The brainstem's ventilatory sensitivity and tolerance to elevated carbon dioxide levels can be trained and adapted through intentional breath-holding exercises.
"if we can get the brainstem to handle high circulating CO2s by practicing exposing that brainstem to these climbing CO2 levels, then we're able to tolerate higher workloads, strip more oxygen off, and not quickly scrub-breathe off that. And now we can actually get more work done because it's the CO2 that's triggering the breathing response, right? And we don't sort of realize that that's very trainable" (said at 0:55:27)
The claim that the brainstem's ventilatory sensitivity to carbon dioxide (the hypercapnic ventilatory response, or HCVR) is easily trained through breath-holding exercises is overstated. Randomized and prospective interventional studies show that while repeated breath-hold practice significantly improves breath-holding time, it does not alter central chemoreflex sensitivity or HCVR slopes over short-to-medium training periods (such as 14 days of daily apneas). Although some cross-sectional studies in long-term breath-hold divers (with years of exposure) have observed blunted ventilatory responses to hypercapnia, multiple controlled trials and physiological assessments of elite apneists demonstrate that resting central chemoreflex sensitivity to CO2 does not significantly differ from non-divers, indicating that increased breath-hold tolerance is primarily driven by psychological tolerance, lung volumes, and peripheral adaptations rather than readily trained brainstem chemosensitivity.
- partial: Ventilatory responses to hypercapnia in divers and non-divers: effects of posture and imme… (European journal of applied physiology 2001)
"The slopes of the linear regression, which characterised the sensitivity to CO2 and were determined with the rebreathing tests, revealed differences between the two populations (ventilation: P < 0.0001; mean inspiratory flow: P < 0.05)... These results confirmed a lower sensitivity to CO2 for trained divers." (abstract, results, passage verified)
pubmedfull study (doi) - contradicts: Central chemoreflex sensitivity and sympathetic neural outflow in elite breath-hold divers… (Journal of applied physiology (Bethesda, Md. : 1985) 2008)
"The slope of the hypercapnic ventilatory and MSNA response was similar in both groups. We conclude that repeated bouts of hypoxemia in elite, healthy breath-holding divers do not lead to sustained sympathetic activation or arterial hypertension." (abstract, results and conclusions, passage verified)
pubmedfull study (doi) - contradicts: Cardio-ventilatory responses to poikilocapnic hypoxia and hypercapnia in trained breath-ho… (Respiratory physiology & neurobiology 2014)
"BHDs presented carbon dioxide sensitivity similar to that of NDs (2.85±1.41 vs. 1.85±0.93Lmin(-1)mmHg(-1), p>0.05, respectively)... These results suggest that the exposure to repeated bouts of hypoxia/hypercapnia frequently experienced by trained breath-hold divers only enhances their chemo-responsiveness to poikilocapnic hypoxia, without altering HCVR." (abstract, results and conclusions)
pubmedfull study (doi) - contradicts: Repeated apneas and hypercapnic ventilatory response before and after apnea training. (Aerospace medicine and human performance 2015)
"Although breath hold time (BHT) increased after apnea training in the NBH group by ~46%, CO2 sensitivity slopes were not different among experimental conditions and groups (2.8 0.3, 2.9 0.4 L min(1) mmHg(1) in the CBH and 2.7 0.5, 2.7 0.3 L min(1) mmHg(1) in the NBH during the second and third visit, respectively). HCVR after five RBHE or 14 d of apnea training was not decreased despite the achieved BHT enhancement." (abstract, results and conclusions, passage verified)
pubmedfull study (doi)