CO₂, critical closing pressure, and CBF before vasovagal syncope

In 2001, Carey and colleagues at the University of Birmingham published a detailed cerebrovascular investigation of vasovagal syncope in the journal Brain. Using transcranial Doppler to monitor cerebral blood flow velocity in real time as subjects underwent provoked tilt-table testing, they documented the hemodynamic pattern in the moments immediately preceding loss of consciousness. Their key finding was not simply that cerebral blood flow fell before syncope. It was that the collapse was asymmetric — diastolic flow fell sharply and early, while systolic flow was relatively preserved until the final pre-syncopal seconds. The mechanism linking these two observations is CO₂, and understanding it changes how recurrent syncope and pre-syncope should be evaluated.

The Study: Measuring Blood Flow to the Brain Through the Faint

The investigators studied patients with documented recurrent vasovagal syncope and healthy controls, monitoring cerebral blood flow velocity continuously via transcranial Doppler of the middle cerebral artery during 70-degree head-up tilt. Heart rate, blood pressure, and end-tidal CO₂ were measured simultaneously. Subjects who developed prodromal symptoms during tilt were kept in the position until syncope occurred or the test was terminated for safety.

The continuous high-resolution recording allowed the researchers to reconstruct the second-by-second hemodynamic sequence rather than comparing pre- and post-tilt values. This temporal resolution is what produced the finding. If the study had only compared baseline to a single standing measurement, the diastolic-specific collapse would have been invisible in the averaged data.

The Finding: Diastolic Flow Collapses First and Earliest

As subjects approached syncope, cerebral blood flow velocity did not fall uniformly. Diastolic flow velocity — the flow occurring between heartbeats, when arterial pressure is at its lowest — began to fall substantially before systolic flow velocity was affected. In the pre-syncopal period, there were intervals in which diastolic flow approached zero while systolic flow was still detectable. The brain was losing flow in the diastolic phase while the heart continued to push blood forward during systole.

End-tidal CO₂ fell during this period. The CO₂ decline tracked with the progressive collapse of diastolic flow, and the timing suggested a causal rather than coincidental relationship. As the tilt continued and symptoms worsened, systolic flow eventually fell as well, at which point consciousness was lost.

In healthy controls undergoing the same tilt protocol, neither the CO₂ decline nor the diastolic flow collapse pattern was observed to the same degree. Controls either maintained cerebral blood flow or were terminated from the protocol before approaching syncope.

Critical Closing Pressure: The Mechanism That Links CO₂ to Diastolic Collapse

The concept that explains this pattern is critical closing pressure — the minimum arterial pressure required to keep a blood vessel open against the elastic recoil of its walls. Below this pressure, the vessel collapses. Flow stops.

Cerebral arterioles are particularly sensitive to CO₂. CO₂ is a direct vasodilator: it causes cerebrovascular smooth muscle to relax, reducing vessel wall tension and lowering critical closing pressure. When CO₂ falls — as it does during hyperventilation — the smooth muscle contracts, vessel wall tension rises, and critical closing pressure increases. More arterial pressure is required to hold the vessels open.

Arterial pressure during a cardiac cycle is not constant. Systolic pressure is the peak, occurring with each heartbeat. Diastolic pressure is the trough, occurring between beats. When critical closing pressure rises due to CO₂-driven vasoconstriction, diastolic pressure — which is lower — is the first value to fall below the new threshold. Diastolic flow collapses. Systolic pressure remains above threshold for longer, so systolic flow continues. The pattern of asymmetric collapse — diastolic failing before systolic — is the direct signature of rising critical closing pressure meeting a low diastolic floor.

This is not a metaphor or a model. It is the mechanical consequence of a specific biophysical relationship. When CO₂ falls and cerebral critical closing pressure rises beyond diastolic arterial pressure, diastolic flow stops. The brain enters intermittent ischemia with each cardiac cycle — receiving flow only during the systolic peak, and receiving none during diastole. Continued CO₂ fall eventually raises critical closing pressure above systolic pressure as well, at which point flow fails entirely and consciousness is lost.

Hyperventilation Is Not a Psychological Response — It Is a Physiological Participant

In clinical settings, hyperventilation in patients who faint or nearly faint is often attributed to anxiety. The patient is nervous about the tilt table. They are anxious about their symptoms. The fast breathing is a psychological reaction, not a physiological event worth measuring.

The Carey 2001 data does not support this interpretation. The CO₂ decline documented in these patients preceded syncope and was temporally associated with the progressive diastolic flow collapse. The hyperventilation is not the consequence of impending faint — it is a participant in the mechanism that produces it. Whether the hyperventilation was initiated by anxiety, by a reflex response to early cerebral hypoperfusion, or by an autonomic trigger, its physiological effect is the same: CO₂ falls, critical closing pressure rises, diastolic flow collapses, cerebral perfusion fails.

This matters because treatment interventions focused only on the hemodynamic side of syncope — volume loading, vasopressors, salt intake — may leave the CO₂ mechanism unaddressed. A patient who is well-volume-loaded but hyperventilates during orthostatic stress will still drive their cerebral critical closing pressure above their diastolic pressure. The volume helps. The CO₂-driven vasoconstriction continues.

Implications for Pre-Syncope, Recurrent Syncope, and POTS

Many patients with POTS or related orthostatic intolerance conditions experience recurrent pre-syncope — the prodromal phase of near-faint — without completing the full syncope event. They may also hyperventilate during upright periods without being aware of it, or breathe in patterns that produce functional CO₂ reduction without obvious rapid breathing. The Carey mechanism applies to these patients in attenuated form: chronic mild CO₂ reduction during upright periods raises cerebral critical closing pressure, increases diastolic flow vulnerability, and reduces the margin between functional cerebral perfusion and intermittent ischemic episodes.

For the subset of patients whose orthostatic symptoms include a pre-syncopal quality — a sense of graying out, sudden cognitive impairment, loss of visual field — without a full faint, the diastolic flow collapse mechanism provides a plausible substrate. The brain is not completely losing flow. It is losing diastolic flow, intermittently, producing a fluctuating perfusion deficit rather than a clean loss of consciousness.

End-tidal CO₂ monitoring during tilt table testing is not a standard element of most clinical protocols. Adding it — along with transcranial Doppler — would identify in real time whether CO₂ is falling during the orthostatic challenge and whether that fall is temporally associated with symptom onset or worsening. This information directly informs whether breathing-focused interventions, such as slow-paced breathing or CO₂ rebreathing techniques, should be considered as part of the management approach.

What This Paper Establishes for Clinical Practice

The Carey 2001 paper is more than two decades old and was published in one of the leading neurology journals. Its mechanism — CO₂-driven elevation of critical closing pressure producing asymmetric diastolic flow collapse before syncope — has been available to clinicians evaluating recurrent syncope for years. The measurement that would confirm it in individual patients, end-tidal CO₂ monitoring during tilt, adds minimal cost and no procedural burden to an evaluation that is already using a tilt table.

What the paper establishes is that the standard syncope evaluation, which captures heart rate, blood pressure, and symptom onset but not CO₂ or cerebral blood flow velocity, is missing two of the three elements of the mechanism it is trying to characterize. The recorded variables are the downstream outcomes. The CO₂ and cerebrovascular components that drive the collapse are upstream, unrecorded, and — for patients who receive normal syncope workups and continue to faint — unaddressed.

If you experience recurrent syncope or pre-syncope during standing, and your evaluation has not included CO₂ monitoring or transcranial Doppler, you have undergone a test that was not designed to see the mechanism described in this paper. That is not a failure of your physiology to produce findings. It is a measurement gap that leaves the most mechanistically specific part of the story unexamined.