Computer control of arterial blood pressure following cardiac surgery

C. Joyce Murchie

Hypertension occurs commonly during the immediate post-operative period in patients who have undergone cardiac surgery. Careful monitoring is necessary and control of blood pressure often involves the use of vasodilator drugs such as sodium nitroprusside (SNP). Sensitivity to intravenous SNP varies from patient to patient and may change during its administration. Accurate delivery, by altering the infusion pump rate, can be difficult for nurses to achieve.

In Glasgow Royal Infirmary a computer-controlled system has been developed which analyses the blood pressure waveforms from a cardiovascular monitor and instructs the infusion pump to deliver the appropriate rate. A study has shown that the computer system does provide better control of patient’s blood pressure than manual control.

Nursing care of a patient following cardiac sur- gery involves an initial stabilising period where careful assessment and intervention are of para- mount importance. Unstable blood pressure is relatively common during this time and careful control is essential to prevent further problems arising. The surgeon will normally set an opti- mum range for each individual patient depend- ing on the pre-operative condition and the type of surgery performed, and it is the nurse’s re- sponsibility to monitor the blood pressure closely, control it, and report any significant change.

BLOOD PRESSURE MONITORING

Manual blood pressure recording using a sphyg- momanometer and stethoscope is not practical when nursing a patient who has just undergone

C Joyce Murchie 6N RGN RM Computing Fellow, University Department of Anaesthesia, Glasgow Royal Infirmary

Manuscript accepted 10 Jan 1987

cardiac surgery. The nurse has so many other ob- servations to make and under certain circum- stances, such as shock, the compensatory mechanism of vasoconstriction and arteriolar constriction can lead to inaccurate cuff measure- ments (Aspinall, 1973). To overcome this prob- lem and provide information during surgery, an arterial cannula is inserted pre-operatively into a radial artery thus enabling blood pressure moni- toring throughout the procedure in theatre and post-operatively in the cardiac intensive care unit. The cannula is connected by manometer tubing to a transducer which converts the mechanical energy produced by the arterial pres- sure into electrical energy. Changes in electrical potential are displayed on a screen and a digital display is available to make observation easier. Care of this arterial line is extremely important if accurate measurement is to be achieved and it is necessary .to keep the line patent using a pressure infusor which provides a continuous slow infusion of heparinised saline. Recordings are initially made every 15 min.

3

4 INTENSIVE CARE NURSING

HYPERTENSION AND THE USE OF SODIUM NITROPRUSSIDE

Hypertension is often a problem in the imme- diate post-operative period. The increase in blood pressure and therefore cardiac work associated with this rise can cause myocardial ischaemia due to increased myocardial oxygen demand. If blood pressure is reduced to normal, myocardial work will decrease and the risk of ischaemia will be less. It is also apparent that post-operative mediastinal blood loss is generally decreased when blood pressure is reduced (Pace & Westernskow, 1983). Short acting vasodilator agents such as sodium nitroprusside (SNP) are used frequently to bring about this reduction and to maintain stability.

SNP is a very powerful drug which acts dir- ectly on the smooth muscle of the resistance and capacitance vessels. A hypotensive response occurs within 30-60 s but resolves just as quickly if the infusion is stopped. Sensitivity to the drug varies from patient to patient and may change during administration. SNP is given intra- venously via an infusion pump and the rate of infusion is usually controlled manually by the nurse, depending on the patient’s blood pressure and the prescribed range set by the surgeon. It is a difficult task to control such a powerful drug accurately while trying to provide total patient care during this crucial post-operative period. In an attempt to relieve nursing-staff of this prob- lem, computer controlled administration of SNP has been developed.

Computer control of I.V. drug infu- sions

Controlling drug administration using a com- puter may seem a little futuristic and frightening to some, but 11 years ago Sheppard and his col- leagues (1975) described the use of a computer to measure arterial pressure via an intra-arterial cannula, calculate a new infusion rate and instruct a pump to deliver the new rate. With the deveiopment of special microprocessor- controlled infusion pumps and the increasing use of microcomputers in medicine and nursing, this

type of system seems likely to become more widely used.

In Glasgow Royal Infirmary, a computer- controlled system has been developed based on the Apple Ile microcomputer and using an Imed 929 volumetric infusion pump. Problems can arise when trying to link the two together but Kenny et al ( 1985) describe a specially developed method of communication which organises trans- mission to and receives data from the pump. By adding a Kone 565A monitor, an arterial pres- sure closed loop control system was developed (Fig. 1).

The computer constantly analyses the intra- arterial pressure waveform using an analogue to digital converter, (ADC; Interactive Structures A102) and then a machine code subroutine analyses the digitised data beat by beat and stores the information in memory where it is read by the main application program.

The main application computer program is written in BASIC language and is responsible for total control of the system. To describe it in more detail the program can be divided into four main sections (Reid & Kenny, In press) :

1. arterial pressure data collection and verifi- cation

2. control algorithm 3. infusion pump routine 4. user interface.

From the machine code subroutine mentioned above, values for systolic and diastolic pressure and heart rate are obtained. If the subroutine does not detect a satisfactory waveform, an error flag is set which the main program then inter- prets. By refering back to the ADC, it can decide whether the line is being ‘zeroed’, flushed or mechanically disturbed, if an arterial blood sample is being taken or if there is damping of the waveform. Appropriate messages then appear on the screen and an audible alarm sounds. If an error in verification of the arterial waveform occurs, the rate of infusion is reduced by 25% until the fault is corrected.

An algorithm is a series of instructions or pro- cedural steps for the solution of a specific prob- lem. In this case the control algorithm was based on the proportional- integral-derivative (PID)

INTENSIVE CARE NURSING 5

Intra-arterial b pressure

Vasodilator drug

Infusion pump

A

Y

Cardiovascular monitor

Computer with controller program*

Fig. 1, Closed loop system for computer control of arterial blood pressure. (Reproduced from Computer Technkues in Clinicel Medicine with kind permission from author and Editor).

controller developed by Sheppard (1977) and modified by Slate (1980). A PID controller tracks the difference between actual and desired pressure in three ways (Pace & Westenskow, 1983) : the absolute value (proportional), the rate of change (derivative) and the accumulated error (integral). These three measurements are aver- aged and then used to control the infusion rate. Modifications in the Glasgow system allow more frequent observation of pressure, the ability to alter the infusion rate review time and more fre- quent alterations to the infusion rate depending on patient response.

The infusion pump subroutine ensures correct data communication between the computer and the infusion pump every 22 s. This makes the sys- tem safer to use because failure of either instru- ment is detected quickly and an alarm sounded.

Ease of use and clarity of display are essential if such a system is to be controlled by staff with little experience of computers. The user interface provides two types of screen page. The first, in menu form gives options which allow control of the infusion and alteration of the target pressure or drug concentration. The second, in graphical form, displays a colour trend graph, the top half

shows how far the systolic arterial pressure varies from the target pressure and the lower half shows how much drug is being infused in pg per kg per min (Fig. 2).

The system can be used in either automatic or advisory mode. In advisory mode, the computer displays the newly calculated infusion rate for the operator to refer to, the rate may be changed if considered appropriate. This is normally only used initially before automatic control is com- menced but may be continuous if required.

Comparing manual and computer control

Reid and Kenny (1986) describe how they evalu- ated the performance of the closed loop control system by comparing nurse and computer con- trol of systolic arterial blood pressure in 60 patients who had undergone cardiopulmonary bypass surgery and who required, sodium nitro- prusside during the early post-operative period.

The study took place in the cardiac intensive care unit in Glasgow Royal Infirmary and the nursing staff involved were post-registration students, undergoing training in intensive care,

6 INTENSIVE CARE NURSING

Fig. 2. The top graph shows how far the systolic arterial pressure varies from the target pressure. The scale markings represent - 10, 0,lO. 20.30 and 40 mmHg from the target pressure. The bottom graph shows the infusion rate of SN P. The scale markings are 1,2, 3 and 4pg kg-l min-1 (Reproduced from the British JournalofAnaesthesia with kind permission from author and Editor).

or experienced sisters who worked full-time in the unit. Thirty patients were chosen randomly to have their arterial pressures controlled by nurses (Group 1) and 30 by the closed loop computer control system (Group.2). Each patient was pre- scribed an optimum range for systolic arterial pressure and the computer program was designed to aim for the midpoint of that range. Systolic, arterial pressure and rate of SNP infu- sion were recorded at intervals of 1 min manually in Group 1 and automatically in Group 2. Analgesia and sedation was provided by intravenous administration of morphine and midazolam, the nursing staff deciding when bolus doses were required.

The data were analysed for the percentage time spent outside the prescribed range in total; outside the range f lOmmHg, f 20mmHg and f 30mmHg. The results were expressed as the percentage time spent outside these limits for each patient. The study showed that systolic pressures in the manually controlled group were more frequently outside the prescribed limits for longer periods of time than those in the computer

controlled group. The results suggest therefore that computer control of systolic arterial pressure is more accurate than manual control. However, the significance of these findings is limited by the size of the sample and the potential effect of including less experienced post basic students with experienced ICN sisters.

Reid and Kenny conclude that the computer system may assist the nursing staff with drug ad- ministration by providing better control and easier evaluation. They do not suggest that the system should take over completely because nurses are ultimately responsible for ensuring that their patients receive the correct therapy.

NURSING OPINION

The computer system has been used in the car- diac intensive care unit in Glasgow Royal In- firmary for 18 months (Fig. 3). It is still necessary to control infusions of SNP manually on occasion but the nursing staff do find that the closed loop system provides more accurate assessment of blood pressure, more gradual changes in drug

INTENSIVE CARE NURSING 7

Fig. 3. Closed loop computer system for the control of arterial blood pressure in use following cardiac surgery.

administration and therefore smoother control. Initially, it was difficult for the nursing staff to accept the idea of a computer controlling drug therapy, however increasing familiarity has proved the system to be a valuable aid which allows the nurses to observe and evaluate blood pressure control at a glance and gives them more time to carry out other aspects of patient care. Due to lack of knowledge, doubts and fears can arise. When faults are detected and alarms sound the tendency is to switch off the system, revert to manual control and seek assistance. Many of the nurses feel that in-service training would help them to deal with such problems and would give them a greater understanding of computers in general.

To conclude, it is clear that use of the closed loop computer system to control blood pressure following cardiac surgery is of benefit to both

patients and nurses. It is important, however, to ensure adequate training and assistance in com- puter usage if such a system is to be fully accepted by the nursing staff.

References Aspinall NJ 1973 Nursing the open-heart surgery patient.

McGraw-Hill Inc., New York, p 106 Kenny G N C, Rennie R, Toal F, Reid J A 1985 Computer

control ofan IMED 929 infusion pump..Journal of Medical Engineering and Techndlogy9_227-228

Pace N L. Westenskow D R 1983 Cornouter reaulated Sodium Nitroprusside infusion for blood prekure control. In: Prakash 0 (ed.) Computing in Anaesthesia and Intensive Care. Martinus Nijhoff Publishers, Boston, p 295

Reid J A, Kenny G N C 1986 Evaluation of closed loop control of arterial pressure after cardiopulmonary bypass. British Journal ofAnaesthesia (in press)

Sheppard L C, Kouchoukos N T, Shotts J F 1975 Regulation of mean arterial pressure by computer control of vasoactive agents in postoperative patients. Computers in Cardiology, Proceedings IEEE