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Acute decompensated heart failure (ADHF) reflects the inability of the heart to maintain end-organ perfusion without the cost of augmenting intracardiac filling pressures. Clinical heart failure phenotypes are commonly classified on the basis of the level of left ventricular ejection fraction (either preserved or reduced) and the presence or absence of hemodynamic compensation. In recent decades, there has been growing recognition that disruptions in hemodynamic balance are further intensified by excessive activation of neurohumoral, inflammatory, and/or fibrotic pathways. Indeed, the inability of the heart to adequately maintain kidney perfusion had been explained over a century ago by James McKenzie in 1908.1 This concept of “forward failure” was popularized as a cardio-centric explanation of inadequate cardiac output (CO) causing a reduction in renal plasma flow (often defined as cardiac index <2.2 L/min per m2 based on the classic Forrester hemodynamic classification).2 This forward failure can lead to clinical signs of hypoperfusion, including cool extremities, decreased urine output, elevated serum lactate levels, or altered mental status. However, the kidneys possess a remarkable ability to autoregulate renal blood flow (RBF), maintaining intraglomerular pressure and GFR within a mean arterial pressure range of 70–150 mm Hg, even when CO is significantly reduced. This autoregulation occurs through adjustments in afferent arteriolar resistance, mediated by tubuloglomerular feedback and the myogenic response. However, if mean arterial pressure falls below this range during the course of ADHF or its treatment, autoregulation fails, leading to a sharp decline in RBF and GFR, which may be especially severe in the presence of reduced CO due to already compromised renal perfusion pressure.3 Thus, prolonged or recurrent episodes of hypotension may irreversibly damage nephrons and accelerate GFR decline beyond the baseline loss associated with heart failure. In addition, the disproportionate decrease in RBF relative to GFR results in an elevated filtration fraction, increasing peritubular oncotic pressure and enhancing sodium, chloride, and water reabsorption in the proximal tubule, which reduces tubular flow to the macula densa.4 The associated reduction in both renal perfusion pressure and chloride delivery to the macula densa also stimulates renin release from the afferent arteriole, a key activator of neurohormonal counter regulation observed in heart failure. As far back as in 1832, Dr. James Hope described how increased venous pressure contributes to peripheral and pulmonary edema.5 This “backward failure” is defined by elevated cardiac filling pressures, either on the left side, estimated by pulmonary capillary wedge pressure (PCWP), or on the right side, estimated by central venous pressure (CVP). A PCWP of ≥18 mm Hg at rest and in a supine position is the current diagnostic threshold for left-sided heart failure.6 Although PCWP serves as a surrogate for left ventricular end diastolic pressure (true cardiac preload), it more effectively predicts poor outcomes by offering greater insight into opposing downstream pulmonary artery pressures. The interplay between the left and right heart chambers significantly influences both PCWP and CVP, as their dynamics are interconnected within the shared pericardial space. The PCWP/CVP relationship reflects right ventricular function and can also indicate the extent of pericardial constraint. For instance, in right ventricular failure, CVP becomes disproportionately elevated, directly affecting left ventricular filling pressures through a leftward shift of the interventricular septum and increased pericardial restraint, a phenomenon known as ventricular interdependence. The elevated CVP can also significantly impair renal perfusion and is one of the most impactful factors affecting renal function. This occurs through increased renal venous pressure and/or congestion-related interstitial pressure. Higher renal venous pressure raises vascular transmural pressure, triggering myogenic vasoconstriction of the afferent arteriole, leading to decreased RBF. Concurrently, renal congestion elevates interstitial pressure, which, in the setting of high CVP, may reduce net glomerular filtration pressure and impair tubular and lymphatic flow, which also may contribute to renal dysfunction and increased sodium reabsorption. It is in this context that Tuttle et al.7 investigated the effect of baseline hemodynamic measurements aortic pulsatility index (APi) and pulmonary artery pulsatility index (PAPi) on renal outcomes in patients with ADHF. Specifically, APi and PAPi incorporate systemic or pulmonary artery pulse pressure (PAPP), respectively (the numerator of the equation), which represents the relationship between cardiac ejection (e.g., stroke volume) and downstream arterial compliance (Figure 1). In other words, both APi and PAPi reflect the interplay of left and right ventricular contractile function (forward flow) with markers of congestion (backward flow) for the respective ventricles. A reduction in left or right ventricular contractile function, resulting in decreased stroke volume, will inherently increase right atrial pressure and PCWP (the denominator of the equation), impeding venous return and elevating renal venous impedance.8 Although Winton in 1931 was one of the first to describe the effect of venous congestion on renal hemodynamics,9 Tuttle et al.7 now demonstrated that low PAPi (but not APi) was associated with lower baseline and more detrimental changes in GFR, as well as higher risk of dialysis. To explain these findings, it is important to point out that PAPP is influenced by right ventricular stroke volume and pulmonary arterial capacitance, the latter of which decreases as pulmonary vascular resistance increases (hyperbolic relationship). Therefore, both pulmonary and systemic pulse pressures are primarily determined by vascular capacitance, which depends on the elasticity of the vascular walls.8 Hence, PAPi might explain the components of backward failure more precisely as it incorporates the dynamic flow patterns of the right heart and its vasculature.Figure 1: Hemodynamics balance to maintain kidney perfusion. ADHF, acute decompensated heart failure; APi, aortic pulsatility index; CVP, central venous pressure; DBP, diastolic BP; HF, heart failure; MAP, mean arterial pressure; N, functional nephrons; PAC, pulmonary artery capacitance; PADP, pulmonary artery diastolic pressure; PAPi, pulmonary artery pulsatility index; PAPP, pulmonary artery pulse pressure; PASP, pulmonary artery systolic pressure; Pb, hydrostatic pressure Bowman Space; π, colloid oncotic pressure; PCWP, pulmonary capillary wedge pressure; Pgc, hydrostatic pressure glomerular capillary; PP, pulse pressure; PVR, pulmonary vascular resistance; RAAS, renin-angiotensin-aldosterone system; RAP, right atrial pressure; RVSV, right ventricular stroke volume; SBP, systolic BP; SV, stroke volume; VAD, ventricular assist device; VC, vascular capacitance.How do the findings of Tuttle et al.7 enhance our physiologic understanding or inform clinical practice in managing ADHF and kidney dysfunction? First, although previous studies have highlighted the potential of hemodynamically guided therapy to optimize PAPi and reduce the risk of end-organ damage,10 its incremental value over traditional CVP measurements remains uncertain. Of note, CVP, as the numerator in the equation, demonstrates a strong negative correlation with PAPi (correlation coefficient approximately −0.75). Thus, it is unsurprising that the risks of AKI, dialysis requirement, and adverse clinical outcomes were diminished when adjusted for CVP (as shown in Model 3 in all the Supplemental Tables).7 Practically, there may also be questions about how best to optimize PAPi when treatments typically target individual components, such as improving stroke volume or pulmonary vasodilation to enhance PAPP or using diuresis to lower CVP. On the other hand, although previous studies suggest that forward failure may not be the primary driver of renal dysfunction in ADHF,11 a declining APi may serve as an early warning of impaired kidney and liver perfusion beyond their abilities to autoregulate, which may warrant timely escalation to temporary mechanical circulatory support devices. Hence, these metrics must be interpreted within the broader clinical context. In fact, PAPi is a metric for right ventricular function, as it integrates right atrial pressure, PCWP, PAPP, and pulmonary arterial capacitance and becomes an oversimplification that inherently neglects heterogenous patient populations and/or dynamic, longitudinal hemodynamic changes. Indeed, PAPi can be similar between patients despite significant differences in the individual variables of the equation. Perhaps the biggest clinical value of these novel hemodynamic assessments in ADHF lies more in their ability to proactively signal the risk of end-organ failure than in serving as direct therapeutic targets. Clinicians should understand that these measurements are not standalone indicators, but complementary tools that reflect highly complex hemodynamic adaptations aimed at preserving kidney perfusion, primarily in response to backward failure.
Vanhentenrijk et al. (Thu,) studied this question.