18th Annual International Confertmce of the IEEE Engineering in Medicine and Biology Society, Amsterdam 1996 7.1.2: Clinical Engineering - Instrumentation

THE EFFECTS OF PULSE RATE, ARTEFACT AND PULSE STRENGTH ON OSCILLOMETRIC NON-INVASIVE BLOOD PRESSURE MEASUREMENTS.

J N Amoore, W B Geake, D H T Scott * Department of Medical Physics and Medical Engineering, Department of Anaesthetics *,

Royal Infirmary of Edinburgh, Edinburgh, EH3 9YW, Scotland

Abstract We examined the effects of pulse rate, pulse strength, and artefact on 20 adult oslcillometric non- invasive blood pressure measurements rising NIBP test simulators. Over a range of pulse rates from 40 to 200 bpm the 20 NIBP monitors studied recorded little change in registered pressures. The monitors achieved consistent readings at low pulse rates by decreasing the deflation rate and hence increasing the determination time (from under 20 sec at 200 bpm to over 30 sec at 40 bpm). High levels of artefact produced artificially high systolic pressures in 12 of the 16 monitors that recorded preswres, 6 of them being over 30 mmHg above the target 120 mmHg; 4 of the monitors signalled failure to record due to excessive artefact. Pulse strengths of 25% of the nominal were coped with well by most monitors with a slight tendency for the monitors to record low systolic pressures.

I. INTRODUCTION

Most of the automated clinical non invasive blood pressure (NIBP) monitors use the oscillotnetric technique [l]. When a cuff placed around a limb is deflated from above the systolic to below the diastolic pressure low amplitude (1 to 2 mmHg) pressure oscillal.ions, generated by beat-to-beat pulsatile arterial displacements, are induced in the cuff. The oscillations reach a maximum amplitude when the cuff pressure equals the mean arterial pressure. The systolic and diastolic pressures can be determined empirically from the relationship between the envelope of the oscillations and the cuff pressure.

The cuff pressure is “sampled” by the oscillations whose frequency is the arterial pulse rate. Hence there is an inherent relationship between the uncertainty in the measurement and the ratio of the rate of change of cuff pressure and the pulse rate [2]. Thie oscillometric technique relies on analysing the amplitude of relatively small pressure oscillations which may be contaminated by artefact caused by movement of the cuff [3]. Weak arterial pulses may lead to low amplitude oscillometric waveforms and hence reduce the signal to noise ratio. Consequently, variations in pulse rate, artefact and weak arterial pulses may all affect the reliability of non invasive blood pressure measurements, and we have examined how well several NIBP monitors cclpe with these

0-7803-381 1-1/97/$10.00 OIEEE

conditions using commercially available NIBP test simulators.

11. METHODS

NIBP test instruments generate simulated oscillometric pulses in response to cuff inflation in such a way as to cover a wide range of conditions including variations in pulse rate, pulse strength, arterial pressure, arrhythmias and artefact [4]. We used either the Dynatech Nevada CuffLink (Dynatech Nevada, P.O. Box 1925, Carson City, Nevada 89706-0403, USA) or the Bio-Tek BP Pump (Bio-Tek Instruments Inc., Highland Park, Box 998, Winooski, Vermont 05404-0998, USA) for our evaluations. We studied how well 20 different adult NIBP instruments responded to variations of pulse rate (from 40 to 200 bpm), pulse strength (varied fiom 100% to 10% of the nominal) and movement and tremor artefact.

Five consecutive recordings were made at each condition and the results analysed statistically by comparing the measured parameter and the set simulator target [5]. The Bias was calculated as the mean of the differences between the monitor’s determination and the simulated target pressure (nominal gold standard). The Variability of success measurements was calculated from the sample standard deviation of these differences. The simulated arterial pressure was kept constant at 120180 with a pulse rate of 80 bpm for all but the pulse rate studies.

111. RESULTS

One of the 20 monitors failed to record at a pulse rate of 200 bpm. The Bias of the systolic pressure registered by 17 of the remaining 19 did not vary by more than 5% when tested at 200 bpm as compared to 40 bpm; one decreased its registered systolic pressure at 200 bpm by slightly over 5% and the other by nearly 10%. By decreasing the deflation rate at low pulse rates (and consequently increasing the determination time from an average of about 20 seconds at 200 bpm - 120/80 mmHg - to 30 seconds at 40 bpm) the Variability of the determinations at 40 bpm was generally kept low. The improving performance of NIBP monitors is demonstrated when comparing the variability of an early model with that

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18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Amsterdam 1996 7.1.2: Clinical Engineering - Instrumentation

of a current model. One early monitor did not adjust its deflation rate with pulse rate (keeping it fixed at about 5 mmHgls) and the average of its systolic, diastolic and mean variabilities increased to over 15 mmHg at 40 bpm from less than 2 mmHg at 200 bpm with determination time remaining about 37 seconds. In comparison the deflation rate of a modem monitor increased to 15 mmHg/s at 200 bpm with mean variability of about 1 mmHg over the range of pulse rates; the determination time decreased from 35 seconds at 40 bpm to 14 seconds at 200 bpm.

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NIBP Monitor I I

Fig 1 YO Change of systolic pressure when presented with high levels of artefact. One monitor recorded a systolic pressure 100% falsely high.

The response of the various monitors to simulated combined low frequency and high frequency (Tremor) artefact included 3 responding well (Bias remaining within 5 mmHg), 4 out of 20 signalling excessive artefact, to those registering high and very high systolic pressures. Figure 1 shows the percentage change in systolic reading with artefact compared to no artefact for a 120/80 mmHg simulated waveform for the 16 monitors that provided readings; one monitor registered a systolic pressure 100% higher than the target.

Most monitors responded well to pulse strengths 25% of the nominal (maximum amplitude of oscillometric waveform nominally 2 mmHg), while a few recorded very weak pulses at 10% of nominal. Of the 15 that recorded at pulse strengths of 25% or lower, 5 reported pressures that were 5% or more lower than at the nominal pulse strength while the remaining were within 5%.

1V. DISCUSSION

In general the monitors tested responded well to changes in pulse rate, keeping low variabilities by decreasing their deflation rates at low pulse rates. However, many monitors did not detect high levels of artefact well, recording artificially high pressures. Most monitors responded well to low pulse strengths, but there was a tendency among some of them to record slightly low systolic pressures.

Ttie lack of ability of several monitors to cope with high levels of artefact is of concern as part of the rationale for the use of NIBP monitors is that they can record blood pressures (accurately) without the attendance of a nurse or other practitioner. By registering false high values in the presence of artefact staff may be led to make incorrect judgements, or to waste the time of maintenance groups with queries about the calibration of the device.

These studies made use of commercially available NIBP test simulators using the levels of pulse strength and artefact available. We arbitrarily analysed the response of monitor to pulse strengths of 25% of nominal; is that a realistic level of what can be expected in patients with weak pulses or is the level higher or lower? A precise knowledge of the strengths encountered would help to strengthen the conclusions drawn from studies such as this.

ACKNOWLEDGEMENTS The financial support of the Chief Scientists

Organisation of the Scottish Office Home and Health Department through the Advisory Panel on Evaluation of Medical and Scientific Equipment and Health Service is gratefully acknowledged.

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