Effects of respiratory muscle training on ventilatory and cerebrovascular responses to hypercapnia during temperate water immersion

SCUBA divers are at risk of developing hypercapnia, and subsequently oxygen (O 2 ) toxicity, due to increased work of breathing and/or a reduced ventilatory response to hypercapnia (HCVR). Some divers have a blunted or exaggerated HCVR, exacerbating diving-induced hypercapnia and/or O 2 toxicity risk. Respiratory muscle training (RMT) can normalize this ventilatory response; however, it is unclear if the benefits of RMT alter cerebral hemodynamics. Purpose: We tested the hypotheses that RMT would decrease cerebrovascular reactivity to hypercapnia (CVR CO2 ) at rest, and modify the cerebrovascular response during temperate water immersion. Methods: Seven healthy participants (age: 26±4 y, 3 women) completed a CO 2 rebreathe and 1-hour of temperate head-out water immersion, on separate occasions, before (PRE) and after 4-weeks of RMT (POST). RMT consisted of a 5 sec inhale, 5 sec exhale, and 20 sec of rest for 60 cycles. Resistance was set to ~50 cm H 2 O at week 1 and increased by 10 cm H 2 O each week (up to 80 cm H 2 O maximum). The CO 2 rebreathe (8% CO 2 , 92% O 2 ) was used to quantify HCVR at PRE and POST-RMT. During immersion visits, middle cerebral artery blood velocity (transcranial Doppler; MCAv), end-tidal CO 2 (capnograph; PETCO 2 ), respiratory rate (capnograph; RR), and mean arterial pressure (finger photo plethysmography; MAP) were continuously recorded and reported before immersion (Baseline) and every 15 min during immersion. CVR CO2 was assessed at Baseline and at 1-hour of immersion using a stepped hypercapnia protocol (2, 4, 6% CO 2 for 3 min each) on a 4-way valve to avoid re-breathing. Between the CO 2 tests, participants breathed on a SCUBA regulator to simulate diving. Data are reported as (mean [min, max]) for HCVR and CVR CO2 , and mean ± SD for all other variables. Results: RMT did not significantly alter HCVR from PRE (2.69 [1.28, 5.25] L/min/mmHg) to POST-RMT (4.01 [2.41, 8.24] L/min/mmHg, p=0.199). During immersion visits, there were no differences at Baseline between PRE and POST-RMT for any variables. MCAv and PETCO 2 did not change during immersion (all p≥0.252). RR decreased (time effect: p=0.021) throughout HOWI while breathing on the SCUBA regulator (minutes 0 to 45). In contrast, MAP increased from Baseline to 60 min of HOWI (p=0.029) at PRE-RMT, but did not change throughout immersion at POST-RMT (p≥0.110). CVR CO2 was not modified following RMT or during immersion (p≥0.651) (PRE, Baseline: 1.39 [0.39, 2.32] vs. Immersion: 1.52 [1.13, 2.34], and POST, 1.48 [0.73, 2.10] vs. Immersion: 1.56 [0.73, 2.68] cm/s/mmHg). Conclusion: These preliminary data demonstrate that 4-weeks of respiratory muscle training do not alter the cerebrovascular response to hypercapnia at rest, or following simulated diving in temperate water, specifically in the middle cerebral artery. Funded by the Office of Naval Research (N000142412041). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.