Effects of lung volume and involuntary breathing movements on the human diving response

Abstract

The effects of lung volume and involuntary breathing movements on the human diving response were studied in 17 breath-hold divers. Each subject performed maximal effort apnoeas and simulated dives by apnoea and cold water face immersion, at lung volumes of 60%, 85%, and 100% of prone vital capacity (VC). Time of apnoea, blood pressure, heart rate, skin capillary blood flow, and fractions of end-expiratory CO2 and O2 were measured. The length of the simulated dives was the shortest at 60% of VC, probably because at this level the build up of alveolar CO2 was fastest. Apnoeas with face immersion at 100% of VC gave a marked drop in arterial pressure during the initial 20 s, probably due to high intrathoracic pressure mechanically reducing venous return. The diving response was most pronounced at 60% of VC. We concluded that at the two larger lung volumes both mechanical factors and input from pulmonary stretch receptors influenced the bradycardia and vasoconstriction, resulting in a nonlinear relationship between the breath-hold lung volume and magnitude of the diving response in the near-VC range. Furthermore, the involuntary breathing movements that appeared during the struggle phase of the apnoeas were too small to affect the diving response.

Cardiovascular responses to dry resting apnoeas in elite divers while breathing pure oxygen

“…At the beginning of apnoea, the immediate response consisted of a rapid fall of SBP and DBP, as previously observed at rest (Andersson and Schagatay, 1998;Palada et al, 2007;Perini et al, 2008) and at exercise (Sivieri et al, 2015). This fall was associated with a reduction of SV, supporting the notion that the fall of SBP may be due to an acute reduction of venous return related to the act of holding the breath at elevated lung volumes (Andersson and Schagatay, 1998).…”

“…This fall was associated with a reduction of SV, supporting the notion that the fall of SBP may be due to an acute reduction of venous return related to the act of holding the breath at elevated lung volumes (Andersson and Schagatay, 1998). If this is so, then the simultaneous increase in f H tries to correct hypotension and compensate for the fall of SV, yet without success, as demonstrated by the reduction ofQ at the minimum of SBP.…”

The human diving response, its function, and its control

“…Heart rate is less for any given intrapleural pressure, the larger the lung volume, and bradycardia is greatest at larger lung volumes (Song et al, 1969). However, these results contradict more current research with animals and humans, which clearly shows that the bradycardic response is greatest with smaller lung volumes (Angell-James et al, 1981;Daly, 1997;Andersson & Schagatay, 1998b). In seals, a close relationship exists between lung volume and output from vagal afferent nerve fibers that innervate pulmonary stretch receptors (Angell-James et al, 1981).…”

“…In dogs, spontaneous lung movements override diving bradycardia (Angell-James & Daly, 1969). In human breath-hold divers, under simulated diving conditions, the heart rate decreases more when the lung volume is held at 60% in comparison with 85% of vital capacity (Andersson & Schagatay, 1998b). Potentially this occurs because, at smaller lung volumes, activity in slowly adapting pulmonary stretch receptors is minimal or absent.…”

Pathophysiology of Drowning

“…The assumption is that, as in animals, the lower heart rate is a by-proxy parameter for the oxygen reduction effects of the diving response. Factors that enhance the refl ex in humans (defi ned as a lower heart rate) are cold (less than 10 °C) water, a large air-water temperature gradient, increased hypoxia, prolonged submersion, a reduced lung volume, and anxiety [ 18 ]. The refl ex starts more rapidly and is stronger during exercise, such as underwater swimming, and is not infl uenced by small involuntary breathing movements during the struggle phase of prolonged apnea.…”