Systolic Blood Pressure Measurement in Critically-ill Patients

Background/Significance:

Hypotension is common and predicts worse outcomes. Arterial hypotension, defined as an arterial systolic blood pressure (BP) less than 90 mmHg in adults, represents the hallmark of critical illness. Accurate arterial BP measurement is essential to deliver effective treatment, to guide resuscitation, and to assess interventions. Non-traumatic hypotension has been documented in as many as 19% of Emergency Department (ED) patients and those patients with hypotension have a 10-fold increased risk of sudden, unexpected in-hospital death.(1) Early hypotension is associated with increased mortality in survivors of cardiac arrest, stroke, and STEMI,(2-4) stressing the need for a tool to rapidly and accurately identify BP trends in order to effectively direct resuscitative measures. Additionally, Doppler waveforms may provide additional clues of impending hemodynamic deterioration before overt hypotension or shock ensues.

Disadvantages of the current system - The current initial standard for BP measurements in most hospitals is via an automated non-invasive BP device that uses oscillometric technology. This method has numerous limitations including non-continuous monitoring, missing or delays in identifying hypotensive episodes, inaccuracy or inability to measure BP due to various reasons including inaccurate cuff size or severity of hypotension. In one study, 34% of patients had a BP discrepancy of ≥ 20 mmHg with the oscillometric device compared to intra-arterial BP monitoring due to incorrect cuff size.(5) Oscillometric devices are sufficient for use in many clinical situations; however, notable exceptions include patients that are severely hypertensive, hypotensive, with arrhythmias, after trauma, and other critical clinical scenarios, whereby manual auscultatory BP measurement is preferred.(6) Even in non-critically ill patients, validation data of oscillometric measurements has been called into question.(7) The alternative to oscillometric devices is an intra-arterial catheter. Although accurate, intra-arterial catheters are invasive, technically difficult to insert, expensive, time-intensive, painful, and with risks including bleeding, infection, and neurovascular injury. Current guidelines for management of critically ill patients suggest intra-arterial BP measurement is preferred over automatic oscillometric non-invasive BP measurement. Despite this, in a recent survey of intensivists, 73% used non-invasive BP measuring devices in hypotensive patients.(8) For the above reasons, researchers have been investigating newer technology to replace the oscillometric technology; examples include Doppler and photoplethysmographic devices.(9;10) Newer technology- The research and application of Doppler technology for measuring BP is in its infancy and has yet to be fully realized. The benefits of such a device would include an accurate, non-invasive means to measure BP in critically-ill patients. Due the high-risk of missing hypotension, the possibility exists to commercialize the prototype for broader clinical use in non-critically ill patients as well. The accuracy of Doppler velocimetry is one important advantage. In an animal study comparing three indirect BP measuring instruments- a Doppler ultrasonic flowmeter, an oscillometric device, and a photoplethysmograph- to direct arterial pressure in cats, the Doppler and photoplethysmographic devices had the highest overall accuracy. (10) In clinical practice, when the initial automatic oscillometric BP device is unable to obtain a measurement due to hypotension, the manual Doppler technique is a common practice due to its reliability and referenced in one study as the "gold standard".(11) However, accurate systolic BP measurement is only one parameter of the device. The Doppler signal received from the arterial flow through the vessel has incredible potential to identify strength of pulse and other auditory queues. This has been well established in peripheral arterial disease, with descriptive terms used such as monophasic, biphasic, or triphasic pulses identified by Doppler. The novel device has the potential to identify a "sick" pulse as determined by changes in the Doppler waveform prior to hypotension ensues, as often, hypotension is very late in the course of the disease process. To date, there is no automatic sphygmomanometer/Doppler apparatus available.

This project will provide data to further the development of a novel blood pressure monitoring device that will enable non-invasive and near-continuous blood pressure monitoring and other hemodynamic information on critically-ill patients. Data from healthy volunteers using a standard 8MHz (fetal) Doppler probe have already been collected in a companion protocol in place at the University of North Carolina at Charlotte (UNCC).

Research Strategy: