Cardiac & Resuscitation

Ditch the Machine to Improve Accuracy in Blood Pressure Measurement and Diagnostics

“Medic 14, respond to an unconscious subject.”

On arrival, two paramedics find a 45-year-old male sitting on a chair under a shade umbrella outside a car wash. The time is approximately 4:00 p.m., and the temperature is 94 degrees F. The patient presents with pale skin and his breathing appears slightly labored.

After the paramedics introduce themselves, the patient reports that he’s feeling faint and says he almost passed out while working in the hot sun. He’s willing to submit to an assessment, but reluctant to accept transport, stating, “I just got this job, and I need to get back to work.”

While completing the primary assessment, the senior paramedic notes the patient’s radial pulse is rapid and weak, and his skin to be cool and moist. The paramedic and his partner move the patient into the ambulance to perform the secondary examination, which begins with removal of the patient’s shirt.

With the patient properly exposed to allow for the interventions of the secondary exam, the paramedic delegates an ECG, vital signs, SpO2, and blood glucose measurement to his partner, and begins to interview the patient. He considers the presentation and apparent history of exertion in a hot environment.

When the senior paramedic inquires about associated symptoms, the patient replies that he’s been experiencing progressive weakness, dizziness and exertional dyspnea throughout the course of the morning. He further reports that he’s been experiencing an intermittent, violent cough over the last several weeks.

He denies any neurological symptoms, chest pain or recent trauma. His family, lifestyle and social history reveal that he’s transitioning from a homeless shelter to a roommate situation. His general medical history is unremarkable.

With the interview now complete, the senior paramedic prepares to conduct a comprehensive physical exam. His partner reports the vital signs, which reveal the patient to be tachycardic, tachypneic and hypertensive, with a blood pressure measured at 153/91 mmHg. SpO2 and blood glucose are within normal limits. On examining the ECG, the paramedic doesn’t note any indication of acute myocardial infarction.

With the presentation of tachycardia, tachypnea, faint pulse and history of recent infection, the senior paramedic’s early considerations of possible causes of the patient’s near-syncope range from heat exhaustion and/or dehydration to sepsis.

Given the patient’s skin signs and the weak quality of the radial pulse, the senior paramedic is suspicious that the blood pressure, which was taken with an automated cuff, might be inaccurate.

The paramedic asks his partner to re-take the blood pressure using an aneroid sphygmomanometer and stethoscope. Obtaining the blood pressure in this fashion reveals an updated finding: the blood pressure is in fact 108/90 mmHg, with pulsus paradoxus noted from 108 mmHg all the way to the diastolic reading of 90 mmHg.

Given this concerning result, the paramedic re-examines the ECG and notes electrical alternans—a hallmark sign of cardiac tamponade.

Electrical Alternans

The comprehensive physical examination further confirms the working diagnosis of cardiac tamponade: The patient’s heart tones are muffled to auscultation, and there’s jugular venous distention noted. The paramedics inform the patient of the seriousness of his condition, and he agrees to be transported.

Manual BP Measurement: A Fundamental Skill

In this age of automated technology, few clinical skills are going the way of the passenger pigeon more thoroughly or decisively than the use of the aneroid sphygmomanometer, more commonly referred to as the “manual blood pressure cuff.”

In spite of the efforts of many well-regarded clinicians, the art of patient assessment continues to be neglected in favor of expedient examinations that rely on digital measurements.1–3

Given the evidence clearly substantiating that automated blood pressure readings are frequently inaccurate, as well as limited in their diagnostic capability, it’s time to revisit one of the most fundamental skills of the field medicine provider: measurement of blood pressure by aneroid sphygmomanometer.4

Pressure within the vascular system is the primary determinant of tissue perfusion. As such, the importance of its accurate measurement during patient assessment can’t be overstated.

In the case above, the senior paramedic recognized that obtaining a correct blood pressure was a crucial part of assessing and treating his patient’s condition. And, when the reading from the automated device appeared to be incongruent with the patient’s overall presentation, he elected to have his partner repeat the assessment using an aneroid sphygmomanometer and stethoscope.

In order to appreciate how this classic time-honored technology provides for optimal accuracy in measurement and diagnostics, we need to understand the principles behind its use.

Basic Physiology of Blood Pressure

The ventricles of the heart fill with blood during diastole, the first phase of the cardiac cycle. Oxygenated blood returning from the lungs enters the left ventricle as it relaxes during this phase.

The amount of blood in the left ventricle at the end of diastole is known as the left ventricular end diastolic volume. The right ventricle also has an end diastolic volume, but it’s the end diastolic volume of the left ventricle that’s significant in the measurement of blood pressure.

Diastole is followed by systole, the phase in which the ventricles contract. During this part of the cycle, the left ventricle discharges a portion of the end diastolic volume into the aorta.

This amount is known as the stroke volume, and is ejected from the ventricle at a pressure that’s reliant on several variables, including the end diastolic volume itself and the amount of force the heart muscle is able to generate.

Once in the aorta, the blood begins its journey through the arterial system toward the tissues. The force the blood exerts on the inside of the walls of the arteries is known as blood pressure, and is dependent upon the following three factors:

  1. Volume of blood in the arterial system;
  2. Peripheral vascular resistance, which is determined by blood vessel diameter, viscosity of the blood, and length of the vessel; and
  3. Cardiac output, which is a function of heart rate x stroke volume.

Blood pressure fluctuates based on whether or not the ventricle is contracting or relaxing; The pressure will be greater during systole, when blood is being pumped out of the ventricle, than during diastole, when it is not.

Noninvasive Blood Pressure Measurement

The earliest known measurement of blood pressure dates back to the 1700s, when English experimental scientist Stephen Hales inserted a catheter into the femoral artery of a horse.5  While palpating the carotid artery, Hales noted fluctuations in the height of the blood in a glass tube connected to the catheter. Every time a pulsation was felt at the carotid artery, the blood rose higher in the tube.

Indeed, the most accurate way to measure blood pressure remains invasive intra-arterial measurement (although the process is thankfully somewhat more benign these days).

Such technology is typically not used in field medicine. Instead, we measure blood pressure noninvasively through the use of an inflatable cuff wrapped around the upper arm. When the cuff is filled with air to sufficient pressure so that the pressure in the cuff exceeds the blood pressure in the brachial artery, blood flow through the brachial artery stops. If we auscultate (i.e., listen with a stethoscope) over the brachial artery in the antecubital fossa (i.e., the anterior aspect of the junction between the upper arm and forearm), just distal to the point of occlusion, no sounds will be present—there’s no blood flowing through the artery.

If we gradually release the pressure inside the cuff, there will come a time when the left ventricle is able to eject blood at a high enough pressure to overcome the pressure of the cuff. At this moment—when the pressure in the artery during systole is equal to the pressure measured inside the cuff—we’ll hear a thumping or tapping sound as the sudden return of blood flow strikes the inner wall of the artery. This is the systolic blood pressure.

The thumping or tapping sounds—named Korotkoff sounds, after the Russian doctor who first discovered them—continue to be heard so long as the pressure in the artery exceeds the cuff pressure during systole; but they’re not heard during diastole, when the pressure inside the artery is much less, and blood is forced out of the artery by the pressure from the cuff.

If we continue to release the cuff pressure, eventually this pressure will become equal to the pressure inside the artery during diastole. When this occurs. blood is no longer forced out of the artery during diastole, and the surge of blood returning to a formerly occluded artery goes away—as do the Korotkoff sounds. Therefore, the first point where we no longer hear Korotkoff sounds is the diastolic blood pressure.

In all, five distinct phases of Korotkoff sounds are acknowledged to be significant in blood pressure measurement:

  • Phase 1: the first appearance of thumping or tapping sounds that gradually increase in loudness;
  • Phase 2: the Korotkoff sounds take on a fainter, swishing sound;
  • Phase 3: the Korotkoff sounds become loud and sharper again;
  • Phase 4: the sounds suddenly become muffled; and
  • Phase 5: the Korotkoff sounds disappear entirely.

It’s worth noting that the very first point at which Korotkoff sounds aren’t heard with systole (i.e., Phase 5) corresponds to the diastolic blood pressure, but due to the difficulty in measuring something that’s absent, accepted practice is to record the diastolic blood pressure as the very last Korotkoff sound during Phase 4. (Some texts do advocate utilizing Phase 5 as a more accurate measure of the actual pressure in the arteries during diastole).6

How is the blood pressure actually measured? The inflatable cuff is connected to a bulb fitted with a two-way valve that allows air to be pumped into a bladder sewn into the cuff. The bladder, in turn, is connected to a calibrated meter that measures the pressure inside the bladder, and expresses this pressure in units known as millimeters of mercury, or mmHg.

The meter itself is known as a sphygmomanometer (“sphygmo” is the Greek word for pulse and a manometer is a pressure meter). When the Korotkoff sounds appear and move though the five phases, they correspond to given readings on the sphygmomanometer.

These readings are noted and documented; typically only the systolic and diastolic pressures—the first Korotkoff sound at phase 1 and the last Korotkoff sound at phase 4, respectively—are recorded.

The Importance of Using an Aneroid Sphygmomanometer

The gold standard in noninvasive blood pressure measurement is the mercury sphygmomanometer, a device in which the bulb and cuff are attached to an inverted glass tube that’s calibrated in millimeters and is designed to hold the liquid metal that gives it (and the unit used for blood pressure measurement) its name.

Although the mercury sphygmomanometer is impractical for use in field medicine, this manometer provides the reference against which the accuracy of all other blood pressure measurement technologies are determined.7

The technology was first introduced in 1881 by Austrian physician Samuel Siegfried Karl Ritter von Basch. Shortly thereafter, the aneroid (i.e., without liquid) sphygmomanometer was developed by French cardiologist Pierre Potain as an alternative to the cumbersome mercury device.8

Potain's sphygmomanometer Wellcome M0018409

It’s through comparison to measurements taken with the mercury sphygmomanometer that the aneroid sphygmomanometer has been shown to be vastly superior to automated blood pressure measurements in terms of accuracy and diagnostics. One recent study demonstrated that aneroid sphygmomanometers are more accurate than automated devices in their ability to measure both the systolic and diastolic blood pressures; diastolic accuracy is particularly improved (98.7% accuracy using an aneroid manometer vs. 67.7% using digital technology).7

Given this result, is EMS-based research demonstrating a particularly high variation between aneroid blood pressure measurements and those taken with automated technology in a condition where accuracy is of paramount importance? For example, profound hypotension is defined as a systolic blood pressure of less than 90 mmHG. 9

Even manufacturers of the technology warn against relying on automatic blood pressures in these situations, stating “ … shock may result in a blood pressure waveform that has a low amplitude, making it difficult for the monitor to accurately determine the systolic and diastolic pressures.”10

Automated blood pressure technology doesn’t make use of auscultation, but relies instead on oscillometry, which measures the amplitude of various pulse pressures in the cuff overlaying the artery. The systolic and diastolic pressures are determined using a complex mathematical algorithm that extrapolates the diastolic and systolic pressures from the mean value of these measurements.11

Arrhythmias, muscle tremors and other disorders commonly encountered in the field skew these values even further beyond the inaccurate readings already inherent in automated blood pressure measurement.4

The American Heart Association has redefined chronic hypertension, a serious, potentially fatal disease, to be systolic blood pressure measurements consistently measured at greater than 130 mmHg and/or diastolic blood pressure measurements consistently measured at greater than 80 mmHg.12

Although management of chronic hypertension is beyond the scope of field providers, these more restrictive parameters underscore the importance of accurate blood pressure management.

Diagnostic Considerations

In the opening case scenario, in addition to obtaining an accurate blood pressure, the use of the aneroid sphygmomanometer yielded another benefit: the discovery of pulsus paradoxus, a finding predictive of cardiac tamponade. Recognition of this condition, defined by a marked and prolonged decrease in systolic blood pressure on inspiration, was integral to making a correct diagnosis.

In a healthy patient, Korotkoff sounds appear as pressure is released from the blood pressure cuff. As the patient breathes in, the muscles of respiration expand the area inside the rib cage and decrease the area inside the abdominal cavity. The resulting pressure changes cause an increase in venous return to the right ventricle, which in turn creates an increase in pressure inside the right ventricle that’s distributed equally across the inner wall and the interventricular septum.

This causes the septum to bulge slightly into the left ventricle, temporarily reducing the capacity of the left ventricle and resulting in a slightly decreased end diastolic volume, decreased stroke volume, and, ultimately, decreased systolic blood pressure during inspiration. As a result, Korotkoff sounds normally diminish or may disappear entirely during the inspiratory phase of the respiratory cycle at the beginning of Phase 1.

In a patient with cardiac tamponade, the increased force on the outside of the heart results in a disproportionate amount of pressure being transmitted to the interventricular septum, with the result being an exaggerated bulge of this structure into the left ventricle.

Consequently, end diastolic volume and stroke volume are decreased even further on inspiration, resulting in a discrepancy greater than the 10 mmHg range normally observed between when Korotkoff sounds disappear during inspiration and when they’re present throughout the entire respiratory cycle.

To assess for pulsus paradoxus, the paramedic inflates the cuff until the brachial artery is completely occluded and then slowly releases the pressure. If the difference between the systolic pressure measured when Korotkoff sounds first disappear during inspiration and when they are present during both inspiration and expiration is greater than 10 mmHg, then clinically significant pulsus paradoxus is present.

Such an observation is easily made by a skilled provider with the use of an aneroid sphygmomanometer and a stethoscope, but won’t be able to be determined through the use of an automated blood pressure device.

For our patient, who was considering signing a release and returning to work, the paramedics’ discovery of pulsus paradoxus was a turning point. A misdiagnosis in this case could have had catastrophic consequences, particularly if the patient had refused treatment based on a presumption of a more benign condition.

The narrow pulse pressure revealed by the now correct systolic and diastolic pressures further increased the index of suspicion for tamponade, which was all but confirmed by the findings obtained during the physical exam.

Case Outcome

The patient is transported Code 3 to the hospital with high-flow oxygen provided, and an IV of normal saline established en route. During transport, the patient’s blood pressure is maintained at 90 mmHg systolic, with measurements taken every five minutes using the aneoroid sphygmomanometer. In the ED, the physician confirms the diagnosis of cardiac tamponade.

Further testing reveals that the patient is positive for a tuberculosis infection. Pericardiocentesis is performed, and the patient is placed on a regimen to include IV antibiotics.

Consistent with the finding of tuberculosis, it’s determined that the patient had developed infectious pericarditis and subsequent tamponade. The patient is admitted for treatment and observation, and discharged the following week without further incident.

Conclusion

For the patient in this case, the decision to forego the convenience of a machine in favor of the skills of a knowledgeable paramedic was lifesaving. Much like the comparison often drawn between the old-fashioned barbell and more sophisticated exercise machines, newer, more complex, and more expensive might make a process more comfortable, but doesn’t always equate to superior results.

As we surrender more and more of our hands-on skills to the ease of automated technology, we risk more than the loss of the aptitudes that form the foundation of sound patient assessment—we place our patients in jeopardy of misdiagnoses and inadequate treatment.

Proper use of the aneroid sphygmomanometer is but one of many practices that’s important for EMS professionals to maintain if we are to claim that we are anything other than accomplices to placing our patients’ lives at the mercy of machines.

Companion Article

Taking a Manual Blood Pressure: Techniques & Pitfalls
Use an aneroid sphygmomanometer for optimal measurement and diagnostic accuracy
By Mark Rock, NRP

References

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3. Asif T, Mohiuddin A, Hasan B, et al. Importance of thorough physical examination: A lost art. Cureus. 2017;9(5):e1212.

4. Medicines and Healthcare Products Regulatory Agency. (December 2013.) Blood pressure measurement devices. Retrieved Oct. 14, 2018, from https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/403448/Blood_pressure_measurement_devices.pdf.

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7. Shahbabu B, Dasgupta A, Sarkar K, et al. Which is more accurate in measuring the blood pressure? A digital or an aneroid sphygmomanometer. J Clin Diagn Res. 2016;10(3):LC11–14.

8. Booth J. A short history of blood pressure measurement. Proc R Soc Med. 1977;70(11):793–799.

9. Melnyk M. (May 31, 2009.) Automatic vs. manual BP. JEMS. Retrieved Oct. 14, 2018, from www.jems.com/articles/2009/05/automatic-vs-manual-bp.html.

10. Physio-Control Inc. (May 2010.) NIBP monitoring considerations. In: LIFEPAK 15 Monitor/Defibrillator Operating Instructions. Retrieved Oct. 14, 2018, from www.physio-control.com/uploadedFiles/Physio85/Contents/Emergency_Medical_Care/Products/Operating_Instructions/LIFEPAK15_OperatingInstructions_3306222-002.pdf.

11. Babbs CF. Oscillometric measurement of systolic and diastolic blood pressures validated in a physiologic mathematical model. Biomed Eng Online. 2012;11:56.

12. MacGill M. (Dec. 11, 2017.) Everything you need to know about hypertension. Medical News Today. Retrieved Oct. 14, 2018, from www.medicalnewstoday.com/articles/150109.php.