ICU Physiology in 1,000 Words: Stroke Volume Variation and the Concept of Dose-Response
Stroke Volume Variation and the Concept of Dose-Response
Awareness of the undulating pattern of an arterial line tracing is high amongst health professionals in the intensive care unit; certainly this is an aftereffect of a cacophony of studies and reviews pertaining to pulse pressure variation and fluid responsiveness in the operating room and ICU. But what exactly is the genesis of pulse pressure variation? What are its limitations and why do these limitations exist? Functional hemodynamic monitoring Functional hemodynamic monitoring [FHM] assesses the functional state of the cardiovascular system by measuring a response to a defined stress . The measured response may be either a ‘venous-side’ [e.g. venae cavae diameter change] or an ‘arterial-side’ [e.g. stroke volume variation (SVV)] variable ; the defined stress is often a change in intra-thoracic pressure [ITP]. A commonly queried hemodynamic state is whether cardiac output will rise in response to a volume challenge. The general principle involved in FHM is a dose-response relationship similar to basic pharmacology. Importantly, if one alters the dose of a medication and observes a different clinical response, the change in response may be due to the altered dose. Similarly, because the diagnostic characteristics of SVV were derived in the context of a specific, predefined stress applied to the cardiovascular system [e.g. ITP change], observing a change in ‘response’ of SVV will be confounded by a change in the ‘dose’ of ITP. Intra-thoracic pressure In the passive, ventilated patient – one not generating any respiratory effort – the change in ITP is related directly to the ventilator-applied tidal volume and indirectly to the compliance of the chest wall [3-5]. Thus, the ITP experienced by the cardiovascular system will increase with higher tidal volume and/or lower chest wall compliance [e.g. obesity, ascites, chest wall edema, etc.], and vice versa . If a patient generates his or her own inspiratory effort with the ventilator or bears down against it, the change in ITP is, essentially, unpredictable. Such fluctuations in ITP will confound any measured cardiovascular response such as SVV. It is the reason why studies of FHM included patients who were completely passive with the ventilator and receiving relatively high tidal volumes [8-10 mL/Kg] [6-8]. Intra-thoracic pressure and cardiovascular physiology Cardiac output is determined at the cross-roads of venous return and cardiac function. These two physiological phenomenon may be plotted simultaneously to generate the Guyton Diagram [9, 10]. The intersection of venous return and cardiac function defines the operating point of the cardiovascular system; the operating point determines the central venous pressure [on the x-axis] and cardiac output [on the y-axis] . A passive, mechanical breath [i.e. an increase in ITP] favors a right shift of the cardiac function curve relative to the venous return curve. Consequently, if the operating point lies upon the ascending [volume-responsive] portion of the cardiac function curve, an increase in ITP transiently reduces right ventricular output. By contrast, if the operating point intersects the plateau of the cardiac function curve [volume-unresponsive], an increase in ITP will not diminish cardiac output. While this physiology applies to the right ventricle, 2-3 cardiac cycles later, the right heart output is experienced by the left heart – a delay imposed by the pulmonary transit time. Therefore, the physiological stress of an increase in ITP may be assayed by beat-to-beat evaluation of left ventricular SVV; the magnitude of SVV is suggestive of where upon the cardiac function curve the venous return curve intersects  and, consequently, alludes to volume responsiveness. The physiology of left ventricular stroke volume variation Because aortic pulse pressure [aortic systolic pressure – aortic diastolic pressure] directly relates to left ventricular stroke volume [as a function of central aortic compliance], pulse pressure variation (PPV) is frequently used as a surrogate of left ventricular SVV . Deflections in pulse pressure during a single, mechanical respiratory cycle are referenced to an end-expiratory baseline. In the patient passive with the ventilator, a mechanical breath causes an in initial increase in systolic blood pressure referred to as reverse pulsus paradoxus, dUP [or delta UP] and is the consequence of multiple mechanisms. Firstly, there may be direct transmission of the increased ITP to the aorta and arterial tree , secondly mechanical insufflation of the lungs in a passive patient favors each of the following: improved left ventricular compliance via reduction of right ventricular volume , reduced left ventricular afterload  and, most prominently, an increase in pulmonary venous return and therefore LV preload [16, 17]. As many of these variables are not related to volume responsiveness, dUP is felt to be a poor marker of fluid responsiveness . By contrast, during mechanical expiration, an inspiratory reduction in RV output reaches the left ventricle. This is reflected as an expiratory reduction in aortic systolic pressure and is referred to as dDown [or delta DOWN]. Caveats and considerations Firstly, in the initial clinical FHM studies, the patients were assumed to have essentially normal chest wall compliance, all received relatively large tidal volumes and all were completely passive with the ventilator. These parameters, in effect, standardized the ‘dose’ of ITP. As a consequence, PPV performs poorly in patients generating spontaneous inspiratory effort [19-22], when smaller tidal volumes are applied [23, 24], or when there are abnormalities in chest wall compliance [4, 25, 26]. Secondly, the primary mechanism of SVV as a marker of fluid responsiveness is the result of an inspiratory reduction in right ventricular output that is transmitted to the left ventricle during mechanical expiration [12, 18]; this is because small changes in ITP affect large changes in venous return [27, 28]. However, increasing lung volume also afterloads the right heart such that diminished right heart output may not reflect preload reserve, but instead afterload sensitivity. As such, reduced pulmonary compliance and pulmonary arterial hypertension may accentuate afterload sensitivity and diminish SVV and PPV as predictors of fluid responsiveness [29-34]. Additionally, if the left ventricle is afterload sensitive [e.g. systolic heart failure], ITP can unload the heart, facilitate dUP and create PPV [4, 35-37]. The predictive value of SVV and PPV is also impaired in patients with cardiac arrhythmia [SVV due to variable diastole length], and excessive respiratory rate . Finally, means of FHM that vary the venous return function may reduce some of the aforementioned uncertainty. Passive leg raising and the end-expiratory occlusion test are two such methods which outperform those that rely on ITP change as the cardiovascular stress in both in patients breathing spontaneously and with arrhythmia [29, 39].
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