Hydraulic Coupling: A New Method for Noninvasive Intermittent Blood Pressure Monitoring

HYPOTENSION during surgery with general anesthesia is associated with postoperative organ injury and death,^1,2  although evidence of a causal relationship remains sparse. But there is little question that excessively low or high blood pressures can cause organ injury. Blood pressure is therefore monitored frequently during anesthesia to guide hemodynamic management.

Effective intraoperative blood pressure management requires sufficiently accurate monitoring methods. In this issue of the Journal, Briegel et al. propose an intriguing new method for noninvasive intermittent blood pressure monitoring, “hydraulic coupling,” which uses a silicone oil-filled sensor pad inside a semirigid conical upper-arm shell wrapped by an inflatable air-filled cuff.³ 

Blood pressure changes through the cardiac cycle are transmitted from arteries into surrounding tissue. The new hydraulic coupling method uses a silicone oil-filled sensor pad to detect and transmit changes in tissue pressure, which reduces the system’s compliance and increases the signal-to-noise ratio compared to conventional oscillometry using air-filled cuffs. The shape of the hydraulic coupling cuff is determined by a semirigid conical shell, the ends of which can move against each other. The cuff is therefore pulled over the arm rather than being wrapped around. The silicone oil-filled sensor pad on the inside of the shell is positioned above the brachial artery. When the surrounding air-filled cuff is inflated by an integrated pneumatic actuator, the semirigid shell compresses the upper arm and raises tissue pressure (as does the cuff of a conventional oscillometric monitor) and couples the silicone oil-filled sensor pad to tissue above the artery, which enhances pressure conduction. Changes in tissue pressure pulsations during cuff inflation are transmitted to the silicone oil-filled sensor pad and thereafter to a pressure transducer via a fluid-filled tubing system (hence, hydraulic coupling). From tissue pressure pulsations, a tissue pressure waveform is derived that is mathematically analyzed to determine systolic and mean blood pressure by an algorithm and calculate diastolic blood pressure.

To validate the new method, the authors—who invented the method and transparently disclose financial and intellectual conflicts of interests—compared pairs of blood pressures simultaneously measured using their new hydraulic coupling method and a femoral arterial catheter (reference method) in 110 patients who had abdominal surgery or neurosurgery. The study was conducted in three centers.

Bland–Altman analysis revealed that the mean of the differences (often called “bias”) between mean, systolic, and diastolic blood pressures measured with the two methods was low—indicating a high accuracy (better called trueness⁴ ) of hydraulic coupling blood pressure measurements. Additionally, narrow 95% limits of agreement around the mean of the differences show a high precision of agreement⁵  between hydraulic coupling and reference blood pressure measurements.

To characterize the clinical importance of measurement differences, the authors additionally performed error grid analysis.^6,7  Error grid analysis classifies the difference between blood pressure measurements by two methods into one of five risk zones depending on the magnitude of the difference and whether differences are likely to put patients at risk for inadequate therapeutic interventions. Error grid analysis showed that mean and systolic blood pressures assessed with the hydraulic coupling method resulted in little or no risk. The hydraulic coupling method also performed well in identifying hypotension defined by mean blood pressures less than 65 mmHg and reliably detected blood pressure changes exceeding 5 mmHg. The authors therefore justifiably claim good absolute and trending agreement between blood pressure measurements obtained using the hydraulic coupling method and reference measurements from a femoral arterial catheter.

The initial validation study by Briegel et al. was performed under strictly controlled conditions by dedicated investigators. However, more than a quarter of enrolled patients were subsequently excluded due to protocol violations, and more than a third of the measurements in the remaining patients were excluded because they did not fulfill predefined quality criteria. The number of excluded measurement pairs and reasons for data exclusion are reported in detail. Excluding artefacts and obviously erroneous blood pressure readings is reasonable since the study sought to describe the measurement performance of the new hydraulic coupling method in comparison with the reference method under optimal conditions. But it does leave open the question of how well the new approach will work under more conventional clinical conditions. Finally, the authors included only five measurements from each patient in their analysis. The pairs were apparently selected objectively, but there was no obvious reason to exclude technically adequate measurements.

The new hydraulic coupling method measures blood pressure intermittently. It therefore cannot substitute for continuous blood pressure monitoring using an arterial catheter (which remains the clinical reference method, but is restricted to high-risk patients)⁸  or noninvasive finger-cuff methods (which are not yet commonly used).^9,10  In the vast majority of patients having surgery, blood pressure is monitored noninvasively and intermittently at 3- to 5-min intervals using conventional oscillometry with an inflatable air-filled cuff. Therefore, the strengths and limitations of the new noninvasive and intermittent hydraulic coupling method should be compared to those of conventional oscillometry.

Conventional oscillometry uses an inflatable and flexible air-filled cuff that is wrapped around an extremity to derive blood pressure from a large artery—usually the brachial artery in the upper arm. Changes in tissue pressure, reflecting blood pressure changes over the cardiac cycle, cause oscillations in the air-filled cuff that are detected by pneumatically coupled piezoelectric sensors. Cuff inflation and deflation alter the amplitude of the tissue pressure oscillations from which an estimate of blood pressure can be derived. But because compliance of the air-filled cuff and tubing system is high, and because of damping by surrounding tissue, the amplitude of the detected oscillations is small compared to changes in tissue pressure. Consequently, the signal-to-noise ratio is low, which compromises accuracy of conventional oscillometric blood pressure measurements. Conventional oscillometry becomes unreliable at blood pressure extremes. It underestimates high and overestimates low blood pressures, thus sometimes failing to detect hypertension and hypotension.^11,12  It remains unclear whether the hydraulic coupling method is superior to conventional oscillometry because conventional oscillometry was not part of the study.

In summary, hydraulic coupling is an innovative method for noninvasive intermittent blood pressure monitoring. The authors show that their hydraulic coupling method exhibits excellent intraoperative measurement performance. Future independent studies are needed to investigate the applicability, measurement performance, and signal stability of the hydraulic coupling method under routine clinical conditions and in challenging patients, including those who are morbidly obese, have cardiac arrhythmias, or are in circulatory shock. If future studies confirm the method’s accuracy and precision, it would be a welcome improvement over conventional oscillometric blood pressure measurements—which are not nearly as good as generally assumed.