oxygenator did not run out of blood and no air was pumped into the patient during this low output episode. After the tube was re-advanced properly into the inferior vena cava, the blood flow to and from the pump automatically returned to normal.
The Non-Sucking Pump Used with a Membrane Oxygenator:
The non-sucking heart replacement pump is ideal for use with a membrane oxygenator. It automatically allows a constant blood volume in the system with this closed-volume oxygenator. There is no need for a reservoir for volume monitoring of changing blood levels, which would occur and require alteration of pump rate with other types of pumps. In effect, the passive filling, continuous inflow pump used with a membrane oxygenator provides a closed automatic bypass system.*
*NOTE: In all clinical applications of passive filling pumps, it is important to use large caliber venous drainage tubing so as not to add to the vascular system's normal inlet impedance to the pumps.
Pulsatile Blood Flow During Bypass Surgery:
Cardiac bypass during heart surgery has given an opportunity to observe the benefit of pulsatile blood flow, by comparing it with techniques using non-pulsatile flow. Pulsatile flow ensures diffuse normal distribution of blood flow to all the organs and tissue of the body, while non-pulsatile flow results in reduced flow to certain vascular beds and excessive flow to others. With non-pulsatile flow, the brain and kidneys receive reduced blood supply. Also, incomplete metabolism from islands of under-perfused tissue results in acidosis, not found when pulsatile flow is provided. When hypothermia is used, the need of pulsatile flow to insure diffuse distribution of circulation is increased. While the adverse effects of non-pulsatile flow can be offset by the use of vaso-relaxers, blood dilution, and other methods, the value of pulsatile flow in normal function is illustrated by comparing findings from the two types of pumping systems.
Ventricular Compliance an Inlet Impedance Factor:
Open-heart surgery on a patient with severe hypertrophy of the left ventricle, from longstanding aortic stenosis, illustrated how ventricular compliance can be a factor in the inlet impedance determination of cardiac output. This patient's left ventricle measured three-fourths of an inch in thickness. After replacement of the aortic valve, the bypass was discontinued. The heart contracted very strongly, with a normal pulmonary venous pressure of 12 cm. water. However, there was practically no cardiac output and the blood pressure remained at only 55/25. The blood volume was, therefore, increased by increments while watching the pulmonary venous pressure go to 16, 18, 20, 25, and 30 cm., with no effect on output or blood pressure. The heart continued to beat very forcibly. Suddenly, at a pulmonary venous pressure of 35 cm., almost at the point of causing pulmonary edema, the heart distended during diastole, cardiac output went beyond normal with a resulting blood pressure of 160/90. The non-compliance of this thick ventricle was a significant impediment to passive filling and cardiac output.
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