The relationship of hypercapnic ventilatory responses to age, gender and athleticism

Nocturnal urination is associated with the presence of higher ventilatory chemosensitivity in patients with obstructive sleep apnea

PURPOSE

Chemosensitivity is an essential part of the pathophysiological mechanisms of obstructive sleep apnea (OSA). This study aims to use the rebreathing method to assess hypercapnic ventilatory response (HCVR) and analyze the association between chemosensitivity and certain symptoms in patients with OSA.

METHODS

A total of 104 male patients with diagnosed OSA were enrolled. The HCVR was assessed using rebreathing methods under hypoxia exposure to reflect the overall chemosensitivity. Univariate and multivariate linear regression were used to explore the association with chemosensitivity. Participants were enrolled in the cluster analysis using certain symptoms, basic characteristics, and polysomnographic indices.

RESULTS

At similar baseline values, the high chemosensitivity group (n = 39) demonstrated more severe levels of OSA and nocturnal hypoxia than the low chemosensitivity group (n = 65). After screening the possible associated factors, nocturnal urination, rather than OSA severity, was found to be positively associated with the level of chemosensitivity. Cluster analysis revealed three distinct groups: Cluster 1 (n = 32, 34.0%) held younger, obese individuals with nocturnal urination, elevated chemosensitivity level, and very severe OSA; Cluster 2 (41, 43.6%) included middle-aged overweighted patients with nocturnal urination, increased chemosensitivity level, but moderate-severe OSA; and Cluster 3 (n = 21, 22.3%) contained middle-aged overweighted patients without nocturnal urination, with a lowered chemosensitivity level and only moderate OSA.

CONCLUSION

The presence of nocturnal urination in male patients with OSA may be a sign of higher levels of ventilatory chemosensitivity, requiring early therapy efforts independent of AHI levels.

Periodized Aerobic Training between Thresholds Improves Submaximal Cardiorespiratory Parameters in Octogenarians

BACKGROUND AND AIMS

The worldwide aging population is expanding, with more individuals living into their 80s. Physiological functions decline gradually with age, compounded by sedentary lifestyles. Incorporating physical activity into daily routine is crucial for maintaining independence. This study aimed to assess a periodized high-intensity aerobic training program (PEZO-BT) in octogenarians, focusing on submaximal ergospirometry effects.

METHODS

A total of 48 non-frail octogenarian subjects (12 females, 36 males) were randomized into control and intervention groups. All subjects underwent submaximal cardiopulmonary exercise testing with gas analysis at baseline, stopping after the respiratory compensation point (RCP). Our intervention group completed a 14-week PEZO-BT aerobic training program. The outcomes were oxygen consumption at first ventilatory threshold (VO2AT), ventilatory efficiency slope (VE/VCO2), oxygen uptake efficiency slope (OUES), cardiorespiratory optimal point (COP), oxygen pulse change (ΔVO2/HR) from anaerobic threshold (AT) to respiratory compensation point (RCP), and power output at anaerobic threshold (POAT).

RESULTS

Mixed ANOVA examined time and treatment effects. If significance emerged, post hoc t-tests were used to compare significances between groups. The homogeneity of variance was assessed using Levene's test. Chi-square tests compared ergospirometry criteria and ventilatory performance within groups. The mean differences at post intervention were significant in VO2AT (p < 0.001), VE/VCO2 (p < 0.001), ΔVO2/HR (p < 0.05), and POAT (p < 0.001), while OUES and COP were not significant (p > 0.05). However, clinical effects were observed in the entire intervention group.

CONCLUSIONS

Training improved exercise capacity and workload. Overall, this periodic aerobic and high-intensity interval training (HIIT) program yielded significant improvements in cardiorespiratory fitness (CRF) in previously untrained octogenarians with and without comorbidities. The findings suggest implications for promoting long-term healthy aging.

Pathophysiology of Acute Respiratory Distress Syndrome and COVID-19 Lung Injury

The pathophysiology of acute respiratory distress syndrome (ARDS) is marked by inflammation-mediated disruptions in alveolar-capillary permeability, edema formation, reduced alveolar clearance and collapse/derecruitment, reduced compliance, increased pulmonary vascular resistance, and resulting gas exchange abnormalities due to shunting and ventilation-perfusion mismatch. Mechanical ventilation, especially in the setting of regional disease heterogeneity, can propagate ventilator-associated injury patterns including barotrauma/volutrauma and atelectrauma. Lung injury due to the novel coronavirus SARS-CoV-2 resembles other causes of ARDS, though its initial clinical characteristics may include more profound hypoxemia and loss of dyspnea perception with less radiologically-evident lung injury, a pattern not described previously in ARDS.

The Pathophysiology and Dangers of Silent Hypoxemia in COVID-19 Lung Injury

The ongoing coronavirus disease (COVID-19) pandemic has been unprecedented on many levels, not least of which are the challenges in understanding the pathophysiology of these new critically ill patients. One widely reported phenomenon is that of a profoundly hypoxemic patient with minimal to no dyspnea out of proportion to the extent of radiographic abnormality and change in lung compliance. This apparently unique presentation, sometimes called "happy hypoxemia or hypoxia" but better described as "silent hypoxemia," has led to the speculation of underlying pathophysiological differences between COVID-19 lung injury and acute respiratory distress syndrome (ARDS) from other causes. We explore three proposed distinctive features of COVID-19 that likely bear on the genesis of silent hypoxemia, including differences in lung compliance, pulmonary vascular responses to hypoxia, and nervous system sensing and response to hypoxemia. In the context of known principles of respiratory physiology and neurobiology, we discuss whether these particular findings are due to direct viral effects or, equally plausible, are within the spectrum of typical ARDS pathophysiology and the wide range of hypoxic ventilatory and pulmonary vascular responses and dyspnea perception in healthy people. Comparisons between lung injury patterns in COVID-19 and other causes of ARDS are clouded by the extent and severity of this pandemic, which may underlie the description of "new" phenotypes, although our ability to confirm these phenotypes by more invasive and longitudinal studies is limited. However, given the uncertainty about anything unique in the pathophysiology of COVID-19 lung injury, there are no compelling pathophysiological reasons at present to support a therapeutic approach for these patients that is different from the proven standards of care in ARDS.

Importance of Cardiopulmonary Exercise Testing amongst Subjects Recovering from COVID-19

The cardiopulmonary exercise test (CPET) provides an objective assessment of ventilatory limitation, related to the exercise minute ventilation (V_E) coupled to carbon dioxide output (V_CO2) (V_E/V_CO2); high values of V_E/V_{CO2 slope} define an exercise ventilatory inefficiency (EVin). In subjects recovered from hospitalised COVID-19, we explored the methodology of CPET in order to evaluate the presence of cardiopulmonary alterations. Our prospective study (RESPICOVID) has been proposed to evaluate pulmonary damage’s clinical impact in post-COVID subjects. In a subgroup of subjects (RESPICOVID2) without baseline confounders, we performed the CPET. According to the V_E/V_{CO2 slope}, subjects were divided into having EVin and exercise ventilatory efficiency (EVef). Data concerning general variables, hospitalisation, lung function, and gas-analysis were also collected. The RESPICOVID2 enrolled 28 subjects, of whom 8 (29%) had EVin. As compared to subjects with EVef, subjects with EVin showed a reduction in heart rate (HR) recovery. V_E/V_{CO2 slope} was inversely correlated with HR recovery; this correlation was confirmed in a subgroup of older, non-smoking male subjects, regardless of the presence of arterial hypertension. More than one-fourth of subjects recovered from hospitalised COVID-19 have EVin. The relationship between EVin and HR recovery may represent a novel hallmark of post-COVID cardiopulmonary alterations.

Silent hypoxaemia in COVID-19 patients

The clinical presentation of COVID-19 due to infection with SARS-CoV-2 is highly variable with the majority of patients having mild symptoms while others develop severe respiratory failure. The reason for this variability is unclear but is in critical need of investigation. Some COVID-19 patients have been labelled with 'happy hypoxia', in which patient complaints of dyspnoea and observable signs of respiratory distress are reported to be absent. Based on ongoing debate, we highlight key respiratory and neurological components that could underlie variation in the presentation of silent hypoxaemia and define priorities for subsequent investigation.

Acute Effects of Using Added Respiratory Dead Space Volume in a Cycling Sprint Interval Exercise Protocol: A Cross-Over Study

Background: The aim of the study was to compare acute physiological, biochemical, and perceptual responses during sprint interval exercise (SIE) with breathing through a device increasing added respiratory dead space volume (ARDS_V) and without the device. Methods: The study involved 11 healthy, physically active men (mean maximal oxygen uptake: 52.6 ± 8.2 mL∙kg¹∙min^-1). During four visits to a laboratory with a minimum interval of 72 h, they participated in (1) an incremental test on a cycle ergometer; (2) a familiarization session; (3) and (4) cross-over SIE sessions. SIE consisted of 6 × 10-s all-out bouts with 4-min active recovery. During one of the sessions the participants breathed through a 1200-mL ARDSv (SIE_ARDS). Results: The work performed was significantly higher by 4.4% during SIE_ARDS, with no differences in the fatigue index. The mean respiratory ventilation was significantly higher by 13.2%, and the mean oxygen uptake was higher by 31.3% during SIE_ARDS. Respiratory muscle strength did not change after the two SIE sessions. In SIE_ARDS, the mean pH turned out significantly lower (7.26 vs. 7.29), and the mean HCO₃^- concentration was higher by 7.6%. Average La^- and rating of perceived exertion (RPE) did not differ between the sessions. Conclusions: Using ARDS_V during SIE provokes respiratory acidosis, causes stronger acute physiological responses, and does not increase RPE.

Why COVID-19 Silent Hypoxemia Is Baffling to Physicians

Patients with coronavirus disease (COVID-19) are described as exhibiting oxygen levels incompatible with life without dyspnea. The pairing-dubbed happy hypoxia but more precisely termed silent hypoxemia-is especially bewildering to physicians and is considered as defying basic biology. This combination has attracted extensive coverage in media but has not been discussed in medical journals. It is possible that coronavirus has an idiosyncratic action on receptors involved in chemosensitivity to oxygen, but well-established pathophysiological mechanisms can account for most, if not all, cases of silent hypoxemia. These mechanisms include the way dyspnea and the respiratory centers respond to low levels of oxygen, the way the prevailing carbon dioxide tension (PaCO2) blunts the brain's response to hypoxia, effects of disease and age on control of breathing, inaccuracy of pulse oximetry at low oxygen saturations, and temperature-induced shifts in the oxygen dissociation curve. Without knowledge of these mechanisms, physicians caring for patients with hypoxemia free of dyspnea are operating in the dark, placing vulnerable patients with COVID-19 at considerable risk. In conclusion, features of COVID-19 that physicians find baffling become less strange when viewed in light of long-established principles of respiratory physiology; an understanding of these mechanisms will enhance patient care if the much-anticipated second wave emerges.

Women candidates for diving with oxygen-enriched gas mixtures have a lower end tidal CO2 than men during moderate exercise

We have previously determined the thresholds for CO2 detection (conscious recognition of elevated CO2) and retention in male divers, beyond which a diving candidate should not continue his diving activity due to an increased risk of CNS oxygen toxicity. The purpose of the present study was to establish whether there is a difference in end tidal PCO2 between male and female divers who use oxygen-enriched gas mixtures. Ventilatory and perceptual responses to variations in inspired CO2 (range 0-42 mm Hg) were assessed during moderate exercise in 18 males and 18 females. End tidal PCO2 was lower in the female divers when breathing oxygen with 42 mm Hg CO2 (58.2±3.0 mm Hg vs. 61.5±4.5 mm Hg, P<0.03). These results suggest that female divers have a lower end tidal CO2 than males when breathing a hyperoxic gas mixture during exercise, which might imply that women are less susceptible to CNS oxygen toxicity than men.

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Tube breathing as a new potential method to perform respiratory muscle training: Safety in healthy volunteers

Normocapnic hyperpnea has been established as a method of respiratory muscle endurance training (RMET). This technique has not been applied on a large scale because complicated and expensive equipment is needed to maintain CO(2)-homeostasis during hyperpnea. This CO(2)-homeostasis can be preserved during hyperpnea by enlarging the dead space of the ventilatory system. One of the possibilities to enlarge dead space is breathing through a tube. If tube breathing is safe and feasible, it may be a new and inexpensive method for RMET, enabling its widespread use. The aim of this study was to evaluate the safety of tube breathing and investigate the effect on CO(2)-homeostasis in healthy subjects. A total of 20 healthy volunteers performed 10 min of tube breathing (dead space 60% of vital capacity). Oxygen-saturation, PaCO(2), respiratory muscle function, hypercapnic ventilatory response and dyspnea (Borg-score) were measured. Tube breathing did not lead to severe complaints, adverse events or oxygen desaturations. A total of 14 out of 20 subjects became hypercapnic (PaCO(2)>6.0 kPa) during tube breathing. There were no significant correlations between PaCO(2) and respiratory muscle function or hypercapnic ventilatory responses. The normocapnic versus hypercapnic subjects showed no significant differences between decrease in oxygen saturation (-0.7% versus -0.2%, respectively, P=0.6), Borg score (4.3 versus 4.7, P=0.9), respiratory muscle function nor hypercapnic ventilatory responses. Our results show that tube breathing is well tolerated amongst healthy subjects. No complaints, nor desaturations occurred. Hypercapnia developed in a substantial number of subjects. When tube breathing will be applied as respiratory muscle training modality, this potential development of hypercapnia must be considered.

Role of acid-base balance in the chemoreflex control of breathing

This paper uses a steady-state modeling approach to describe the effects of changes in acid-base balance on the chemoreflex control of breathing. First, a mathematical model is presented, which describes the control of breathing by the respiratory chemoreflexes; equations express the dependence of pulmonary ventilation on Pco(2) and Po(2) at the central and peripheral chemoreceptors. These equations, with Pco(2) values as inputs to the chemoreceptors, are transformed to equations with hydrogen ion concentrations [H(+)] in brain interstitial fluid and arterial blood as inputs, using the Stewart approach to acid-base balance. Examples illustrate the use of the model to explain the regulation of breathing during acid-base disturbances. They include diet-induced changes in sodium and chloride, altitude acclimatization, and respiratory disturbances of acid-base balance due to chronic hyperventilation and carbon dioxide retention. The examples demonstrate that the relationship between Pco(2) and [H(+)] should not be neglected when modeling the chemoreflex control of breathing. Because pulmonary ventilation controls Pco(2) rather than the actual stimulus to the chemoreceptors, [H(+)], changes in their relationship will alter the ventilatory recruitment threshold Pco(2), and thereby the steady-state resting ventilation and Pco(2).

CO2 retention in lung disease; could there be a pre-existing difference in respiratory physiology

Some patients with lung disease retain CO(2), while others with similar lung function do not. This could be explained if CO(2) retainers had a pre-existing low hypercapnic ventilatory response (HCVR) and, from this, a tendency to retain CO(2). To test if such a phenomenon exists in healthy people, we determined the change in end-tidal P(CO(2)) (deltaPET(CO(2))) produced by the addition of a dead-space (DS), during wakefulness and sleep, and related this to the HCVR measured awake. The group mean (n=14) HCVR slope was 2.2+/-1.1 (S.D.) L min(-1) mmHg(-1). The deltaPET(CO(2)) with the application of DS was 1.6+/-1.6 mmHg awake and 2.6+/-2.2 mmHg asleep. During wakefulness the deltaPET(CO(2)) with DS did not correlate with the HCVR slope. However, during sleep, four subjects had a marked increase in the deltaPET(CO(2)) (3.7, 4.3, 6.2, 8.0 mmHg) and a relatively low HCVR (slope 1.5, 1.7, 1.5, 1.7 L min(-1) mmHg(-1), respectively). We speculate that such individuals, should they develop lung disease, may be predisposed to retain CO(2).

[Measuring hypoxic and hypercapnic respiratory response in patients with obstructive sleep apnea]

The gas composition of breathing air is a very important stimulus for the control of breathing. The different partial pressures of O2 and CO2 independently trigger individually different reactions (respiratory response), which can be measured as a change of respiratory minute volume. Investigations of the respiratory control in patients with obstructive sleep apnoea (OSA) have up to now been restricted to an analysis of the breathing patterns at night. Therefore we have developed a computer-controlled device which allows a flexible composition of the air to be inhaled using a regulated feet-back circle. With this system it is possible to produce a hypercapnia test as well as a hyperoxia and an isocapnic hypoxia test. The simultaneous recording of all relevant respiratory parameters (AF, AMV, ETCO2, SpO2, FiO2) and the parallel recording of continuous blood pressure allow a quantitative description of the respiratory regulation of patients with OSA with exactly defined tests.

Gender differences in workload effect on coordination between breathing and cycling

PURPOSE

To investigate the gender differences in the effect of increasing workload level and thus of an increasing metabolic drive to ventilation on the degree of coordination between breathing and cycling rhythms.

METHODS

Twenty-one men and 21 women cycled on an electromagnetically braked ergometer while breathing through a pneumotachograph at workloads corresponding to 55, 75, and 95% of V0(2peak) (WL1, WL2, and WL3). Leg movements, respiratory parameters, and heart rate were continuously recorded. The degree of coordination (%coord) was quantified as the percentage of breaths starting during the same phase of leg movement.

RESULTS

In men, %coord increased with increasing exercise intensity (WL1: mean +/- SE = 18.8 +/- 2.6%, WL2: 30.9 +/- 4.9%, WL3: 40.9 +/- 5.6%), whereas in women exercise intensity had no influence on %coord (WL1: 25.0 +/- 5.0%, WL2: 29.7 +/- 5.1, WL3: 31.7 +/- 4.7%). There were no gender differences in breathing pattern during high metabolic demands. A major effect on %coord came from the regularity of the breathing rhythm, whereas cycling frequency, fitness level, or cycling experience exerted no influence.

CONCLUSIONS

The present study demonstrates that the effect of exercise intensity on the occurrence of coordination between breathing and cycling rhythms differs between men and women.

Acid-base balance during repeated cycling sprints in boys and men

The aim of this study was to investigate the acid-base balance during repeated cycling sprints in children and adults. Eleven boys (9.6 +/- 0.7 yr) and ten men (20.4 +/- 0.8 yr) performed ten 10-s sprints on a cycle ergometer separated by 30-s passive recovery intervals. To measure the time course of lactate ([La]), hydrogen ions ([H(+)]), bicarbonate ions ([HCO(3)(-)]), and base excess concentrations and the arterial partial pressure of CO(2), capillary blood samples were collected at rest and after each sprint. Ventilation and CO(2) output were continuously measured. After the 10th sprint, concentrations of boys vs. men were as follows: [La], 8.5 +/- 2.1 vs. 15.4 +/- 2.0 mmol/l; [H(+)], 43.8 +/- 1.3 vs. 66.9 +/- 9.9 nmol/l (P < 0.001). Significant correlations showed that, for a given [La], [H(+)] was lower in the boys compared with the men (P < 0.001). Significant relationships also indicated that, for a given [La], [HCO(3)(-)] and base excess concentration were similar in the boys compared with the men. Moreover, significant relationships revealed that, for a given [H(+)] or [HCO(3)(-)], arterial partial pressure of CO(2) was lower in the boys compared with the men (P < 0.001). The ventilation-to-CO(2) output ratio was higher in the boys during the first five rest intervals and was then higher in the men during the last five sprints. To conclude, during repeated sprints, the ventilatory regulation related to the change in acid-base balance induced by lactic acidosis was more important during the first rest intervals in the boys compared with the men.

A model of the chemoreflex control of breathing in humans: model parameters measurement

We reviewed the ventilatory responses obtained from rebreathing experiments on a population of 22 subjects. Our aim was to derive parameter estimates for an 'average subject' so as to model the respiratory chemoreflex control system. The rebreathing technique used was modified to include a prior hyperventilation, so that rebreathing started at a hypocapnic P(CO2) and ended at a hypercapnic P(CO2). In addition, oxygen was added to the rebreathing bag in a controlled manner to maintain iso-oxia during rebreathing, which allowed determination of the response at several iso-oxic P(O2) levels. The breath-by-breath responses were analysed in terms of tidal volume, breathing frequency and ventilation. As P(CO2) rose, ventilation was first steady at a basal value, then increased as P(CO2) exceeded a breakpoint. We interpreted this first breakpoint as the threshold of the combined central and peripheral chemoreflex responses. Above, ventilation increased linearly with P(CO2), with tidal volume usually contributing more than frequency to the increase. When breathing was driven strongly, such as in hypoxia, a second breakpoint P(CO2) was often observed. Beyond the second breakpoint, ventilation continued to increase linearly with P(CO2) at a different slope, with frequency usually contributing more than tidal volume to the increase. We defined the parameters of the variation of tidal volume, frequency and ventilation with P(O2) and P(CO2) for an average subject based on a three-segment linear fit of the individual responses. These were incorporated into a model of the respiratory chemoreflex control system based on the general scheme of the 'Oxford' model. However, instead of considering ventilatory responses alone, the model also incorporates tidal volume and frequency responses.

The effect of hypoxia on the ventilatory response to carbon dioxide in man

We used rebreathing with prior hyperventilation to measure ventilatory responses to CO2 at iso-oxic PO2's of 100, 80, 60 and 40 mmHg in seven subjects. The mean sub-threshold ventilation (S.E.) of 7.60 (1.31) L min-1 did not vary with iso-oxic PO2. The mean peripheral-chemoreflex threshold of 41 (0.6)) mmHg PCO2 at an iso-oxic PO2 of 100 was greater than 39 (1.2) and 39 (0.6) at 60 and 40, respectively. The mean peripheral-chemoreflex sensitivity of 11.5 (5.2) L min-1 mmHg-1 at an iso-oxic PO2 of 40 was significantly greater than 3.0 (1.3), 2.7 (1.2) and 2.4 (1.2) at 60, 80 and 100, respectively. The mean central-chemoreflex threshold of 45 (1.5) mmHg PCO2 at an iso-oxic PO2 of 40 was significantly less than 48 (0.4) and 48 (0.7) at 80 and 100, respectively. The mean central-chemoreflex sensitivity of 5.0 (1.1) L min-1 mmHg-1 did not vary with iso-oxic PO2. These findings provide insights into the control of breathing in humans, including the implication that CO2 must exceed its peripheral-chemoreflex threshold before hypoxia can effectively increase ventilation.