Abstract
Ultrasonography is becoming the favored hemodynamic monitoring utensil of emergentologists, anesthesiologists and intensivists. While the roles of ultrasound grow and evolve, many clinical applications of ultrasound stem from qualitative, image-based protocols, especially for diagnosing and managing circulatory failure. Often, these algorithms imply or suggest treatment. For example, intravenous fluids are opted for or against based upon ultrasonographic signs of preload and estimation of the left ventricular ejection fraction. Though appealing, image-based algorithms skirt some foundational tenets of cardiac physiology; namely, (1) the relationship between cardiac filling and stroke volume varies considerably in the critically ill, (2) the correlation between cardiac filling and total vascular volume is poor and (3) the ejection fraction is not purely an appraisal of cardiac function but rather a measure of coupling between the ventricle and the arterial load. Therefore, management decisions could be enhanced by quantitative approaches, enabled by Doppler ultrasonography. Both fluid ‘responsiveness’ and ‘tolerance’ are evaluated by Doppler ultrasound, but the physiological relationship between these constructs is nebulous. Accordingly, it is argued that the link between them is founded upon the Frank–Starling–Sarnoff relationship and that this framework helps direct future ultrasound protocols, explains seemingly discordant findings and steers new routes of enquiry.
Highlights
All happy circulations are alike; each unhappy circulation is unhappy in its own way.That is to say, the circulation in its normal, untroubled state maintains constant blood flow and pressure to ensure adequate tissue oxygen delivery
The foremost implication for current practice is that certainty around the slope of the cardiac function curve stipulates a dynamic assessment
While it is tempting to use left ventricular ejection fraction (LVEF) as a surrogate for cardiac function, this too falls short; the LVEF can appear normal when cardiac function is reduced. This assertion is illustrated by an investigation by Mahjoub and colleagues, where 83, septic, critically ill patients were studied for fluid responsiveness with concomitant echocardiographic parameters of diastolic function [93]
Summary
All happy circulations are alike; each unhappy circulation is unhappy in its own way. With respect to the LVEF, because it conveys the interaction between ventricular function and arterial load, in a patient with depressed inotropy (e.g., septic cardiomyopathy) and proportionally diminished arterial load (e.g., septic vasodilation), the observed LVEF can be remarkably preserved [37,38,39] Such a patient may not tolerate additional IV fluids, despite normal-appearing ventricular behavior. With impaired LV lusitropy, the shape of the diastolic pressure–volume relationship is such that incremental volume significantly augments filling pressure [42]. In these patients, preload risks pulmonary edema, with little SV benefit [43]. To better weigh the risks and benefits of IV fluids, Doppler ultrasound may be deployed
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