Cardiopulmonary Responses of SCUBA Divers in Static and Exercise Conditions

Sixteen professional male SCUBA divers were recruited. All volunteers were healthy and non-smokers and had no history of cardiovascular or pulmonary disease. Each gave written informed consent for participation in this study. The characteristics of these subjects were as follows (mean ± SD): age 34.4 ± 12.1 year, height 1.84 ± 0.12 m, and body weight 68.4 ± 7.7 kg. All experimental procedures were conducted in line with the Declaration of Helsinki, and the study protocol was approved by the local Ethics Committee (Comité de Protection des Personnes-CPP Sud Méditerranée V, ref 16.077). The methods and potential hazards were explained to participants in detail before beginning the experiments.
Each diver completed four 30-min air-breathing dives in a 29 °C freshwater pool, at shallow depth (≈ 1 m). The four sessions were randomly allocated and 72 h apart. The divers refrained from exercise and any dive for 24 h before each experimental session. On each dive, they wore trunks without a wetsuit and used the same closed-circuit rebreather SCUBA setting (Triton®, MS3, Tourves, France) and remained in prone position.
The static conditions (Static) consisted in floating at rest, breathing with a positive pressure when the rebreather was attached anteriorly (StPPB), and with a negative pressure when the rebreather was attached posteriorly (StNPB) (Fig. 1a, b). During exercise (Exercise), subjects were asked to fin swim throughout the 30 min of immersion while maintaining a heart rate (HR) of 110 ± 10 bpm (monitored with a Polar® V800, Finland) to achieve constant moderate work intensity.Fig. 1

Tidal volume loop during each dive condition in one diver. A positive transpulmonary pressure gradient (or positive static lung load: SLL+) is set when the rebreather is worn anteriorly (on the abdomen) by the diver in prone position (a). A positive pressure breathing (PPB) condition is created. Conversely, when the rebreather is worn posteriorly (b), the transpulmonary pressure gradient is negative in the prone position (negative static lung load, SLL−), and the diver is in condition of negative pressure breathing (NPB). In each condition, the diver completed two 30-min dives, one simply statically floating (static), and one with continuous fin swimming (exercise). Examples of tidal pressure-volume loops are sketched during both static and exercise in each PPB and NPB condition. The dashed lines indicate the SLL level in each condition. Peak insp. press., peak inspiratory pressure during; peak expir. press., peak expiratory pressure. Of note, in each PPB and NPB, Vt lengthening carried the main rest to exercise change, while pressure ranges were very similar during static and exercise dives

Prior to immersion, resting cardiovascular indices and the absence of EVLW were assessed based on cardiopulmonary ultrasonography. During immersion, ventilatory flow and pressure were continuously measured in the mouthpiece. Transthoracic echocardiography was performed immediately after exertion while still submerged. Lung ultrasound was used to assess for the presence of EVLW, and a single-breath gas transfer capacity of the lung (TLCO) was measured. Pulmonary artery pressure was assessed from the tricuspid regurgitant jet. Two venous blood samples were taken from the antecubital vein before and immediately after immersion.