Beyond the numbers: hypotension, algorithms, and the elusive promise of renal protection

Postoperative acute kidney injury (AKI) after major abdominal surgery continues to represent a major determinant of short- and long-term outcomes. Even mild postoperative creatinine elevations are independently associated with increased mortality and chronic kidney disease progression (1). Intraoperative hypotension has repeatedly been identified as a potentially modifiable exposure associated with this complication. Large observational datasets have demonstrated a graded relationship between the duration and depth of mean arterial pressure (MAP) below commonly used thresholds—most frequently 65 mmHg—and postoperative AKI. This consistent association has shaped contemporary perioperative hemodynamic thinking (2).

The introduction of machine-learning–based predictive algorithms such as the Hypotension Prediction Index (HPI) appeared to offer a logical progression from this epidemiologic signal (3). If hypotension contributes causally to renal injury, and if impending hypotension can be identified before it manifests, then anticipatory intervention should mitigate renal insult (4). Early validation studies confirmed that HPI can predict arterial pressure decline several minutes in advance. Subsequent interventional trials and meta-analyses reported reductions in hypotension metrics, particularly time-weighted average MAP below predefined thresholds (5).

The multicenter randomized clinical trial by Ripollés-Melchor and colleagues provides a rigorous test of whether this physiological plausibility translates into renal benefit. In 917 patients undergoing moderate- to high-risk elective abdominal surgery, HPI-guided goal-directed therapy did not reduce moderate-to-severe AKI within 7 days compared with standard care. Secondary outcomes—including overall postoperative complications, renal replacement therapy, length of stay, and 30-day mortality—were likewise unaffected (6). The implications of this neutral result extend beyond the performance of a specific algorithm. They compel a reassessment of the causal architecture linking intraoperative hypotension to postoperative AKI, reminding us that simple algorithms alone cannot improve patient outcomes. Such tools may create an illusion of control if they are not accompanied by careful interpretation of the collected data and individualized, physiology-guided management (4).

The association between hypotension and AKI is robust. Yet association does not equate to mediation. Hypotension may function as a marker of systemic vulnerability rather than a dominant causal driver. Patients who experience intraoperative hypotension often exhibit diminished cardiovascular reserve, heightened inflammatory response, or greater surgical complexity—each independently contributing to renal susceptibility. The strength of observational correlations may reflect shared pathophysiology rather than direct hemodynamic causation (7).

Renal physiology under anesthesia further complicates the pressure-centric paradigm. Renal perfusion is not determined solely by MAP. Effective renal perfusion pressure is the difference between arterial inflow and venous outflow pressures (8). In abdominal surgery, pneumoperitoneum and fluid shifts may elevate renal venous pressure, reducing filtration pressure despite apparently adequate MAP. Autoregulatory thresholds differ between individuals and are shifted upward in chronic hypertension (9). Moreover, the renal medulla operates at the margin of hypoxia under physiological conditions; modest perturbations in microvascular flow distribution, oxygen extraction, or inflammatory endothelial dysfunction may precipitate tubular injury independently of global arterial pressure (10).

The concept that preventing MAP <65 mmHg uniformly protects renal tissue presumes that global pressure is the principal determinant of medullary oxygenation. Experimental evidence from Brezis et al. demonstrated that acute controlled hypotension paradoxically increased medullary oxygen tension despite reductions in cortical perfusion, likely due to decreased glomerular filtration rate and reduced tubular transport workload in the outer medulla (11). These findings provided part of the physiological rationale underlying the traditional 65 mmHg threshold and suggest that renal medullary oxygenation may not correlate linearly with systemic arterial pressure. Experimental and clinical evidence suggests otherwise (Figure 1). Microcirculatory heterogeneity, capillary shunting, endothelial glycocalyx disruption, and leukocyte–endothelial interactions contribute substantially to perioperative renal vulnerability. Restoration of systemic arterial pressure does not guarantee restoration of microvascular oxygen delivery (12,13).

Figure 1 Conceptual model of the causal architecture linking intraoperative hypotension to postoperative AKI. AKI, acute kidney injury; HPI, Hypotension Prediction Index; MAP, mean arterial pressure.

Moreover, recent guidelines from the European Society of Intensive Care Medicine (ESICM) emphasize the importance of individualizing MAP targets according to patient comorbidities and baseline physiology (14). Baseline kidney function, renal functional reserve, and concomitant pharmacological interventions may further modify the relationship between arterial pressure and renal oxygenation. Observational data suggest that renin-angiotensin system blockade and loop diuretics interact with hypotension and renal functional reserve, potentially influencing both AKI susceptibility and recovery in critically ill patients (15). These factors may partly obscure any isolated protective effect of postoperative blood pressure control strategies. Strategies such as performing a MAP challenge or “MAP test” to identify the optimal perfusion threshold have also been proposed (e.g., in the context of studies such as ANDROMEDA-SHOCK 2). These concepts of individualized perfusion targets were not incorporated into the algorithm evaluated in the present study (14,16).

The therapeutic pathway activated by HPI alerts also deserves scrutiny. In the trial, the HPI-guided group received more frequent vasopressor therapy, including higher cumulative exposure to ephedrine and norepinephrine (6). Vasopressors raise MAP primarily through increased systemic vascular resistance. However, renal cortical and medullary circulations respond differently to catecholaminergic stimulation. Norepinephrine may improve renal perfusion in vasodilatory shock by restoring perfusion pressure relative to vasoplegia; its physiological role in normovolemic anesthetized patients differs (17). Ephedrine’s mixed α- and β-adrenergic effects may augment cardiac output transiently but also increase myocardial oxygen demand and alter regional blood flow distribution (18).

If earlier and more frequent vasopressor administration reduces hypotensive exposure yet simultaneously modifies intrarenal hemodynamics, the net renal effect becomes uncertain. Retrospective analyses in surgical cohorts have linked higher vasopressor doses to increased AKI incidence, although residual confounding cannot be excluded. Nonetheless, the physiological trade-off is real: increasing arterial pressure pharmacologically is not equivalent to restoring balanced renal perfusion (19,20).

Another interpretative challenge arises from the absence of detailed intraoperative hypotension exposure data in the published report. Without comprehensive reporting of time-weighted average MAP, area under threshold, alert-to-intervention latency, and adherence to the therapeutic algorithm, the degree of exposure separation between groups cannot be fully appreciated. In interventional hemodynamic trials, outcome divergence is contingent upon meaningful physiological separation. If standard care already maintains relatively tight blood pressure control in experienced centers, incremental benefit from predictive alerts may be intrinsically limited (6).

Comparison with prior randomized trials is instructive. The INPRESS study demonstrated reduced postoperative organ dysfunction when MAP targets were individualized to preoperative baseline values in high-risk patients, suggesting that arterial pressure may be relevant when aligned with patient-specific autoregulatory physiology (21). Conversely, multiple goal-directed therapy trials focusing on stroke volume or cardiac output optimization have produced heterogeneous effects on renal outcomes (22). Kidney Disease Improving Global Outcomes definition (KDIGO)-based preventive bundles underscore that renal protection in high-risk surgical patients requires integrated strategies encompassing hemodynamic optimization, avoidance of nephrotoxins, glycemic control, and volume stewardship. These data collectively suggest that isolated manipulation of a single hemodynamic variable—whether predicted or reactive—may be insufficient in a multifactorial injury landscape (23).

The neutral findings of the present trial therefore do not negate the importance of blood pressure management. Rather, they question the sufficiency of threshold-based paradigms and the assumption that reducing predicted hypotension, in isolation, modifies downstream renal biology. Machine-learning-derived prediction is an analytical achievement. Clinical impact depends on the effectiveness, safety, and physiological coherence of the response it triggers (24).

The trial by Ripollés-Melchor and colleagues represents an important inflection point in perioperative hemodynamic medicine. It reminds us that translation from epidemiologic association to therapeutic efficacy demands rigorous causal testing. Predicting a decline in arterial pressure is valuable; demonstrating that such prediction alters renal outcomes is the true benchmark (6).

In contemporary abdominal surgery performed within optimized perioperative pathways, algorithm-driven anticipation of hypotension does not appear sufficient to reduce postoperative AKI. Protecting the kidney requires a broader physiological strategy—one that accounts for perfusion pressure, venous congestion, microcirculatory integrity, inflammatory stress, individualized autoregulatory thresholds, and preservation of renal functional reserve (4).

Future research should move beyond pressure-centric frameworks toward integrated hemodynamic phenotyping. Identification of patients with impaired autoregulation, elevated venous congestion, or early tubular stress may allow more targeted interventions. Incorporation of renal biomarkers such as TIMP-2·IGFBP7 or NGAL could refine risk stratification and clarify whether hypotension reduction mediates biological protection. Detailed mediation analyses linking exposure reduction to biomarker dynamics and clinical endpoints would strengthen causal inference (25).

Prediction is an enabling technology. Protection remains a biological challenge.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Translational Medicine. The article has undergone external peer review.

Peer Review File: Available at https://atm.amegroups.com/article/view/10.21037/atm-2026-0056/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-2026-0056/coif). O.H. reports consulting fees from Bayer and honoraria for lectures from BD APM, AOP Healthcare, and Viatris. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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