CBF changes come first, tachycardia comes after
A 2014 paper by Del Pozzi and colleagues, published in Hypertension, reorders the causal sequence of events during orthostatic stress in POTS. The study was built around a specific patient subset: POTS patients whose primary complaint was upright shortness of breath rather than simple lightheadedness or tachycardia alone. By instrumenting these patients with transcranial Doppler, end-tidal CO₂ monitoring, respiratory inductance plethysmography, and continuous heart rate and blood pressure recording during head-up tilt, the researchers captured the moment-by-moment sequence of hemodynamic and respiratory changes as they unfolded. The sequence they documented is not the one standard POTS testing is designed to see.
The Study Design and Why This Subset Mattered
The investigators recruited POTS patients with documented orthostatic dyspnea — breathlessness that occurs when upright and resolves when supine. This symptom is frequently attributed to anxiety, deconditioning, or functional causes when it appears in the context of chronic illness. Selecting patients with this specific complaint allowed the researchers to examine whether the respiratory symptom was causing the hemodynamic changes, or whether the hemodynamic changes were causing the respiratory symptom.
Patients were tilted to 70 degrees head-up. All physiological parameters were recorded continuously at high temporal resolution to preserve the sequence of events. Transcranial Doppler measured middle cerebral artery blood flow velocity — the closest available real-time proxy for cerebral blood flow in a clinical research setting. End-tidal CO₂ served as a continuous measure of the partial pressure of carbon dioxide in expired air, which tracks closely with arterial CO₂ and therefore with the CO₂-dependent tone of cerebrovascular smooth muscle.
The Sequence: Cerebral Hypoperfusion Comes First
When the patients were tilted upright, cerebral blood flow velocity dropped first. Not tachycardia. Not hyperventilation. The cerebral perfusion deficit was the initiating event that preceded everything else in the cascade.
Following the CBF velocity drop, the patients began to hyperventilate. The increase in respiratory rate and tidal volume drove end-tidal CO₂ downward. As CO₂ fell, cerebrovascular smooth muscle constricted further — CO₂ is a direct vasodilator of cerebral vessels, and its removal causes vasoconstriction that is not mediated by systemic blood pressure. This additional vasoconstriction compounded the initial perfusion deficit, sustaining and deepening the cerebral hypoperfusion.
Only after this sequence — CBF drop, hyperventilation, CO₂ fall, further cerebral vasoconstriction — did sympathetic activation follow, and the tachycardia appear. The heart rate elevation that defines POTS on a tilt table is arriving late in the cascade. It is the nervous system's escalating attempt to drive more blood upward to the brain. It is not the first event. It is closer to the last one.
What This Means for the Standard POTS Diagnostic Test
Standard tilt table testing monitors heart rate and blood pressure. These are the primary outputs it is designed to capture, and the diagnostic criteria for POTS are based on them. A 30 bpm heart rate rise is diagnostic. Blood pressure change is tracked for orthostatic hypotension. The study ends, the cardiologist interprets the trace, the patient receives or does not receive a POTS diagnosis.
What the Del Pozzi sequence demonstrates is that the event that initiates the entire cascade — the drop in cerebral blood flow velocity — is invisible to this protocol. The CBF drop occurs before tachycardia and before blood pressure change. It is only visible if transcranial Doppler is running simultaneously with the tilt. Standard tilt table testing does not include transcranial Doppler.
This means the test that diagnoses POTS is measuring the fourth or fifth event in a causal chain. The first event is already over by the time anything diagnostic is recorded. A clinician watching a POTS tilt table trace is watching the downstream compensation and identifying it as the condition, when the upstream failure has already come and gone unobserved.
The CO₂ Amplification Loop
The CO₂ component of this cascade deserves specific attention because it creates a self-sustaining loop that is not obvious from the standard hemodynamic picture.
When cerebral blood flow drops and the body attempts to compensate by hyperventilating — whether consciously or reflexively — CO₂ is driven lower. The brain's arterioles respond to CO₂ directly: lower CO₂ equals more vasoconstriction, higher CO₂ equals vasodilation. This response is rapid and substantial. Cerebrovascular reactivity to CO₂ in healthy subjects produces roughly a 3% change in cerebral blood flow per 1 mmHg change in arterial CO₂. In the POTS patients studied by Del Pozzi, hyperventilation-driven CO₂ reduction was adding a second layer of cerebral vasoconstriction on top of whatever hemodynamic factors were already reducing perfusion.
The result is a loop: CBF drops, hyperventilation begins, CO₂ falls, cerebral vessels constrict further, CBF drops more. The hyperventilation and its CO₂ consequences are not the initiating event — but once the initial CBF drop triggers them, they amplify and sustain the cerebral hypoperfusion beyond what the hemodynamic failure alone would produce. Patients in whom this loop is active may experience worse and more prolonged symptoms than patients who do not mount a hyperventilatory response, because the CO₂ component adds vasoconstriction that outlasts the immediate hemodynamic stress.
This also explains why patients can feel profoundly symptomatic — confused, cognitively impaired, exhausted — during orthostatic challenges that produce only modest heart rate changes. The heart rate reflects how hard the sympathetic system is compensating. The cerebrovascular response to CO₂ is occurring upstream and is not captured in that number.
Clinical and Diagnostic Implications
For patients who have undergone tilt table testing and received normal or borderline results, this paper provides a mechanistic explanation for why that testing may have missed a real physiological problem. The test is designed to detect the compensatory response (tachycardia, hypotension). It is not designed to detect the initiating event (cerebral hypoperfusion) or the amplifying mechanism (CO₂-driven vasoconstriction). A patient with orthostatic cerebral hypoperfusion that resolves before the tachycardia threshold is reached, or that produces a tachycardia of only 25 bpm, may be told their tilt table was normal. Their cerebral blood flow velocity may have dropped 20% before the trace showed anything diagnostic.
Transcranial Doppler during tilt table testing captures what standard testing cannot. The combination allows investigators to observe whether CBF drops precede the tachycardia, how large that drop is, whether CO₂ is falling in parallel, and whether the magnitude of the CO₂ change correlates with symptom intensity. Without it, the diagnostic picture is missing its most mechanistically informative data.
The finding from Del Pozzi 2014 has a direct implication for how POTS and orthostatic intolerance should be evaluated: the test that ends at heart rate and blood pressure is not seeing the beginning of the story. It is watching the ending, and drawing conclusions about cause from the effect.
What You Now Know That Changes the Clinical Picture
If you have POTS, or suspect you do, and your primary symptoms when upright are cognitive — brain fog, difficulty with word retrieval, slowed processing, fatigue that is specifically worse when standing — this paper provides the mechanism. Those symptoms are downstream of inadequate cerebral blood flow when upright. The brain fog is not a psychological consequence of feeling unwell. It is a physiological consequence of the brain not receiving adequate perfusion during the minutes or hours you spend standing.
The tachycardia your clinician is tracking is the body's attempt to fix that problem by increasing cardiac output. It is a compensation. The question your evaluation should be asking is what is causing the cerebral perfusion deficit that triggered the compensation — and that question is only answerable if cerebral blood flow is being measured at all.