The Frank-Starling-Sarnoff curve is clinically important and describes the relationship between cardiac preload and output. This report illustrates a novel method of simultaneous jugular venous and carotid arterial Doppler velocimetry as transient surrogates of cardiac preload and output, respectively; this approach is enabled by wireless, wearable Doppler ultrasound.
A preload challenge (PC) is a clinical maneuver that, first, increases the cardiac filling (i.e., preload) and, second, calculates the change in cardiac output. Fundamentally, a PC is a bedside approach for testing the Frank-Starling-Sarnoff (i.e., “cardiac function”) curve. Normally, this curve has a steep slope such that a small change in the cardiac preload generates a large change in the stroke volume (SV) or cardiac output. However, in various disease states, the slope of this relationship flattens such that increasing the volume into the heart leads to little rise in the SV. In this pathological scenario, additional cardiac preload (e.g., intravenous fluid) is unlikely to be physiologically effective and could lead to harm if organ congestion evolves. Therefore, inferring both the cardiac preload and output is clinically useful as it may guide intravenous (IV) fluid resuscitation. Accordingly, the goal of this protocol is to describe a method for contemporaneously tracking the surrogates of cardiac preload and output using a novel, wireless, wearable ultrasound during a well-validated preload challenge.
At its foundation, the Frank-Starling-Sarnoff curve describes the relationship between cardiac preload and output1,2,3,4. Historically, this curve is depicted by plotting the right atrial pressure on the abscissa and the cardiac output or stroke volume (SV)5 on the ordinate. Assessing the slope of this curve is clinically important because the relationship between cardiac filling and output is dynamic; thus, the slope of the curve informs the resuscitation strategy1,4. Specifically, if the slope of the Frank-Starling-Sarnoff (i.e., "cardiac function") curve is steep, then increasing the preload (e.g., administering intravenous fluid) augments the output. By contrast, if the slope of the cardiac function curve is shallow, then providing intravenous (IV) fluid does not increase the SV2.
Knowing when IV fluid does or does not increase the SV is important so that the treating clinician can avoid physiologically ineffective fluid4,6, in other words, the scenario in which giving IV fluid to a patient does not increase the SV7,8. Identifying this relatively common clinical state is achieved via a preload challenge (PC), which is a clinical maneuver that "tests" the slope of the cardiac function curve3. A PC is achieved by rapidly increasing the cardiac filling and measuring the change in SV9. As above, IV fluid can act as a PC, as can gravitational maneuvers such as moving the head below the level of the heart (i.e., Trendelenburg positioning)10 or moving from a semi-recumbent position to supine with the legs elevated (i.e., a passive leg raise)11. In fact, the passive leg raise (PLR) is a well-accepted and well-validated PC that is employed in modern intensive care units and recommended by experts prior to IV fluid administration during sepsis resuscitation4,12. Importantly, it is suggested that during the PLR, the clinician should measure both the cardiac preload (e.g., the change in right atrial pressure) and the output (e.g., the change in SV) to adequately test the cardiac function curve13. However, the former is rarely performed as simultaneous measures are cumbersome and an invasive catheter placed into the right atrium is often required.
Ultrasonographic surrogates of cardiac filling and output have grown in popularity over the last few decades, especially in emergency departments and intensive care units2,14. Specifically, the simultaneous assessment of both a great vein and large artery acts as a surrogate for cardiac preload and output, respectively2,15. For example, morphological changes in great vein Doppler have been found to track right atrial pressure-this is true for the internal jugular16,17,18, hepatic, and portal veins19, superior vena cava20, inferior vena cava21, femoral veins22, and even intrarenal veins23. Thus, great vein Doppler velocimetry operates as a surrogate for cardiac filling2. However, the Doppler of a large artery can transiently track changes in cardiac output. For instance, measures of common carotid artery systolic time24,25, velocity26,27,28, and flow 29,30 have shown promise for detecting SV changes.
A novel, wireless, wearable, continuous wave Doppler ultrasound that simultaneously insonates both the internal jugular vein and common carotid artery has previously been described14,15,27,28,31,32,33,34,35,36. Herein, a method using this device during a commonly employed, clinical PC-the passive leg raise-is illustrated. Further, the internal jugular and common carotid arterial Doppler morphologies during the PC are described as possible surrogates of cardiac preload and output, respectively. This protocol is clinically important because it provides both a practical and physiological foundation for future patient study. For example, inpatients (e.g., perioperative setting, sepsis, critically ill) and outpatients (e.g., congestive heart failure, dialysis) could be monitored by the method, or modifications thereof, described below.
The main purpose of this visual experiment is to describe a protocol for contemporaneously tracking the surrogates of cardiac preload and output during a well-validated PC using a wireless, wearable ultrasound. The goal is not to describe a specific study protocol in patients, per se. However, the description of continuous venous and arterial Doppler serves as a practical and physiological foundation for designing studies in patients both in need of resuscitation (e.g., perioperative period, sepsis) or de-resuscitation (e.g., congestive heart failure, dialysis, failure to liberate from mechanical ventilation)15,36.
The method described employs a wearable, continuous wave Doppler ultrasound that simultaneously insonates a major vein and artery to infer the cardiac function during a PC15. Critical to this method is the selection of an appropriate, cooperative patient and ensuring a minimal angle change between the vessels and the transducer throughout the assessment. Furthermore, assuring a clear and consistent dicrotic notch velocity is paramount to allow for the consistent measurement of the systolic time. Finally, the user must appreciate the venous Doppler morphology and its variation across a spectrum of jugular venous pressure (JVP), as discussed above in the representative results.
As a modification to the method described, instead of a PLR, the PC might consist of a rapid infusion of intravenous fluid9, moving a completely supine patient from horizontal to head down by 15-30° (i.e., Trendelenburg positioning)10, or respiratory maneuvers such as end-expiratory occlusion34. These approaches are beneficial in that there is less patient movement and, ostensibly, a reduced risk of angle change during the assessment. In general, troubleshooting all PCs with the wearable ultrasound requires stable neck positioning, extra adhesive to secure the insonation angle, the prolongation of the assessment when phonation or deglutition artifacts occur, the repositioning of the device, or the addition of ultrasound gel to optimize the acoustic coupling to the patient31.
There are limitations to the method of cardiovascular inference described within this manuscript. With regard to the jugular venous signal, the Doppler morphology is a surrogate of the jugular venous pressure, which itself is a surrogate of the right atrial pressure37,38,39,40. Therefore, there is no certainty that the cardiac preload is increased based on the venous Doppler changes alone. Nevertheless, the venous Doppler waveform varies its morphology based upon the pressure deflections of the right atrium17,18,41; this has been observed in multiple great veins in addition to the jugular. For example, evaluations of the superior and inferior vena cava and the hepatic, portal, intrarenal, and femoral veins all qualitatively estimate the venous pressure42. More specifically, the prominent venous velocity wave during systole is formed by the x-descent of the right atrial pressure and the diastolic velocity wave by the y-descent of the right atrial pressure. The velocity nadir between systole and diastole is due to the right atrial pressure "v wave"16,17,18,42.
Additionally, while the duration of mechanical systole is directly proportional to the stroke volume, the systolic time, similar to SV, is mediated by the heart rate, preload, afterload, and contractility43. While the ccFT equation corrects for heart rate, a limitation of the ccFT as a surrogate for the stroke volume is that it is determined by other hemodynamic inputs. Nevertheless, increases in the ccFT by at least 7 ms24 or by +2%-4% have been shown to accurately detect a 10% rise in the SV in critically ill patients24, healthy volunteers performing a preload modifying maneuver44,45, and healthy volunteers undergoing simulated moderate-to-severe hemorrhage resuscitation27. Furthermore, ccFT has been used to accurately track changing SVs in the elective surgical population during respiratory maneuvers46. Thus, assuming that afterload and contractility are relatively constant during a focused PC, the ccFT varies primarily due to changes in the SV.
Furthermore, the absolute and relative contraindications for this approach have yet to be elaborated, especially in patients. As noted above, the most common contraindication is likely an inability to cooperate (e.g., delirious, speaking, movement, rigors). This is true for many modern vital sign monitors, though the wearable ultrasound is particularly sensitive to phonation and neck movement. Accordingly, the device works very well in intubated and paralyzed patients in the operating room; a study using the device on patients receiving elective coronary artery bypass grafting is currently enrolling. Physiological variation between the opposing carotid arteries in a particular patient is possible; however, this concern is mitigated because, in the PC paradigm, the patient acts as their own control (i.e., a pre-post intervention). Accordingly, we anticipate that while the different sides of the neck (Figure 5) may produce slightly different venous and arterial Doppler signals, the change should be consistent barring any significant unilateral abnormalities (e.g., stenosis). Physical limitations may also pose problems (e.g., central lines, cervical-spine collars, tracheotomy straps, trauma, short necks, or severe cervical kyphosis). Physiological contraindications such as moderate-to-severe carotid stenosis, aortic stenosis, arrhythmia, and abnormal respiratory patterns are also of potential concern. Generally, however, a PLR with real-time measures of cardiac output is resistant to many of these issues, including arrhythmia4,11. The device is currently being studied in both spontaneously breathing emergency department patients and in the operating room; the proportion with unusable signals will be gleaned from this data.
The significance of the method described above is that the adhered ultrasound can sample minutes of continuous data, while hand-held approaches are typically limited to a few cardiac cycles48,49. Additionally, the software for the wearable ultrasound measures the arterial Doppler coefficient of variation. From this, a "smart window" is implemented to sample a sufficient number of cardiac cycles at baseline and during the intervention; this statistical instrument tailors the measurement precision for each preload challenge47. Moreover, given that the wearable ultrasound remains affixed to the patient, the risk of human factors50,51 that increase the measurement variability is diminished; this holds for both arterial and venous insonation. Another significant aspect of this method is that contemporaneous venous and arterial Doppler assessment allows the clinician to indirectly assess the cardiac preload during a dynamic maneuver; this is recommended by experts in the field13 but rarely performed because measuring the right atrial pressure is cumbersome. Accordingly, continuous venous-arterial Doppler during a PC gives a deeper picture of the cardiac function at the bedside. While this method described above may be used to judge intravenous fluid resuscitation, it also holds promise for gauging "de-resuscitation"15,52 or predicting weaning from mechanical ventilation53 and should be explored in future clinical research. For example, the diuresis of patients with volume overload may be revealed by signs of falling right atrial pressure within the venous Doppler signal as the volume removal progresses. Further, should the patient receive a PLR before and after dialysis, the change in arterial Doppler measures should indicate increased cardiac function, as previously reported52.
A method of continuous venous-arterial Doppler during a PC is best accomplished by following the six general steps outlined above in the protocol section. A novel, wireless, wearable Doppler ultrasound system assists this paradigm by adhering to a patient and enabling a relatively fixed insonation angle during the preload change. Fundamentally, simultaneous, instantaneous, venous-arterial Doppler may elaborate the two axes of the Frank-Starling-Sarnoff relationship and, therefore, give new insights into cardiac function. This is especially important when managing acutely ill patients; both volume administration and removal could be refined by this new approach. While the above discussion is largely limited to inpatient applications, additional outpatient uses within the spheres of congestive heart failure, chronic renal failure, and pulmonary hypertension are also possibilities. Accordingly, continuous venous-arterial Doppler may unlock unforeseen channels of exploration within hemodynamics and related medical disciplines.
The authors have nothing to disclose.
None.