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.

Adolescent↗

PubMedDate not supplied

The evolution of asphyxial defense.

From the time animals became dependent upon molecular oxygen as an integral part of their energy-producing processes, they have remained in the shadow of acute asphyxial threat--the blocking of respiratory exchange resulting in the intracellular triad of hypoxia, hypercapnia and acidosis. The most commonly occurring precipitant of acute asphyxia has always been the transfer between air and water environments. Over the last one hundred years studies on a wide range of living organisms, from single cells to complex multicellular organisms like mammals, have demonstrated the presence of well-defined metabolic and cardiovascular-respiratory mechanisms for protecting living things against acute asphyxia. Single-celled animals depend upon anaerobiosis and secondarily hypometabolism. In addition to these processes, animals with gills or lungs utilize "passive" protection such as increased oxygen storage and the "dynamic" cardiovascular adjustments of bradycardia and selective ischemia. These latter changes decrease overall oxygen consumption and hence utilize the oxygen stores in the most economical way to protect the cardiac and cerebral tissue, which are most sensitive to hypoxia and vital to continued survival of the organism. In this article an attempt is made to place these processes into an evolutionary context. As through a glass darkly we glimpse asphyxial defense running like a paleophysiological thread through hundreds of millions of years, being accentuated here and muted there, depending upon the particular needs of individual species.

Animals↗

PubMedDate not supplied

Mechanism of the human diving response.

The diving response in human beings is characterized by breath-holding, slowing of the heart rate (diving bradycardia), reduction of limb blood flow and a gradual rise in the mean arterial blood pressure. The bradycardia results from increased parasympathetic stimulus to the cardiac pacemaker. The reduction in limb blood flow is due to vasoconstriction resulting from increased activity of the sympathetic nerves supplying arteries in the arms and legs. Essentially the response is produced by the combination of water touching the face and either voluntary or involuntary (reflex) arrest of breathing. The nervous inputs and outputs for the response are coordinated in the brain stem by the respiratory, vasomotor and cardioinhibitory "centers." The diving response in human beings can be modified by many factors but the most important are water temperature, oxygen tension in the arterial blood and emotional factors.

Cardiovascular Physiological Phenomena↗

PubMedDate not supplied

The effect of alcohol and barbiturate on the frequency of atrial and ventricular premature beats during sleep.

Repeated EEG and ECG monitoring during sleep was carried out in two subjects with ischemic heart disease and one presumably healthy control. All three displayed frequent ectopic heart beats. Two had ventricular and the third atrial premature beats. The study had two objectives: (1) to ascertain the distribution of ectopic beats among the stages of sleep; and (2) to ascertain the effect of alcohol and barbiturates on the frequency of premature beats. Both drugs are known to alter the pattern of sleep stages. It was found that neither atrial nor ventricular premature beats favored a particular sleep stage. Neither drug had a consistent effect on their frequency and there was no correlation with heart rate. For the most part premature beats were fewer on the second night of a procedure although a day filled with anxiety appeared to greatly increase the frequency of premature contractions.

Adult↗