Height matters: re-thinking blood pressure targets through physics, physiology and evolution

Current clinical thinking increasingly endorses lowering blood pressure (BP) as much as a patient can safely tolerate to prevent major adverse cardiovascular events. The roots of this paradigm date to the Veterans Affairs I trial (1967)—the first randomized, placebo-controlled study of antihypertensive therapy [1]. Despite enrolling only 143 veterans and employing the limited drug armamentarium of the era, the trial showed striking reductions in events among patients whose diastolic BP ranged from 115 to 129 mmHg [1]. The enormous treatment effect reflected the extreme cardiovascular stress of malignant hypertension, and it paved the way for the now-standard goal of keeping office BP below 140/90 mmHg [2].

Lowering systolic targets to <130 mmHg or even <120 mmHg remains contentious. The residual cardiovascular risk within the 120–140 mmHg range is modest compared with earlier eras, making benefit harder to demonstrate without large cohorts, extended follow-up and rigorous safety monitoring [2]. In ACCORD-BP, intensive control (<120 mmHg) in type 2 diabetics offered no added benefit over <140 mmHg and increased treatment burden [3]. The BPROAD trial in China showed only modest gain: 1 cardiovascular event prevented per 65 treated over 4.2 years, at the cost of 1 case of hyperkalemia per 125 and hypotension per 1250 [4]. By contrast, SPRINT found a significant benefit in non-diabetic adults, with 1 event prevented per 57 treated, but also 1 case of acute kidney injury per 63, hypotension per 71 and electrolyte imbalance per 125—emphasizing the need for careful patient selection and monitoring [5].

A uniform systolic threshold of 120 mmHg therefore widens the net for pharmacologic therapy, yet it is not anchored in an intrinsic physiologic cutoff; it is a population-level statistical pivot. This raises a critical question: do all individuals with BPs of 120–140 mmHg truly need treatment, or could this band be physiologically normal—and even optimal—for some? Answering requires stepping outside classical epidemiology and revisiting the evolutionary and biophysical context of human circulation.

EVOLUTIONARY AND PHYSICAL IMPERATIVES

The shift to bipedalism forced the human cardiovascular system to maintain cerebral perfusion against gravity [6]. With the brain elevated above the heart, arterial pressure had to rise—especially in taller bodies—to overcome the hydrostatic column. While this vertical challenge is not exclusive to bipeds—certain quadrupeds like giraffes and camelids also exhibit significant heart-to-brain height differentials—the transition to upright posture in humans necessitated a fundamental cardiovascular adaptation [7]. Quadrupeds, whose heart and brain lie nearly on the same plane, face no such pressure-induced end-organ injury. Thus, baseline systolic pressure in humans rose not as a marker of disease but as a prerequisite for upright life [6]. Body mass certainly loads the heart, yet hydrostatic pressure, which scales almost linearly with height, sets the irreducible floor for cerebral perfusion.

Because we lack an early, patient-specific biomarker of pressure-induced end-organ injury, contemporary trial design still relies on fixed cuff measurements. Adopting indexing strategies grounded in evolutionary biology and first-principle physics could refine both study interpretation and bedside care—an imperative now that the therapeutic window has narrowed to just a few millimetres of mercury.

HEIGHT-BASED SCALING: A SIMPLE CALCULATION

Ignoring hydrostatics introduces a systematic bias in how we interpret BP. Standardized BP measurements are performed with the patient seated and the cuff placed at heart level, minimizing any hydrostatic difference between the brachial artery and the heart. However, this setup does not account for the vertical distance from the heart to the brain, which must be overcome to ensure adequate cerebral perfusion. Anatomical data indicate that the heart lies approximately 27.5% of total stature below the vertex of the skull, making the vertical distance from heart to brain approximately:

Using the hydrostatic pressure equation:

where blood density (ρ) ≈ 1060 kg/m³ and gravitational acceleration (g) ≈ 9.81 m/s², one can estimate the pressure required to perfuse the brain above the heart. This pressure is not reflected in cuff measurements yet is essential to maintaining cerebral blood flow, particularly in taller individuals. The physiological importance of vertical pressure gradients is also evident in routine clinical practice: in the upright position, systolic pressure measured at the ankle is often 80–115 mmHg higher than at the brachial artery, purely due to gravitational hydrostatic load. This familiar discrepancy illustrates how vertical distance alone—not vascular pathology—can drive significant pressure variation across the human column. Failing to account for this vertical gradient may lead to underperfusion risks during aggressive BP lowering, especially when relying on a uniform treatment threshold for all body sizes.

After converting from pascals to mmHg (1 mmHg ≈ 133.322 Pa), a 1.90 m adult must overcome a hydrostatic gradient of approximately 41 mmHg, while a 1.60 m adult faces a gradient of about 34 mmHg—a 7 mmHg difference that remains invisible to standard brachial measurements. This gap represents nearly one-third of the systolic BP differential tested in landmark clinical trials. The physiological logic is not unique to humans: giraffes, for example, maintain systemic pressures approaching 250 mmHg simply to perfuse their brains which is located >2 m above the heart [8].

Assuming a reference systolic BP of 120 mmHg for a 1.70 m adult (corresponding to a heart-to-brain hydrostatic gradient of ∼36 mmHg), we can estimate the systolic pressures needed to maintain equivalent cerebral perfusion across different statures (Table 1). These provisional targets are not clinical recommendations but illustrative estimates based on hydrostatic principles, intended to highlight physiological variation by height and to inform future research—not replace validated guidelines.

Table 1:

Height-indexed hydrostatic pressure gradient and provisional systolic BP targets.

Height (m)	h = 0.275 × height (m)	ΔP (Pa) = ρgh	ΔP (mmHg)	Provisional SBP target (mmHg)
1.50	0.4125	4290.68	≈32.18	116
1.60	0.4400	4582.56	≈34.37	118
1.70	0.4675	4874.45	≈36.56	120
1.80	0.4950	5166.34	≈38.74	122
1.90	0.5225	5458.22	≈40.93	124
2.00	0.5500	5750.11	≈43.12	126

Estimated hydrostatic pressure differences (ΔP) based on the vertical heart-to-vertex distance (h = 0.275 × height), assuming blood density (ρ) = 1050 kg/m³ and gravitational acceleration (g) = 9.81 m/s². ΔP is expressed in Pascals and converted to mmHg. Provisional SBP targets adjust for height-dependent central pressure load.

SBP, systolic blood pressure.

In addition to hydrostatic differences, vascular biomechanics further widen the disparity between short and tall individuals. Shorter arterial trees lead to earlier return of reflected pressure waves to the aortic root, amplifying central systolic pressures in shorter adults [9]. Of course, hydrostatic load is not the sole determinant of BP physiology. Arterial stiffness, autonomic nervous system regulation, vascular compliance and microvascular remodeling all influence the effective transmission of pressure to end organs. While height-dependent hydrostatics may define the minimum systolic requirement for cerebral perfusion, these additional, often nonlinear contributors must be factored into any comprehensive understanding of risk. Together, these physiological effects—hydrostatic gradient and arterial wave reflections—can easily account for 5–10 mmHg of variation in systolic load [10]. That variation, while smaller than the ≈11–15 mmHg differences achieved in intensive treatment arms of landmark trials like ACCORD-BP, BPROAD and SPRINT, still approaches the scale of clinical intervention effects and underscores the need for height-normalized BP targets to minimize measurement bias and optimize risk stratification.

Whether indexing BP targets to height will ultimately improve clinical outcomes remains to be tested, and if so, the optimal anthropometric parameter—whether total height, sitting height or another measure—also remains uncertain. Nonetheless, body height serves as a pragmatic and universally recorded surrogate that approximates the hydrostatic column from heart to brain, allowing for a physiologically anchored hypothesis to be explored using existing datasets.

CLINICAL AND RESEARCH IMPLICATIONS

Seasoned clinicians already sense this heterogeneity: a cuff BP of 90/60 mmHg may suit a petite young woman yet precipitate syncope in a tall older man. Conversely, many tall, otherwise healthy adults tolerate systolics in the 130s without evidence of organ damage. While the proposed systolic targets are hypothetical and not intended as prescriptive thresholds, they serve to illustrate the magnitude of physiological variation imposed by height.

Until a reliable molecular marker of pressure-induced endothelial injury emerges, incorporating physical height into both trial analyses and guideline thresholds offers a pragmatic step toward precision. Re-examining datasets from ACCORD-BP, SPRINT, BPROAD and future trials through a height-normalized lens could uncover treatment-effect modifiers now buried by physiologic noise—at a fraction of the cost of launching new megatrials. While there is no proven explanation for the discrepancy in outcomes between the ACCORD-BP and BPROAD trials, the striking similarities in study design and achieved BP levels make height differences in the populations a compelling lens through which to re-evaluate the results. Indexing targets to height (and thus to hydrostatic load) harmonizes our therapeutic ambitions with the evolutionary and physical realities that sculpted human cardiovascular architecture and may resolve the conflicting evidence that currently perplexes clinicians.

CONFLICT OF INTEREST STATEMENT

T.S. has received speaker honorariums from AstraZeneca, Amgen, Sanofi, Nobel Ilac, Baxter, Boehringer Ingelheim, Abdi Ibrahim-Otsuka, Alexion and Astellas, none of which is associated with this work.