About Non-Invasive Blood Pressure

This application note discusses non-invasive (NBP) blood pressure measurements and techniques, how the HeartStart Intrepid monitor/defibrillator uses the oscillometric method to measure blood pressure, and how to reduce errors and obtain an accurate NBP measurement.

NBP Techniques

Non-invasive blood pressure measurements are quick and simple to perform, causing minimal stress to the patient.

Auscultatory (Manual Cuff)

This technique uses a sphygmomanometer and an occluding cuff, along with a sound detector or stethoscope placed over the occluded artery to detect sounds emitted as the cuff deflates. The cuff pressure at which the sound is first emitted indicates the systolic pressure. When the sound disappears or changes quality, the cuff pressure approximates the diastolic pressure.

Auscultation Assisted with Doppler Flow Detectors

This technique requires a sphygmomanometer and a handheld Doppler device. The pulse detector is placed over the brachial or radial artery distal to the cuff. A characteristic hushing sound, signifying blood flow and vascular motion, is listened for as the cuff pressure is reduced. The pressure at which the first hushing sound is heard is recorded as the systolic pressure. The hushing sounds continue as long as the device is held over the open artery.

Oscillometric

Oscillometric devices measure the amplitude of pressure changes in the occluding cuff as it deflates from above systolic pressure. The amplitude suddenly increases as the pulse breaks through the artery's occlusion, indicating systolic pressure. As the cuff pressure decreases further, pulsations increase in amplitude, reach a maximum (approximating mean pressure), and then diminish rapidly. The diastolic pressure index is taken where this rapid transition begins.

How NBP Works in the HeartStart Intrepid

The HeartStart Intrepid is equipped with the Philips oscillometric non-invasive blood pressure measuring system. It measures blood pressure for both adult and infant/child patients using the oscillometric method. Initial cuff inflation pressure is based on the configured patient type. Measurement ranges and default alarms can be adjusted by changing the patient category. Systolic, diastolic, and mean measurements are provided, with alarms available to alert clinicians to changes in the patient's condition. NBP measurements can be taken in Monitor, Manual Defibrillation, and Pacing modes. Accurate readings can be obtained at heart rates of 40 to 200 beats per minute, systolic pressures of 30 to 270 mmHg, and diastolic pressures of 10 to 245 mmHg.

NBP measurements can be taken:

Manually: One measurement is taken each time the Start NBP soft key is pressed.
Automatically: The measurement repeats at the specified interval from the time the Start NBP key is pressed. Additional manual measurements can be taken without affecting the automatic schedule by pressing the Start NBP soft key.

While an NBP measurement is in progress, the current cuff pressure is displayed. Once complete, the systolic, diastolic, and mean pressure values are shown, along with the measurement schedule and a timestamp. If NBP alarms are enabled, alarm limits appear next to the NBP value, and the alarm source is displayed above the limits. If alarms are off, an 'Alarms Off' symbol replaces the limits.

The parameter area displays NBP values, time, and temperature.

How the NBP Algorithm Works

The first time an NBP measurement is taken, the cuff's initial inflation pressure is 165 mmHg for adults or 130 mmHg for infant/child. As the cuff deflates, pulsatile oscillometric pulses begin as pressure on the brachial artery diminishes. The amplitude of these pulses increases until a maximum is reached, correlating with mean arterial pressure (MAP), and then decreases. Oscillometric pulses (heart beats) are analyzed. When two qualified pulses are acquired, the cuff deflates to the next step. An oscillometric curve is created from all pulses and used to calculate systolic, diastolic, and mean values.

The safety circuit pressure time-out is 180 seconds for adults and 90 seconds for infant/child, with automatic cuff overpressure detection at 300 mmHg for adults and 150 mmHg for infant/child. For elevated systolic blood pressure, the device may re-inflate to a maximum of 300 mmHg.

Maintaining the NBP Feature

NBP should be calibrated yearly. To calibrate the NBP feature, contact Philips service or refer to the HeartStart Intrepid Service Manual.

How to Reduce Errors and Obtain an Accurate NBP Measurement

Measurement Errors

Both manual cuff and oscillometric techniques are subject to errors. Table 1 lists major factors influencing accuracy.

Reducing Errors

Cuff Size

Selecting the correct cuff size is crucial for accuracy. The AHA recommends the cuff width be 40% of the upper arm circumference. Philips' cuffs are recommended as they are verified for use with the HeartStart Intrepid.

Cuff Placement

Correct cuff placement over the artery is critical. The cuff should be at heart level, or a correction factor applied for height differences. Ideally, the lower edge of the cuff should be approximately 2 cm above the antecubital fossa (bend of the elbow).

Table 1: Major Sources of Errors Affecting NBP Measurements
Auscultatory Cuff Technique	Oscillometric Technique
Too rapid deflation of the cuff ^a	Incorrect cuff size ^a
Misinterpretation of Korotkoff sounds ^a	Incorrect cuff application ^a
Incorrect cuff size ^a	Arrhythmias ^b
Incorrect cuff application ^a	Rapid changes in pressure ^b
Calcification of the artery ^b	Patient movement (twitching, shivering, etc.) ^b
Arrhythmias ^b	Cuff not at heart level ^a
Rapid changes in pressure ^b
Operator's hearing acuity ^a
Pressure waveform with high spikes ^b (high dp/dt)
Cuff not at heart level ^a

^a Technical sources of errors minimized by the operator.
^b Clinical or physiological sources of errors not minimized by the operator.

Figure 2 illustrates correct cuff placement, showing the cuff positioned on the upper arm with the artery indicated.

Table 2: Incorrect Cuff Size and Placement Issues
Problem	Cause	Rationale
False high reading	Cuff too small	Small cuff does not adequately disperse pressure over the arterial surface.
	Cuff not centered over the brachial artery	More external pressure is needed to compress the artery.
	Cuff not applied snugly	Uneven and slow inflation results in varying tissue compression.
	Arm below heart level	Hydrostatic pressure imposed by the weight of the intra-arterial blood column above the site of auscultation is additive to arterial pressure. Reposition arm to heart level.
False low reading	Cuff too large	Pressure is spread over too large an area, damping oscillometric pulses.
	Arm above heart level	Hydrostatic pressure in the elevated arm resists pressure generated by the heart.

Clinical Considerations for NBP Monitoring

Consider the following when monitoring blood pressure:

Do not perform NBP measurements on patients with sickle-cell disease or conditions with expected or occurring skin damage.
Use clinical judgment for automatic NBP measurements on patients with severe blood clotting disorders due to the risk of hematoma.
Avoid applying the cuff on the same side as a mastectomy to prevent lymphedema risk. For bilateral mastectomies, weigh the benefits against the risks.
Prolonged automatic NBP measurements may be associated with purpura, ischemia, and neuropathy. Frequently examine the limb's extremities for normal color, warmth, and sensitivity. Stop measurements immediately if abnormalities are observed.

Summary

Non-invasive blood pressure measurements are well-tolerated by patients and contribute to reduced patient safety risks. The HeartStart Intrepid monitor defibrillator's NBP feature, developed for transport situations, employs the oscillometric method to accurately and reliably measure mean arterial pressure and calculate systolic and diastolic pressure.

References

Pickering Thomas, G., Hall John, E., Appel Lawrence, J., Falkner Bonita, E., Graves, J., Hill Martha, N., ... Roccella Edward, J. (2005). Recommendations for blood pressure measurement in humans and experimental animals. Hypertension, 45(1), 142-161. doi:10.1161/01.HYP.0000150859.47929.8e.
Darovic, G. O. (1995). Hemodynamic monitoring : Invasive and noninvasive clinical applications (2nd ed.). Philadelphia: Saunders.

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