28 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-31, NO. 1, MARCH 1982

A Reliable Microprocessor-Based Defibrillator Analyzer

VINOD K. SHARMA, Y. C. CHOPRA, J. S. BAJWA, BALWINDER SINGH SOHI, AND RAJESH KUMAR TIWARI

AbstractA scheme for a reliable microprocessor-based defibrillator analyzer for accurate measurement of various parameters of a dc defibrillator is proposed. A defibrillator, being a life saving device, needs routine calibration since any errors can have deleterious effects on the myocardium.

I. INTRODUCTION

FIBRILLATION is a cardiac arrhythmia characterized by uncoordinated activity of individual myocardial fibers. This causes random contractions of the myocardial fibers which do not pump blood effectively. This condition is normally not reversible, and thus it is almost fatal unless corrective measures are immediately applied. Some sudden stimulus such as chemical, thermal, or electrical is required in order to de-fibrillate the heart. A method of defibrillation by inducing an electric shock to the chest is the most accepted one. Hooker et al. [1] and Gurvicz et al. [2] observed that there is a critical electric current below which fibrillation can be produced (as in electrocution) and above which defibrillation is produced, but never fibrillation.

A defibrillator is a device for applying, in a controlled manner, the energy pulse necessary to restore the normal contraction rhythm of the heart. It involves the application of a high-voltage and high-current short duration impulse through two electrodes placed directly on the heart muscle or across the closed chest. This causes simultaneous contraction of all myocardial cells forcing the heart to resume the normal contraction rhythm. All the commercially available defibril-lators are presently dc type with a capacitor discharging the stored energy through the patient. A defibrillator circuit is shown in Fig. 1 and the discharge current waveform in Fig. 2. For routine calibration and preventive maintenance of the defibrillators, it is essential to measure various parameters: viz. energy delivered, peak voltage, pulsewidth, etc.

This paper describes the significance of these parameters and their accurate measurement with the microprocessor-based defibrillator analyzer. The microprocessor is suitably programmed in order to take care of the various parameters of the defibrillator.

II. SIGNIFICANCE OF VARIOUS PARAMETERS

The amount of energy actually delivered to the patient by a defibrillator can be very different (average 60 percent) from the indicated value according to Koning [3]. Since too little

Manuscript received September 24, 1981. The authors are with the Electronics and Electrical Communication En

gineering Department, Punjab Engineering College, Chandigarh, India.

Fig. 1. Basic circuit diagram of a defibrillator.

I PEAK

TIME-t

Fig. 2. Typical output waveform.

energy will not defibrillate and too large a dose may cause myocardial damage, it is very important to know what the defibrillator is putting out. The following parameters need accurate measurement.

A. Energy Delivered

DC defibrillators are calibrated in terms of energy (joules) stored in the capacitor. Due to finite output impedance of the instrument, the displayed energy can be different from the actual delivered value. Another important factor for this variation can be the change in the component values due to aging. ISI standards state that the delivered energy to a load resistance of 50 shall not deviate by more than 4 J or 15 percent whichever is greater from the indicated output. The defibrillator should be regularly checked for any drift between selected and delivered energies.

B. Peak Voltage and Peak Current

It has been found the defibrillation with short-duration high-peak currents can decrease the contractility of the myocardium and can cause an electrolytic imbalance between the heart and the electrodes. The use of low values of capacitance charged to high voltages can be effective but the associated high peak current can again cause considerable loss of contractile force according to Goddes et al. [4]. Repeated defibrillation with such high-intensity shocks can have commu-lative deleterious effects.

C. Pulsewidth

The optimum pulsewidth for defibrillation is about 4 ms. The threshold energy, charge, and peak current required for defibrillation increase rapidly with increase in pulsewidth.

0018-9456/82/0300-0028$00.75 1982 IEEE

SHARMA et al.: MICROPROCESSOR-BASED DEFIBRILLATOR ANALYZER 29

O / P F O R OSCILLOSCOPE

RANGE SELECTOR

SQUARING

CIRCUIT

PADDLE ELECTRODES

SAMPLE

SWITCH

PULSE SENSING CIRCUIT

H3 ENERGY INDICATOR

Fig. 3. Block diagram of an analog defibrillator analyzer.

Moreover, if the pulse duration is prolonged, the heart may refibrillate. A rise time of less than 1 ms can damage the cardiac tissues.

D. Waveshape

The physiologically most accepted current waveform is shown in Fig. 2. Any deterioration of the waveshape can be indicative of the future potential failure of the instrument under test according to Finlay [5].

E. Delay In order to obtain the desired results, the defibrillator is

required to fire at a particular instant of the electrocardiogram (ECG) waveform. Towards this end, the hardware of the defibrillator detects the /?-peak of the ECG waveform and provides the correct amount of delay.

F. Spikes

Due to various defects in the components, the ECG waveform is not smooth, but has spikes of short duration. The spikes do not alter the amount of energy delivered. However, proper analysis of the spikes indicates the faulty components to be repaired or replaced.

Thus to avoid the possibility of failure to arrest fibrillation and the risk of myocardial damage, a reliable defibrillator pulse analyzer, featuring accurate measurement and display of all the above parameters, is required.

III. DEFIBRILLATOR A N A L Y Z E R

A. General Approach

The block diagram of a typical defibrillator analyzer is shown in Fig. 3. The paddle electrodes of the charged defibrillator are placed against the input contact plates. The defibrillator is then discharged into the 50- standard load. The range selector provides the required attenuation for the squaring circuit. The output of the squaring circuit, on integration, gives a voltage proportional to the energy delivered as follows:

W e\t) ~ Jo R dt (1)

where W is energy in joules, e(t) is voltage in volts, R is the resistance in ohms, and T is the pulsewidth in seconds.

The pulse detector helps in reducing the effect of zero offset of the squaring circuit and the sample switch makes the pulse integrator an analog storage device, thus making the reading stable for a long period. The output of the integrator is read on a meter calibrated in joules. The main drawback of such analog type of analyzer is that the accuracy depends entirely

on the stability of components and errors may result from their aging. Further, only the amount of energy delivered is displayed in such analyzers.

B. New Approach

To overcome the above problems, the use of a versatile microprocessor-based defibrillator analyzer having facilities for the measurement of delay, energy delivered, peak voltage, pulsewidth, rise time, and waveform storage is the real answer.

The defibrillator analyzer has been developed around a system design kit similar to SDK 85 [6]. The hardware on the kit includes a keyboard, six 7-segment displays, 2K bytes ROM, 4K bytes RAM, and 3 I/O ports. The software of the kit has a necessary monitor to examine/modify RAM and registers and to run/single step through the program along with some utility programs. Thus only a limited amount of hardware, i.e., analog-to-digital converter and digital-to-analog converter are necessary to complete the defibrillator analyzer.

IV. SOFTWARE

The steps followed for the programming of the microprocessor are as follows:

a) Initialization: The defibrillator under test is connected to the system as shown in Fig. 4, and the system design kit is switched on. The command is entered through the keyboard to run the TEST program, which initializes the stack pointers, registers I/O ports, and enables interrupts. It then waits for the interrupts in the HALT state. This procedure is shown in the flowchart given in Fig. 5. Immediately after giving command to run the TEST program, the defibrillator is started using a push button which also generates an interrupt input to the microprocessor at the RST 7.5 level. The microprocessor then jumps to the interrupt service routine RST 7.5, the flowchart of which is given in Fig. 6.

A digitized typical ECG waveform is already stored in the memory. The RST 7.5 subroutine sends out ECG waveform samples through an 8-bit digital-to-analog converter to the defibrillator under test. The defibrillator hardware detects the /?-peak and after a predetermined lapse of time gives out a pulse. As soon as the defibrillator pulse starts it interrupts the microprocessor at the RST 6.5 level and the microprocessor starts servicing interrupt service routine RST 6.5. The flowchart of this routine is given in Fig. 7.

b) Data Collection: First, the routine saves the registers which are later used to check if the defibrillator fired after appropriate delay from the /?-peak. It stores the first sample as zero. Then it starts sampling and storing the defibrillator waveform. The pulse required for the initiation of analog-to-digital conversion is generated through software. The delays required for the conversion and sampling are provided using a delay subroutine. The end of the waveform is assumed to have been reached when a sufficient number of samples having zero value have occurred or the number of samples exceed a preassigned value. A counter registers the number of samples stored.

c) Check for Delay: After storing the defibrillator waveform in RAM, a check is made to see if the defibrillator has fired at the appropriate time by comparing the actual delay

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-31, NO. 1,.MARCH 1982

DEFIRILATOR

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Fig. 5. Flowchart for test program.

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caused by the hardware after occurrence of the 7?-peak with the desired delay.

d) Spikes: The data are then scanned to check for the possible spikes in the defibrillator output. The object is not to detect all the spikes. This would necessitate a very high sampling rate and consequent increase in memory space. The purpose is effectively served if there is a reasonably large probability of detecting some spikes if they occur. These are spotted by detecting an unusually large magnitude of the slope between samples.

Care has been taken to keep the negative slope in its true magnitude form. Such sample values, in order to obtain correct value of energy, are replaced by the averaged sample values; and the total number of spikes and their locations are stored. The modified data are used for further calculations.

e) Peak Value, Rise Time, and Pulsewidth: The data are then processed to find out the maximum (peak) value of the defibrillator output. The time of occurrence of the peak, called the peak time, is also determined and stored. The rise time has been defined as the time taken by the output to go from the zero value to the peak value. The width of the defibrillator pulse is determined next. The end of the pulse is considered to have occurred when the output of defibrillator goes below the zero level for the first time after the peak value.

f) Energy: The energy delivered by the defibrillator to the load R is obtained as follows:

/ = i

e2(tj)L\t R

(2)

Fig. 6. Flowchart for interrupt service routine, RST 7.5.

where e(ti) is the value of sample i, TV is the total number of samples, and / is the sampling period. In order to compute the energy contents of the waveform, the sample values are squared using a "shift-and-add" multiplication routine. The

S H A R M A et al.: MICROPROCESSOR-BASED DEFIBR1LLATOR A N A L Y Z E R 31

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