1. Short-term cardiovascular effects of partial and total immersion of eighteen human subjects in the horizontal plane have been examined. Brachial arterial pressure, heart rate, forearm blood flow and respiratory movements were monitored simultaneously throughout the experiments. Forearm vascular resistance was calculated from the mean blood pressure and mean flow.2. Total immersion, including the face, with breath-holding resulted in a 61 ± 43% increase in forearm vascular resistance with an associated 29 ± 15% reduction in forearm blood flow. The concurrent bradycardia was significantly different from the heart rate changes during breath-holding with the torso only immersed, or during total immersion with snorkel-breathing. Neither breath-holding in air or with only the torso immersed, nor total immersion with snorkel-breathing produced such a diving response.3. Breath-holding, after several minutes of total immersion and snorkel-breathing, produced an attenuated diving response. It therefore appears that a full diving response can be obtained only when the apnoea commences at the moment of face immersion.4. The present investigation supports the concept that in man face immersion is an essential predisposing factor for the diving response, and cortical inhibition of the respiratory centre is important for its initiation and maintenance.
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