The Trigeminocardiac Reflex, A Comparison with the Diving Reflex in Humans
Authors: Frédéric Lemaître, Tumul Chowdhury, Bernhard Schaller
DOI / Source: https://doi.org/10.5114/aoms.2015.50974
Date: 01 April 2015
Reading level: Intermediate
Why This Matters for Freedivers
A lot of what freedivers call the “diving reflex” is probably part of a bigger family of reflexes that start with trigeminal nerve stimulation in the face and nose. Understanding that helps explain why cold water on the face can instantly drop heart rate, why stress and temperature change the response so much, and why—rarely—an exaggerated reflex could be risky in susceptible people.
Synopsis
This review compares two closely related automatic reflexes in humans:
- The diving reflex (DR): the classic freediver package—breath-hold, slower heart rate (bradycardia), reduced blood flow to the limbs, and a gradual rise in blood pressure.
- The trigeminocardiac reflex (TCR): a medical reflex seen when the trigeminal nerve is mechanically stimulated (for example during facial or neurosurgery). It typically causes bradycardia and can also cause a drop in blood pressure, plus stomach activity changes.
The authors argue that these two reflexes are not “separate tricks,” but likely different expressions of the same underlying trigeminal–brainstem–vagus circuitry. In that view, the human diving reflex can be seen as a peripheral form of the TCR, triggered through receptors in the face and nose (especially with cold water).
What they share
Both reflexes: - depend on a trigeminal-to-brainstem pathway, - can produce strong bradycardia via parasympathetic (vagal) output, - can involve sympathetic effects that tighten blood vessels and redirect blood toward vital organs, - can increase oxygen efficiency by prioritizing the brain and heart.
Where they differ (and why it matters)
A key difference is blood pressure behavior: - In many diving-reflex situations, mean arterial blood pressure tends to rise gradually. - In “classical” (especially central) TCR situations, blood pressure can drop sharply.
The authors suggest this may partly be a timing issue: in surgery, the stimulus might be sudden and then stopped quickly, while in breath-hold immersion the stimulus can persist and evolve.
The brain-protection angle
One of the most interesting points in the paper is that both reflexes may act as “oxygen-conserving reflexes” for the brain, partly by increasing cerebral blood flow. They cite evidence that breath-hold divers can show a much larger increase in brain blood flow during apnea than untrained people, implying this response may be trainable.
The “autonomic tug-of-war”
The review also highlights that these reflexes can involve simultaneous vagal slowing of the heart and sympathetic vessel tightening. That combination can be efficient for pumping blood into a constricted system—but it also helps explain why heart rhythm changes can appear during apnea, cold water exposure, or strong facial stimulation.
Clinical and safety notes
The paper discusses how these reflexes show up beyond freediving: - cold-face stimulation has been used as a maneuver to help terminate certain fast heart rhythms (SVT), - exaggerated reflex patterns have been discussed in relation to sudden infant death syndrome mechanisms, - and rare reports exist of extreme responses in adults under specific circumstances.
Overall, this is a “big-picture wiring diagram” paper: it places the freediver’s diving response inside a broader trigeminal reflex family, and it explains why cold face immersion is such a powerful switch for heart rate and circulation.
Abstract
This review compares the trigeminocardiac reflex (TCR) and the diving reflex (DR) in humans and proposes that they are closely linked neurogenic oxygen-conserving reflexes sharing a trigeminal–brainstem pathway. Both can produce bradycardia and peripheral vasoconstriction, influence blood pressure, and increase cerebral blood flow, although typical blood pressure patterns differ between central TCR presentations and the DR. The authors discuss similarities, differences, proposed functional significance, developmental and phylogenetic perspectives, and clinical implications, suggesting that the diving reflex may represent a peripheral subtype of the trigeminocardiac reflex.