Inferring the Frank-Starling Curve From Simultaneous Venous and Arterial Doppler: Measurements From a Wireless, Wearable Ultrasound Patch

Abstract

The Frank-Starling relationship is a fundamental concept in cardiovascular physiology, relating change in cardiac filling to its output. Historically, this relationship has been measured by physiologists and clinicians using invasive monitoring tools, relating right atrial pressure (P _ra) to stroke volume (SV) because the P _ra-SV slope has therapeutic implications. For example, a critically ill patient with a flattened P _ra-SV slope may have low P _ra yet fail to increase SV following additional cardiac filling (e.g., intravenous fluids). Provocative maneuvers such as the passive leg raise (PLR) have been proposed to identify these "fluid non-responders"; however, simultaneously measuring cardiac filling and output via non-invasive methods like ultrasound is cumbersome during a PLR. In this Hypothesis and Theory submission, we suggest that a wearable Doppler ultrasound can infer the P _ra-SV relationship by simultaneously capturing jugular venous and carotid arterial Doppler in real time. We propose that this method would confirm that low cardiac filling may associate with poor response to additional volume. Additionally, simultaneous assessment of venous filling and arterial output could help interpret and compare provocative maneuvers like the PLR because change in cardiac filling can be confirmed. If our hypothesis is confirmed with future investigation, wearable monitors capable of monitoring both variables of the Frank-Starling relation could be helpful in the ICU and other less acute patient settings.

Keywords: Doppler ultrasound; corrected flow time; fluid tolerance; frank-starling mechanism; passive leg raise; velocity time integral; venous doppler signals.

Figure 1

Depiction of normal and abnormal Frank–Starling relationships. The abscissa is P_ra or right atrial pressure as classically measured (e.g., by a pulmonary artery catheter). The venous Doppler morphology is depicted as a surrogate for P_ra (2, 3). The ordinate is stroke volume as classically measured (e.g., by a pulmonary artery catheter). The change in carotid arterial Doppler is shown as a surrogate for changing stroke volume (4, 5). Two theoretical curves are depicted with equal change in P_ra, for example from a passive leg raise (arrow Δx). Two potential responses in SV are observed: (1) abnormal, unhealthy (Δy₁) and (2) normal, healthy (Δy₂) such that two semiquantitative slopes (Δy/Δx) are inferred.

Figure 2

Picture of wireless, wearable Doppler ultrasound.

Figure 3

Example of simultaneous carotid arterial and jugular venous Doppler in healthy subject during passive leg raise. At baseline, in semirecumbent, the subject has continuous (low P_ra) jugular venous Doppler morphology. Upon passive leg raise (PLR), the venous morphology transitions to a pulsatile morphology suggesting a rise in P_ra. There is augmentation of carotid arterial Doppler metrics during PLR and SV by non-invasive volume-clamp. Compare this response to the “normal” P_ra-SV relationship in Figure 1. VTI is velocity time integral in centimeters, cm. FTc is corrected flow time in milliseconds, ms. SV is stroke volume in milliliters, ml. cm/s is centimeters per second.

Figure 4

Example of simultaneous carotid arterial and jugular venous Doppler in a patient with septic shock during passive leg raise. At baseline, in semirecumbent, the patient has continuous (low P_ra) jugular venous Doppler morphology. Upon PLR, the venous morphology transitions to a pulsatile waveform on expiration. On inspiration, the velocity is high, suggesting inspiratory collapse of the jugular vein. There is no augmentation of carotid arterial Doppler metrics during PLR. Compare this response to the “abnormal” P_ra-SV relationship in Figure 1. VTI is velocity time integral in centimeters, cm. FTc is corrected flow time in milliseconds, ms. SVI is stroke volume index in milliliters, ml. cm/s is centimeters per second.