Introduction
This article describes how arrhythmias affect the blood pressure measurement with custo screen 300/400. In this context, a difference is made between arrhythmias that are uncomplicated, those that affect the measurement slightly and those that make measurement impossible.
Blood pressure measurement devices depend on heartbeats that are as regular as possible, in order to achieve a precise result. Arrhythmias can considerably disturb the determination of blood pressure, depending on how the intensity of heartbeats varies in rhythm and in strength. custo screen 300/400 uses a method that considerably improves measurement accuracy in the event of arrhythmias.
Functioning of the custo screen 300/400 recorders
Measurement mechanism
custo screen 300/400 inflates the cuff to approximately 30 mmHg above systole. Afterwards, the air escapes with 4 to 5 mmHg/s. Each heartbeat produces an oscillation in the cuff via the arm artery. The measured amplitude of the oscillation depends on the pressure in the cuff and on the blood pressure of the single heartbeat. Above the systolic pressure, oscillations are very small. If the cuff pressure corresponds to the systolic pressure, oscillations have already increased. If mean arterial pressure is equalled, oscillations will reach a maximum approximately and they drop down again in the range of diastolic pressure.
Determination of measured value
Systole and diastole are determined on the basis of rising or falling amplitudes of the oscillations. As to custo screen 300/400, this is done by means of an envelope curve calculation (charts 1-8, green) in order to compensate fluctuations. As long as custo screen 300/400 is capable of calculating an envelope curve on the basis of the oscillations, blood pressure can be determined very precisely. This has been confirmed by the validation according to ESH-IP 2010 [1]. The envelope curve is determined by means of a numerical method, the oscillations being the basis for calculation. By using the envelope curve, the analysing algorithm of custo screen 300/400 considers the oscillations over the total measurement curve. Other blood pressure recorders only consider the local range of systole and diastole in the measurement curve and, as a consequence, are considerably less precise. If a patient has blood pressure fluctuations, the envelope curve method automatically leads to the average of blood pressure values.
Effects of arrhythmia on the measurements
| Flow without arrhythmias: | |
| The variation of oscillations in the cuff (charts 1-8, black arrows) depending on pressure (blue line) is fluent in the ideal case, as in chart 1. The heartbeats of the person who is measured must be relatively similar. |
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| Arrhythmias without noteworthy effects on the measurement result: | |
| Many people have fluctuating blood pressure from beat to beat. This affects the oscillations whose amplitude depends both on cuff pressure and blood pressure (chart 2). |
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| There are also strong arrhythmias (chart 3), mostly occurring in combination with blood pressure fluctuations (chart 4). For example, heartbeat is stronger after a long break and the subsequent beat is weaker because not that much blood has flown into the heart chambers. |
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| In the examples from charts 2 to 4, custo screen 300/400 is able to determine the blood pressure without any noteworthy loss of precision. Attention should be paid to the fact that with these arrhythmias, according to their intensity, a manual measurement with stethoscope and manometer cannot be precise either. |
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| Arrhythmias that may distort the measurement result: | |
| If there are no oscillations in the measurement curve in a coherent range of more than 3 seconds, the measure-ment may fail. If the oscillations are lacking in a range that is not significant for calculation, the determination of blood pressure is basically still possible (chart 5). |
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| If there are no oscillations in the range of systole or diastole, there must necessarily be oscillations without strong fluctuations in the other ranges (chart 6). The measurement becomes less precise, a maximum deviation of less than 10 to 15 mmHg to be expected. |
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| Arrhythmias that do not allow any measurement: | |
| If additionally the oscillations vary in the amplitude (chart 7) or if the beat intervals vary, the envelope curve cannot be calculated any more. Here, the recorder cannot determine any blood pressure. |
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| The same applies if an insufficient number of heartbeats have been detected. This can also be due to artefacts (chart 8). |
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Summary
The new custo screen 300/400 can handle arrhythmias better than other ABPM recorders. It uses a complex analysing algorithm that is able to compensate fluctuations well by means of a numerical method. In order to be able to determine blood pressure, custo screen 300/400 requires a measurement signal with oscillations in sufficient quality. In the event of arrhythmias, custo screen 300/400 is able to measure significantly more precisely than older ABPM recorders.
References
[1] Peter Bramlage, Cornelia Deutsch, Ralf Krüger, Andreas Wolf, Peter Müller, Thomas Zwingers, Beate Beime, Thomas Mengden: Validation of the custo screen 400 ambulatory blood pressure-monitoring device according to the European Society of Hypertension International Protocol revision 2010. Vascular Health and Risk Management 2014;10:303-309
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