the apparatus disclosed herein is constructed and arranged primarily for supplying blood to the cardiac area of a patient under conditions such that the blood pressure proximate the point of supply remains within limits selected by the surgeon . it should be noted that , in contrast to the prior art , it is the blood pressure which is selectively controlled and not the feed rate . nevertheless , the feed rate is controlled in such a way as to maintain the blood pressure proximate the point of supply within said selected limits . moreover , the feed rate is adjusted automatically without sudden changes , and without &# 34 ; hunting &# 34 ; in the control system , thereby eliminating rapid wear of the control elements . as shown in fig1 roller pump 11 transfers oxygenated blood from supply tank 12 to port 13 of solenoid valve 14 . solenoid valve 14 , in preferred form , is a three - way valve having exit ports 16 and 17 . in first condition , the blood passing through said solenoid valve 14 is recycled through exit port 16 back to supply tank 12 . in second condition , said solenoid valve 14 transfers the blood through exit port 17 toward the patient . solenoid valve 14 is under the control of pressure switch indicated generally by the reference numeral 18 , said pressure switch consisting of a pressure transducer which may be a displacement diaphragm , as shown in fig1 and an electric switch 21 . as shown in fig1 electric switch 21 is mechanically coupled to pressure transducer 19 . this arrangement is satisfactory where pressure transducer 19 is a pressure diaphragm as shown in fig1 . however , pressure transducer 19 can also be a variable capacitor or a strain gauge or a variable inductance , in which case a relay or other appropriate accessories are provided as well known to those skilled in the art . preferably , pressure switch 18 is so connected electrically with solenoid valve 14 that in the absence of an electric signal to the solenoid valve , i . e ., when the solenoid valve is non - energized , the valve is in a condition such that entry port 13 is connected with exit port 17 , in other words , so that blood is transferred toward the patient rather than recycled to surge tank 12 . roller pump 11 , pressure switch 18 and solenoid valve 14 may be powered from an external source ( not shown ), connection being made thereto through main switch 22 . that power is being supplied to the apparatus is shown by the indicator lamp 23 connected in series with resistor 27 across hot line 24 and ground 26 . that power is reaching roller pump 11 is shown by the same indicator lamp 23 . power to pressure switch 18 passes through mode selector switch 28 which can be set for operation of the apparatus in any of three modes . when mode selector switch 28 is positioned to make contact at point 29 , the blood flows continuously under the drive of roller pump 11 toward the point of supply for perfusing the heart . this mode of delivery serves both for filling the system with blood and for high - rate delivery where such high - rate delivery is necessary for meeting an emergency . it will be noted that pressure switch 18 is by - passed when mode selector switch 28 makes contact with point 29 , this position being indicated by neon - glow lamp 31 and its associated resistor . as a result , solenoid valve 14 is non - energized when mode selector switch 28 is at position 29 . mode selector switch 28 is brought into contact with point 32 to place the apparatus in normal operational condition , power then being supplied to pressure switch 18 and , depending upon the condition of pressure switch 18 , to solenoid valve 14 . indicator lamp 33 shows when power is being supplied to pressure switch 18 and indicator lamp 34 shows whether solenoid valve 14 , under the control of pressure switch 18 , is in energized or non - energized condition . throwing mode - selector switch 28 to point 36 bypasses pressure switch 18 and provides power continuously to solenoid valve 14 , thereby connecting entry port 13 with exit port 16 for recycling of blood from and to supply tank 12 . movement of switch 28 to position 36 cuts off the blood supply to the patient without shutting down the apparatus . movement of switch 28 to position 36 is indicated by indicator lamp 35 . the fact that the solenoid valve 14 is in the continuously - energized condition is shown by indicator lamp 34 . this mode of operation is provided since the surgeon at certain stages of the surgical procedure may require that the supply of blood to the patient be suspended or the pressure be reduced . in the transfer of blood from solenoid valve 14 to the patient , the blood traverses conduit 37 and enters surge tank 38 through port 39 . since the blood pressure at the point of supply to the patient is substantially above atmospheric , usually about 80 mm of mercury , resistance to flow of blood to the point of supply results in compression of the air initially present in surge tank 38 and development of a pool of blood 41 in the lower part 42 of said surge tank . the air or other gas 43 in upper part 44 is compressed by introduction of blood driven by roller pump 11 until it rises sufficiently above the pressure at the point of supply so that flow of blood for perfusing the heart begins . however , the pressure at the aortic root varies with a specific patient so that the general pressure level should be adjustable over a range between about 50 to 150 mm . also , during one phase of the cardiac cycle ( systolic ), blood flow slows or stops due to constriction of coronary vessels . the heart therefore receives a pulsating flow under normal conditions . the flow occurs when the heart is in the relaxed phase ( diastolic ), that is , when the heart is not pumping . accordingly , the apparatus disclosed herein simulates natural flow , since when the pressure drops in the aortic root , the flow rate increases , and vice versa . for reasons which will be presented below , the total volume of surge tank 38 is preferably between about 2 and 5 liters . blood leaves surge tank 38 through exit port 46 and traverses conduit 47 to a cannula 48 indicated only diagrammatically in fig1 . this cannula is preferably placed in the aortic root of the patient or , alternatively , in a coronary artery . as pressure - sensing means , a second cannula 49 is provided for entry into the aortic root , said cannula 49 functioning as a sensor . cannula 49 is connected through three - way valve 51 and conduit 52 to pressure switch 18 . the combination of sensor , pressure switch and solenoid valve is shown more generally in fig2 in schematic form . as shown in fig2 sensor 53 is operatively connected by connecting means 54 to pressure switch 56 which controls solenoid valve 57 in accordance with the signal received from pressure switch 56 . connecting means 54 can be hydraulic , pneumatic , mechanical or electrical , depending upon the nature of sensor 53 and pressure switch 56 . in the embodiment of pressure switch 18 shown in fig1 the pressure transducer 19 consists essentially of a displacement diaphragm 58 dividing a housing 59 into first chamber 61 , maintained at atmospheric pressure , and second chamber 62 which holds a pressure corresponding to that of the cardiac region proximate the point of supply . it will be noted that in general , no blood enters conduit 52 since this portion of the system has only a single opening , namely that at the end of cannula 49 . conduit 52 may be filled with fluid compatible with blood . pressure transducer 19 , preferably has a dead - band of about 10 mm , the dead - band being the difference in pressure between an upper limit at which pressure switch 18 is closed and a lower limit at which said switch is open . preferably , the range over which the pressure switch operates can be adjusted either upwardly or downwardly to maintain the pressure at the point of supply at any desired level . the presence of the dead - band in the pressure switch prevents hunting , that is , frequent cycling of the pressure switch and , as a consequence , of solenoid valve 14 . frequent change in the condition of solenoid valve 14 is undesirable since it causes trauma to the blood and , of course , is associated with wear of the switch as well as wear of the solenoid valve itself . a cycle time of about one minute is desirable and appropriate in that trauma to the blood and wear of the device are minimized and yet the variation in blood pressure at the point of supply is small . the combination of a dead - band of about 10 mm and a total volume of about 3 liters for the surge tank provides this cycle time . as is evident , other combinations of dead - band and surge tank volume can also be found to provide an appropriate cycle time . the way in which the surge tank minimizes changes in supply rate and blood pressure at the point of supply can be seen from the nature of an air - cushion . considering the situation when blood is being supplied to the surge tank and thence to the patient at a rate such that the blood pressure at the point of supply is rising , said blood pressure eventually rises to the point where the selected upper pressure limit for closing pressure switch 18 is reached . power is then supplied to solenoid valve 14 connecting entry port 13 of said valve to exit port 16 and the blood transferred by roller pump 11 from supply tank 12 is recycled to same . although the supply of blood to surge tank 38 is thus interrupted , air - cushion 43 is still at a pressure high enough so that flow of blood from surge tank 38 to the point of supply continues . as the flow of blood continues , the volume of the air - cushion increases and the pressure of the air decreases . the decrease in flow of blood to the point of supply causes a decrease in the blood pressure at the point of supply . however , the blood pressure at the point of supply is being continuously monitored by sensor 49 and when the pressure at this point drops below the lower pressure limit at which pressure switch 18 is opened , the solenoid valve becomes de - energized and supply of blood to surge tank 38 is re - initiated . it is significant that there is a pressure drop between surge tank 38 and the point of supply , this pressure drop resulting from flow through conduit 47 and cannula 48 , cannula 48 necessarily being relatively small in diameter . it is this pressure drop which is responsible for the pressure in tank 38 being sufficiently greater than the blood pressure at the point of supply so that flow continues , although at a decreasing rate , when the solenoid valve is in a condition such that blood supply to the surge tank is interrupted . further , there is no pressure drop between sensor 49 and pressure switch 18 because there is no flow between these points . accordingly , even when pressure transducer 19 is of the displacement diaphragm type the pressure in chamber 62 corresponds closely to that at the point of supply . in certain cases it is desirable that the number of openings made in the patient be minimized . for this purpose , a double cannula as shown in fig3 can be used . fig3 shows schematically the entry end of such a double cannula , outer tube 63 being connected for introducing blood in the patient and inner tube 64 serving for sensing the pressure at the point of supply in a manner corresponding to that of sensor 49 . the apparatus of fig1 is constructed and arranged for coping with contingencies in addition to that of failure of power at solenoid valve 14 . should the valve jam or lock in the condition such that entry port 13 is connected with exit port 16 , manually - operable three - way valve 65 provides for transferring the blood to surge tank 38 . needless to say , although three - way valves are most convenient for making changes in flow paths , other valve combinations are possible , specifically a pair of two - way valves . wherever the use of the three - way valve is specified , it is to be understood that this term also includes other valve combinations which provide the same flow control . such a combination of 2 - way valves is shown in fig4 in which supply line 74 is connected to exit lines 76 and 77 through two - way valves 78 and 79 respectively . these valves may be linked together , as indicated by dashed line 81 , so that only one of said valves is open at any given time . the valves may be operated by solenoid 82 connected to mode selector switch 28 and pressure switch 18 . a second exit port 66 in surge tank 38 leads through 2 - way valve 67 , conduit 68 and three - way valve 69 to pressure gauge 71 . this makes it possible to monitor the pressure in surge tank 38 and to control the rate of supply of blood to the patient should pressure switch 18 fail in either the open or the closed position . in such circumstances , complete control can be provided by operating roller pump 11 manually at a rate such as to maintain the pressure in surge tank 38 at a level as read on pressure gauge 71 . alternatively , 3 - way valve 69 can be set to register the pressure in conduit 52 on pressure gauge 71 . if the pressure switch fails in the closed position , three - way valve 65 can be positioned manually for directing blood to surge tank 38 . as aforenoted , roller pump 11 should be of a capacity which is at least twice that normally required by a patient and preferably up to about 5 times that normally required . when the capacity is about twice that normally required , then the solenoid valve will be in the supply condition and in the recycle condition for approximately equal periods of each duty cycle . although this is a desirable arrangement , situations may occur during surgical procedures in which a very rapid supply of blood is desired . for this purpose , it is preferable that the roller pump be capable of supplying up to about 5 times the normal rate . increased blood - supply rate is needed in the event of a hyperemic reaction . under such conditions , the blood flows out of the aortic root more rapidly than usual and the feed rate must increase to hold the blood pressure at a desired level . where unusually rapid flow is desired , regardless of the specific pressure at the supply point , valves 67 , 69 and 72 are positioned for transfer of blood from surge tank 38 to conduit 52 and thence through valve 51 and cannula 49 to the patient , thus providing a parallel flow path in addition to that through conduit 47 and cannula 48 for supply of blood to the patient . valves 51 , 72 , 73 and 75 are used for back - filling lines , flushing lines and removing air from lines . as will be seen from the above description , the introduction of surge tank 38 with its air - cushion serves a variety of purposes , these including elimination of surges in the rate of blood supply resulting from mis - match in the capacity of roller pump 11 and the demand of the patient , elimination of frequent cycling of the pressure switch , continuity of blood supply in the case of interruption of power and the possibility of parallel blood - supply lines . the entire apparatus is extremely simple since elaborate monitoring of the blood pressure or of blood flow rates is unnecessary , and yet protection for emergency situations and contingencies is provided . the apparatus can be constructed of readily available components which are relatively low in cost and the apparatus is , in addition , sufficiently light so that it is readily portable . it will thus be seen that the objects set forth above , among those made apparent from the preceding description , are efficiently attained and , since certain changes may be made in the above constructions without departing from the spirit and scope of the invention , it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense . it is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described , and all statements of the scope of the invention , which , as a matter of language might be said to fall therebetween .
an apparatus for perfusing the heart of a patient with oxygenated blood detects the blood pressure at the point of supply , usually the aortic root or a coronary blood vessel , and transmits this pressure to a pressure switch which controls a solenoid valve for recycling blood driven by a roller pump to the oxygenator , or , selectively , through a surge tank and to the point of supply within the patient &# 39 ; s vascular system . the surge tank has an air cushion therein for holding essentially constant the pressure at which the blood is supplied to the patient , and , thus , for holding constant the pressure at the point of supply . the system is fail - safe since the pressure switch can be connected to the solenoid so that blood is pumped to the surge tank and thence to the patient in the event of the failure of an electrical connection to the solenoid valve . provision is made for continuous feed of blood to the patient at a greater - than - normal rate should an emergency arise or for cutting off the flow of blood to the patient when desired .