While stationed at a remote mining site in northwest Western Australia, The Resilience Lab’s Co-Director, Joe Knight, conducted a fascinating personal experiment: he performed an electrocardiogram (ECG) on himself while stimulating the vagus nerve via the Dive Reflex, through an extended exhalation breath-hold.
Though unconventional, the aim was to gain insights into the influence of the Dive Reflex on cardiac function and to objectively capture its effects using electrocardiography.
Understanding the Dive Reflex
The Dive Reflex is a potent activator of the vagus nerve, triggered by three primary stimuli:
Exposure to cold water on the face.
Hydrostatic pressure.
Elevated carbon dioxide (CO₂) levels in the blood.
The intensity of this reflex varies depending on individual factors such as previous exposure to these stimuli, nervous system health, age, sex, and hormonal influences (Ackermann et al., 2023).
The Dive Reflex manifests through four primary adaptations:
Bradycardia (slowing of the heart rate)
Selective vasoconstriction
Blood shift
Splenic emptying (Pendergast et al., 2015)
Cardiovascular System Fundamentals
To understand the Dive Reflex's impact on cardiac function, it is essential to revisit the basics of cardiovascular physiology. The cardiovascular system serves three primary purposes:
Oxygenation and decarbonation of bodily tissues.
Removal of metabolic waste products.
Transportation of nutrients, water, and gases from the environment to tissues to maintain homeostasis (Silverthorn et al., 2019).
The Heart
The heart comprises three tissue layers:
Epicardium: The outer epithelial layer.
Myocardium: The contractile middle layer made of cardiomyocytes and connective tissue.
Endocardium: The inner endothelial layer (Silverthorn et al., 2019).
Mammalian hearts, with their four chambers—the left atrium, right atrium, left ventricle, and right ventricle—are optimised for high metabolic demands and oxygen requirements (Silverthorn et al., 2019).
Blood Vessels
The circulatory system consists of three primary blood vessel types:
Arteries: With three muscular layers (tunica interna, media, and externa), enabling constriction and dilation.
Veins: Equipped with valves to prevent backflow.
Capillaries: Microscopic vessels, one cell thick, facilitating diffusion (Silverthorn et al., 2019).
Cardiac Circulation
Venous blood enters the right atrium via the superior and inferior vena cava. It passes through the tricuspid valve into the right ventricle, then flows to the pulmonary artery through the pulmonary valve. In the lungs, CO₂ is offloaded, and O₂ is absorbed via alveolar capillaries.
Oxygenated blood returns to the left atrium via the pulmonary vein, proceeds to the left ventricle through the mitral valve, and is pumped through the aortic valve into the aorta for systemic distribution (Silverthorn et al., 2019).
Cardiac Electrical Conduction
The heart’s electrical activity originates in the sinoatrial (SA) node, the natural pacemaker, which sends action potentials (APs) to the atrioventricular (AV) node. These APs propagate via the Bundle of His, travel through the left and right bundles in the interventricular septum, and spread through Purkinje fibers, inducing ventricular contraction. Cardiac cells, characterized by self-excitatory properties and efficient mitochondria, contract continuously without external innervation (Silverthorn et al., 2019).
Electrocardiography (ECG)
An ECG graphically represents the heart's electrical activity. Basic ECG interpretation includes:
P wave: Atrial electrical activity.
QRS complex: Ventricular electrical impulses.
ST segment: Ventricular contraction without electrical flow.
T wave: Ventricular repolarization (Sklavos, 2024).
Experiment Results
Two ECGs were recorded:
Baseline ECG (control).
During extended exhalation breath-hold.
Observations
Heart Rate: Decreased from 47 bpm to 37 bpm during breath-hold due to elevated CO₂ levels.
Blood Pressure: Increased from 118/79 mmHg to 134/72 mmHg.
Heart Rate Variability (HRV): Increased, interpreted as a first-degree heart block by the ECG machine (none clinically present).
PR Interval: Lengthened from 186 ms to 212 ms.
QRS Duration: Increased from 97 ms to 102 ms.
QT Interval: Extended from 415 ms to 429 ms.
Physiological Implications
The Dive Reflex slows the heart rate while increasing peripheral vascular resistance through selective vasoconstriction and blood shift. This raises blood pressure, allowing the heart to pump a greater volume of blood per contraction, compensating for the reduced heart rate.
The Frank-Starling Law provides insight: increased right ventricular filling pressure enhances cardiac output by stretching the cardiac muscle, thus increasing the left ventricular ejection volume (Turner & Johnson, 2024).
Conclusion
This brief experiment highlights the profound influence of the Dive Reflex on cardiac physiology.
While this overview avoids delving into the intricate mechanisms, it demonstrates the Dive Reflex’s ability to regulate heart function, which - at it’s core - is an essential tool for remaining cool, calm and collected under stress (resilient, in other words!).
References
Ackermann, S. P., Raab, M., Backschat, S., Smith, D. J. C., Javelle, F., & Laborde, S. (2023). The diving response and cardiac vagal activity: A systematic review and meta-analysis. Psychophysiology, 60(3), e14183. https://doi.org/10.1111/psyp.14183
Pendergast, D. R., Moon, R. E., Krasney, J. J., Held, H. E., & Zamparo, P. (2015). Human Physiology in an Aquatic Environment. Comprehensive Physiology, 5(4), 1705-1750. https://doi.org/10.1002/cphy.c140018
Silverthorn, D. U., Johnson, B. R., Ober, W. C., Ober, C. E., Impagliazzo, A., & Silverthorn, A. C. (2019). Human physiology: An integrated approach (8th ed.). Pearson Education.
Sklavos, D. T. (2024). ECG Interpretation. Life in the Fast Lane. https://litfl.com/ecg-interpretation-video-lectures/
Turner, A., & Johnson, J. (2024). Foundational Cardiology. Australasian College of Paramedicine. https://paramedics.org/elearning/foundational-cardiology
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