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An automatic flow control system for extracorporeal circulation
An automatic flow control system for extracorporeal circulation Murray N. Andersen, M.D., James F. Ulrich, P.E., and Christian V. Mouritzen, M.D.,* Buffalo, N. Y.
A n the usual artificial pump-oxygenator system, continuous visual inspection of oxygenator blood level is required to balance arterial pump rate with venous return, a method which imposes a constant demand on the technician's attention and does not provide any inherent safety mechanism. The wish to achieve a simpler method for main taining a smoothly functioning, balanced perfusion has, therefore, led to continued interest in development of methods of auto matic flow control for extracorporeal circu lation.1"9 However, most methods previously described have not seemed sufficiently simple, adaptable, and efficient to have gained widespread acceptance. This report presents a description of the physical design and functional character istics of a recently developed automatic arterial pump control unit which is simple and effective in use and provides adequate flexibility in control and adjustment. This unit was designed for a disc oxygenator, roller-pump perfusion unit, but could be readily adapted to other types of oxygenators. A system of flow control in which the arterial pump is controlled by the rate of venous return, as originally described by From the Department of Surgery of the State University of New York at Buffalo, and the E. J. Meyer Memorial Hospital, Buffalo, N. Y. Received for publication March 25, 1965. ♦Present address: Aarhus Kommunehospital, Aarhus, Denmark.
260
Crafoord,2 seemed to us to offer the most practical advantages, including the follow ing: (1) oxygenator blood level remains relatively constant at optimum levels since a decrease in venous return is promptly reflected in reduction in arterial output, (2) priming volume for the extracorporeal circuit is reduced by eliminating the need for a reservoir of blood, (3) a significant safety factor is gained by elimination of the possibility of allowing oxygenator blood volume to fall to unsafe levels, and (4) the pump output adjusts to the patient's venous return rather than to an arbitrary figure, thus maintaining intravascular volume in a more physiological manner. In the Crafoord-Senning pump oxygena tor, continuously variable regulation of arterial pump output is obtained through a pneumatic system activated by changes in the level of blood in the oxygenator.2 To adapt the same principle of flow control to a roller pump and disc oxygenator unit, an electronic system which functions in a similar fashion has been designed. Description of control unit Electrical system. The pump oxygenator employs a roller-type arterial pump driven by a direct current motor. The automatic unit superimposes an alternating current on the main arterial pump control to modify rotation speed. A signal voltage derived as a function of oxygenator blood level is compared to a balance voltage derived as a
Volume 50 Number 2
Automatic flow control system
261
August, 19(55
function of pump speed and fed back from the motor load circuit. The direction and magnitude of any error is sensed and ap plied to a transistor-amplified differential relay. This relay electrically positions a motor-driven autotransformer to produce a balanced condition, thus effectively con trolling pump speed as a direct function of blood level. Having adjusted the system for desired rotation speed (pump output), any increase in oxygenator blood level will thus result in an increase of pump speed and any decrease of blood level will in turn re duce pump speed; a stable blood level results in a stable speed controlled by that blood level. Provision is made for the pump to stop automatically if the blood level falls below a pre-set point. The electrical design provides for cancellation of line voltage changes in order to eliminate con trol fluctuations caused by this factor. Level sensor. A small detachable levelsensing chamber is fitted to the arterial end plate of the disc oxygenator with the inlet lying just above the bottom of the oxygena tor (Fig. 1); since this is an open system, the level of blood in the sensing chamber changes to the same degree as the level in the oxygenator. The sensing chamber con
tains two stainless steel electrodes whose position is adjustable vertically by a knurled screw. The depth of immersion of these electrodes modifies the strength of the controlling signal which is applied to the arterial pump. The lower limit cutoff and the flow rate at any given oxygenator blood level can be varied by changing the vertical position of the electrode probes. Further electrical adjustment of these func tions is also provided by separate adjust ment as described below. Control unit. "Manual" and "automatic" switch positions are provided. When turned to "automatic," the unit assumes control, cancelling any effect from the main pump control; switching to "manual" returns the arterial pump rate to the control of the main unit. A speed control is incorporated which will increase or decrease the signal to the arterial pump at any given level of immer sion of the sensor-chamber electrodes (Fig. 2 ) , and thereby adjust the basic pump rota tion speed. This control does not appreciably modify the sensitivity of response to changes in oxygenator blood level; there fore, the removal of a given volume of blood from the oxygenator will decrease
Fig. 1. Physical arrangement of automatic control unit. A is the level sensing chamber on the arterial end plate of a disc oxygenator. B is the control panel of the automatic unit, shown in greater detail in Fig. 2.
262
Andersen,
Journal of
Ulrich, and Mouritzen
Thoracic and Cardiovascular Surgery
Fig. 2. Control panel of automatic flow control unit. The switch labeled "Maximum Speed Con trol" is calibrated in reference numbers which do not directly indicate pump rotation speed.
the arterial pump speed by approximately equal percentages at different speed control settings. Method of adjustment and use The oxygenator and all tubing are initially filled to correct levels, with the use of either the manual or automatic control unit of the arterial pump. Recirculation through a shunt between arterial and venous lines is employed to facilitate ad justment of the automatic control system. With the electrodes immersed to the desired depth, the speed control is adjusted to pro duce a basic flow rate appropriate to the patient's size. When the arterial and venous lines are opened and the shunt line clamped, arterial output will be maintained at the same rate as the venous return. In the event of lower venous return than de sired, or if blood is unexpectedly lost, flow rate may be increased by adjustment of the speed control or by giving small transfusions directly into the filling chamber of the circuit to restore the oxygenator blood level. Initiation of flow with partial bypass, or change from complete to partial bypass at the end of perfusion, can be accomplished
readily by occluding one of the vena caval catheters on the operating table. Pump out put will adjust automatically to the reduced venous return. To adjust blood balance at the end of perfusion, it may be convenient to return to manual control for supple mentary transfusions from the pump oxygenator. Increased venous return at the beginning of perfusion, with a higher than anticipated flow rate, usually indicates that the pre dicted flow rate was inadequate for the patient and the higher flow rate should be permitted to continue. This variation from predicted occurs most commonly in children in whom the formulas applied to adults are not as reliable and higher relative flow rates seem desirable. Fig. 3 illustrates the ratio of change in arterial pump rate to change in oxygenator blood level. With the settings of electrode position (fully immersed) and speed control (75) employed, reduction in oxygenator volume of 22 per cent produced a 50 per cent decrease in pump output. This ratio of change, as well as the cutoff level, can be altered by changing the sensitivity of the electrodes and speed control by separate screw adjustments. Table I lists the changes in pump rotation speed produced by removal of different
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100-
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m => — li-
o a«
si
UJ
^
X
>
«> «
o
80604020-
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ARTERIAL PUMP OUTPUT % Fig. 3. Relation of arterial pump rotation speed to oxygenator priming volume. As described in the text, this ratio may be changed, if desired, by appropriate adjustment of sensitivity controls.
Volume 50
Automatic flow control system
Number 2
263
August, 1965
Table I. Reduction in arterial pump output with removal of varying amounts of blood from oxygenator* Oxygenator blood volume 2,300 2,200 2,000 1,800 1,600 1,400 1,350
21-inch Oxygenator ml. (full) ml. ml. ml. ml. ml. ml.
1,600 1,500 1,300 1,100 1,050
ml. (full) ml. ml. ml. ml.
Arterial pump output 100% 85% 68% 56% 38% 23% off
13-inch Oxygenator 100% 80% 53% 33% off
*This relationship can be altered by adjustment of sensi tivity controls and "cutoff" level control.
volumes of blood from the oxygenator (or loss from the total system during recircula tion). With acute loss or addition of blood, adjustment of arterial pump rate is com pleted within a few seconds. The slight time lag is due to the delay in reflection of blood loss at the arterial end of the oxygenator since response time of the electronic circuit is extremely rapid. The amount of decrease in oxygenator blood volume from normal operating levels to the cutoff point at which the arterial pump is automatically stopped is also given in Table I. This point may be modified independently by a separate screw control if it is desired to establish a higher cutoff level for additional safety. Discussion Since the advent of clinical use of artificial pump oxygenators, there has been a steady interest in the development of more auto matic function to increase safety and simplicity of operation of the extracorporeal circuit. The principles of the system de scribed here are not unique and others have recognized the advantages of use of venous return or oxygenator level as the controlling factor in arterial pump output.2· 5 The
present unit appears simpler and more flexible than others previously described and has the virtue of continuously variable arterial pump regulation rather than an "off-on" mechanism for maintaining oxy genator level. The physical characteristics of the sensing element pose no problems in assembly of the system since no special modifications are necessary and the unit is easily attached or removed. In clinical and experimental use the unit has been reliable and accurate. As long as the operative procedure is not complicated by unexpected major blood loss, flow rate is maintained at a stable level once perfu sion has been established. In the event of sudden major blood loss or unexpected interference with venous return it provides an automatic safety factor and gives prompt warning of abnormalities in venous return which may not otherwise be apparent to the surgeon or technician. Maintenance of a stable flow during per fusion is predicated on return of aspirated blood from the operative field to the pump oxygenator so that the volume of circulating blood remains relatively constant, a method which appears to be in general use. Al though presently employed with gravity drainage of venous blood, this would be equally effective with pump return; in this arrangement, adjustment of the venous pump only would be necessary and the arterial pump rate would automatically stay in balance. Examination of the table illustrating the rate of change in output with decreasing oxygenator blood volume shows that a shift to partial perfusion with decrease in rate of venous return results in the transfer of a certain amount of blood from the oxygena tor to the patient to effect equivalent reduction in arterial pump speed. This degree of change in oxygenator volume has generally seemed satisfactory since virtually all patients require transfusion from the pump oxygenator at the completion of by pass period. Since the amount of blood transferred to the patient in this circum stance is proportionately reduced in smaller
Journal of
2 64
Andersen,
Ulrich, and Mouritzen
Thoracic and Cardiovascular Surgery
oxygenators, this relationship has been satis factory in patients of different size. If desired, the amount of blood transferred to the patient during reduction of venous return may be reduced by appropriate ad justment of the sensitivity controls as described previously. It has also been noted that during hypothermia there is some tendency to reduction in sensitivity so that arterial output may decrease slightly (approximately 10 per cent with reduction of temperature from 37° to 25° C ) . This has seemed satisfactory since modest reduc tion of flow rate frequently is employed during hypothermia and the rate returns to the original level during rewarming. Finally, it is important to note that the described system does not add to the com plexity of the unit in assembly, sterilization, or operation, and does not require any in crease in size of the basic pump-oxygenator unit. Summary 1. An automatic pump control unit has been developed for use in extracorporeal circulation systems employing roller-type arterial pumps. The arterial pump output is controlled by rate of venous return (oxygenator blood level). 2. The physical design, functional char acteristics, and method of use and adjust ment of this system have been described. 3. This unit appears to be accurate and
reliable and to have considerable simplicity in construction and function. REFERENCES 1 Cartwright, R. S., and Palich, W. E.: Simplified Flow Determination, Volume Control, and Blood Replacement in the Rotating Disc Oxygenator, J. THORACIC & CARDIOVAS. SURG. 39:
623, 1960. 2 Crafoord, C , Norberg, B., and Senning, Â.: Clinical Studies in Extracorporeal Circulation With a Heart-Lung Machine, Acta chir. scandinav. 112: 220, 1957. 3 Jones, R. E., Donald, D. E., Swan, H. J. C , Harshbarger, H. G., Kirklin, J. W., and Wood, E. H.: Apparatus of the Gibbon Type for Mechanical Bypass of the Heart and Lungs, Proc. Staff. Meet., Mayo Clin. 30: 105, 1955. 4 Kantrowitz, A., Reiner, S., and Abelson, D.: An Automatically Controlled, Inexpensive Pump-oxygenator, J. THORACIC & CARDIOVAS. SURG. 38: 586, 1959.
5 Lewis, F. J., Horwitz, S. J., and Naines, J. B.: Semi-automatic Control for an Extracorporeal Blood Pump, J. THORACIC & CARDIOVAS. SURG.
43: 392, 1962. 6 Moss, G.: A Device to Maintain Automatically and Continuously an Absolute or Relative Con stant Weight of a Subject or Container During Perfusion, Surgery 49: 743, 1961. 7 Olmsted, F., Kolff, W., and Effler, D. B.: Three Safety Devices for the Heart-Lung Machine, Cleveland Clin. Quart. 25: 169, 1958. 8 Roche, J., Ungar, I., and Coleman, H.: An Electrical Apparatus for Rapid and Precise Regulation of the Venous Blood Reservoir Height on Heart-Lung Machines, Surgery 56: 561, 1964. 9 Schild, R., and Wesson, N.: Servo Circuit Con trols Artificial Heart, Electronics (Engineering Edition) 31: 73, 1958.
A n the usual artificial pump-oxygenator system, continuous visual inspection of oxygenator blood level is required to balance arterial pump rate with venous return, a method which imposes a constant demand on the technician's attention and does not provide any inherent safety mechanism. The wish to achieve a simpler method for main taining a smoothly functioning, balanced perfusion has, therefore, led to continued interest in development of methods of auto matic flow control for extracorporeal circu lation.1"9 However, most methods previously described have not seemed sufficiently simple, adaptable, and efficient to have gained widespread acceptance. This report presents a description of the physical design and functional character istics of a recently developed automatic arterial pump control unit which is simple and effective in use and provides adequate flexibility in control and adjustment. This unit was designed for a disc oxygenator, roller-pump perfusion unit, but could be readily adapted to other types of oxygenators. A system of flow control in which the arterial pump is controlled by the rate of venous return, as originally described by From the Department of Surgery of the State University of New York at Buffalo, and the E. J. Meyer Memorial Hospital, Buffalo, N. Y. Received for publication March 25, 1965. ♦Present address: Aarhus Kommunehospital, Aarhus, Denmark.
260
Crafoord,2 seemed to us to offer the most practical advantages, including the follow ing: (1) oxygenator blood level remains relatively constant at optimum levels since a decrease in venous return is promptly reflected in reduction in arterial output, (2) priming volume for the extracorporeal circuit is reduced by eliminating the need for a reservoir of blood, (3) a significant safety factor is gained by elimination of the possibility of allowing oxygenator blood volume to fall to unsafe levels, and (4) the pump output adjusts to the patient's venous return rather than to an arbitrary figure, thus maintaining intravascular volume in a more physiological manner. In the Crafoord-Senning pump oxygena tor, continuously variable regulation of arterial pump output is obtained through a pneumatic system activated by changes in the level of blood in the oxygenator.2 To adapt the same principle of flow control to a roller pump and disc oxygenator unit, an electronic system which functions in a similar fashion has been designed. Description of control unit Electrical system. The pump oxygenator employs a roller-type arterial pump driven by a direct current motor. The automatic unit superimposes an alternating current on the main arterial pump control to modify rotation speed. A signal voltage derived as a function of oxygenator blood level is compared to a balance voltage derived as a
Volume 50 Number 2
Automatic flow control system
261
August, 19(55
function of pump speed and fed back from the motor load circuit. The direction and magnitude of any error is sensed and ap plied to a transistor-amplified differential relay. This relay electrically positions a motor-driven autotransformer to produce a balanced condition, thus effectively con trolling pump speed as a direct function of blood level. Having adjusted the system for desired rotation speed (pump output), any increase in oxygenator blood level will thus result in an increase of pump speed and any decrease of blood level will in turn re duce pump speed; a stable blood level results in a stable speed controlled by that blood level. Provision is made for the pump to stop automatically if the blood level falls below a pre-set point. The electrical design provides for cancellation of line voltage changes in order to eliminate con trol fluctuations caused by this factor. Level sensor. A small detachable levelsensing chamber is fitted to the arterial end plate of the disc oxygenator with the inlet lying just above the bottom of the oxygena tor (Fig. 1); since this is an open system, the level of blood in the sensing chamber changes to the same degree as the level in the oxygenator. The sensing chamber con
tains two stainless steel electrodes whose position is adjustable vertically by a knurled screw. The depth of immersion of these electrodes modifies the strength of the controlling signal which is applied to the arterial pump. The lower limit cutoff and the flow rate at any given oxygenator blood level can be varied by changing the vertical position of the electrode probes. Further electrical adjustment of these func tions is also provided by separate adjust ment as described below. Control unit. "Manual" and "automatic" switch positions are provided. When turned to "automatic," the unit assumes control, cancelling any effect from the main pump control; switching to "manual" returns the arterial pump rate to the control of the main unit. A speed control is incorporated which will increase or decrease the signal to the arterial pump at any given level of immer sion of the sensor-chamber electrodes (Fig. 2 ) , and thereby adjust the basic pump rota tion speed. This control does not appreciably modify the sensitivity of response to changes in oxygenator blood level; there fore, the removal of a given volume of blood from the oxygenator will decrease
Fig. 1. Physical arrangement of automatic control unit. A is the level sensing chamber on the arterial end plate of a disc oxygenator. B is the control panel of the automatic unit, shown in greater detail in Fig. 2.
262
Andersen,
Journal of
Ulrich, and Mouritzen
Thoracic and Cardiovascular Surgery
Fig. 2. Control panel of automatic flow control unit. The switch labeled "Maximum Speed Con trol" is calibrated in reference numbers which do not directly indicate pump rotation speed.
the arterial pump speed by approximately equal percentages at different speed control settings. Method of adjustment and use The oxygenator and all tubing are initially filled to correct levels, with the use of either the manual or automatic control unit of the arterial pump. Recirculation through a shunt between arterial and venous lines is employed to facilitate ad justment of the automatic control system. With the electrodes immersed to the desired depth, the speed control is adjusted to pro duce a basic flow rate appropriate to the patient's size. When the arterial and venous lines are opened and the shunt line clamped, arterial output will be maintained at the same rate as the venous return. In the event of lower venous return than de sired, or if blood is unexpectedly lost, flow rate may be increased by adjustment of the speed control or by giving small transfusions directly into the filling chamber of the circuit to restore the oxygenator blood level. Initiation of flow with partial bypass, or change from complete to partial bypass at the end of perfusion, can be accomplished
readily by occluding one of the vena caval catheters on the operating table. Pump out put will adjust automatically to the reduced venous return. To adjust blood balance at the end of perfusion, it may be convenient to return to manual control for supple mentary transfusions from the pump oxygenator. Increased venous return at the beginning of perfusion, with a higher than anticipated flow rate, usually indicates that the pre dicted flow rate was inadequate for the patient and the higher flow rate should be permitted to continue. This variation from predicted occurs most commonly in children in whom the formulas applied to adults are not as reliable and higher relative flow rates seem desirable. Fig. 3 illustrates the ratio of change in arterial pump rate to change in oxygenator blood level. With the settings of electrode position (fully immersed) and speed control (75) employed, reduction in oxygenator volume of 22 per cent produced a 50 per cent decrease in pump output. This ratio of change, as well as the cutoff level, can be altered by changing the sensitivity of the electrodes and speed control by separate screw adjustments. Table I lists the changes in pump rotation speed produced by removal of different
o o O
100-
_J -1 _l
m => — li-
o a«
si
UJ
^
X
>
«> «
o
80604020-
0-
ARTERIAL PUMP OUTPUT % Fig. 3. Relation of arterial pump rotation speed to oxygenator priming volume. As described in the text, this ratio may be changed, if desired, by appropriate adjustment of sensitivity controls.
Volume 50
Automatic flow control system
Number 2
263
August, 1965
Table I. Reduction in arterial pump output with removal of varying amounts of blood from oxygenator* Oxygenator blood volume 2,300 2,200 2,000 1,800 1,600 1,400 1,350
21-inch Oxygenator ml. (full) ml. ml. ml. ml. ml. ml.
1,600 1,500 1,300 1,100 1,050
ml. (full) ml. ml. ml. ml.
Arterial pump output 100% 85% 68% 56% 38% 23% off
13-inch Oxygenator 100% 80% 53% 33% off
*This relationship can be altered by adjustment of sensi tivity controls and "cutoff" level control.
volumes of blood from the oxygenator (or loss from the total system during recircula tion). With acute loss or addition of blood, adjustment of arterial pump rate is com pleted within a few seconds. The slight time lag is due to the delay in reflection of blood loss at the arterial end of the oxygenator since response time of the electronic circuit is extremely rapid. The amount of decrease in oxygenator blood volume from normal operating levels to the cutoff point at which the arterial pump is automatically stopped is also given in Table I. This point may be modified independently by a separate screw control if it is desired to establish a higher cutoff level for additional safety. Discussion Since the advent of clinical use of artificial pump oxygenators, there has been a steady interest in the development of more auto matic function to increase safety and simplicity of operation of the extracorporeal circuit. The principles of the system de scribed here are not unique and others have recognized the advantages of use of venous return or oxygenator level as the controlling factor in arterial pump output.2· 5 The
present unit appears simpler and more flexible than others previously described and has the virtue of continuously variable arterial pump regulation rather than an "off-on" mechanism for maintaining oxy genator level. The physical characteristics of the sensing element pose no problems in assembly of the system since no special modifications are necessary and the unit is easily attached or removed. In clinical and experimental use the unit has been reliable and accurate. As long as the operative procedure is not complicated by unexpected major blood loss, flow rate is maintained at a stable level once perfu sion has been established. In the event of sudden major blood loss or unexpected interference with venous return it provides an automatic safety factor and gives prompt warning of abnormalities in venous return which may not otherwise be apparent to the surgeon or technician. Maintenance of a stable flow during per fusion is predicated on return of aspirated blood from the operative field to the pump oxygenator so that the volume of circulating blood remains relatively constant, a method which appears to be in general use. Al though presently employed with gravity drainage of venous blood, this would be equally effective with pump return; in this arrangement, adjustment of the venous pump only would be necessary and the arterial pump rate would automatically stay in balance. Examination of the table illustrating the rate of change in output with decreasing oxygenator blood volume shows that a shift to partial perfusion with decrease in rate of venous return results in the transfer of a certain amount of blood from the oxygena tor to the patient to effect equivalent reduction in arterial pump speed. This degree of change in oxygenator volume has generally seemed satisfactory since virtually all patients require transfusion from the pump oxygenator at the completion of by pass period. Since the amount of blood transferred to the patient in this circum stance is proportionately reduced in smaller
Journal of
2 64
Andersen,
Ulrich, and Mouritzen
Thoracic and Cardiovascular Surgery
oxygenators, this relationship has been satis factory in patients of different size. If desired, the amount of blood transferred to the patient during reduction of venous return may be reduced by appropriate ad justment of the sensitivity controls as described previously. It has also been noted that during hypothermia there is some tendency to reduction in sensitivity so that arterial output may decrease slightly (approximately 10 per cent with reduction of temperature from 37° to 25° C ) . This has seemed satisfactory since modest reduc tion of flow rate frequently is employed during hypothermia and the rate returns to the original level during rewarming. Finally, it is important to note that the described system does not add to the com plexity of the unit in assembly, sterilization, or operation, and does not require any in crease in size of the basic pump-oxygenator unit. Summary 1. An automatic pump control unit has been developed for use in extracorporeal circulation systems employing roller-type arterial pumps. The arterial pump output is controlled by rate of venous return (oxygenator blood level). 2. The physical design, functional char acteristics, and method of use and adjust ment of this system have been described. 3. This unit appears to be accurate and
reliable and to have considerable simplicity in construction and function. REFERENCES 1 Cartwright, R. S., and Palich, W. E.: Simplified Flow Determination, Volume Control, and Blood Replacement in the Rotating Disc Oxygenator, J. THORACIC & CARDIOVAS. SURG. 39:
623, 1960. 2 Crafoord, C , Norberg, B., and Senning, Â.: Clinical Studies in Extracorporeal Circulation With a Heart-Lung Machine, Acta chir. scandinav. 112: 220, 1957. 3 Jones, R. E., Donald, D. E., Swan, H. J. C , Harshbarger, H. G., Kirklin, J. W., and Wood, E. H.: Apparatus of the Gibbon Type for Mechanical Bypass of the Heart and Lungs, Proc. Staff. Meet., Mayo Clin. 30: 105, 1955. 4 Kantrowitz, A., Reiner, S., and Abelson, D.: An Automatically Controlled, Inexpensive Pump-oxygenator, J. THORACIC & CARDIOVAS. SURG. 38: 586, 1959.
5 Lewis, F. J., Horwitz, S. J., and Naines, J. B.: Semi-automatic Control for an Extracorporeal Blood Pump, J. THORACIC & CARDIOVAS. SURG.
43: 392, 1962. 6 Moss, G.: A Device to Maintain Automatically and Continuously an Absolute or Relative Con stant Weight of a Subject or Container During Perfusion, Surgery 49: 743, 1961. 7 Olmsted, F., Kolff, W., and Effler, D. B.: Three Safety Devices for the Heart-Lung Machine, Cleveland Clin. Quart. 25: 169, 1958. 8 Roche, J., Ungar, I., and Coleman, H.: An Electrical Apparatus for Rapid and Precise Regulation of the Venous Blood Reservoir Height on Heart-Lung Machines, Surgery 56: 561, 1964. 9 Schild, R., and Wesson, N.: Servo Circuit Con trols Artificial Heart, Electronics (Engineering Edition) 31: 73, 1958.