In young, healthy people the alveolar-arterial P O 2 difference (A-aDO 2 ) is small at rest, but frequently increases during exercise. Previously, investigators have focused on ventilation/perfusion mismatch and diffusion abnormalities to explain the impairment in gas exchange, as significant physiological intra-pulmonary shunt has not been found. The aim of this study was to use a non-gas exchange method to determine if anatomical intra-pulmonary (I-P) shunts develop during exercise, and, if so, whether there is a relationship between shunt and increased A-aDO 2 . Healthy male participants performed graded upright cycling to 90%V O 2 max while pulmonary arterial (PAP) and pulmonary artery wedge pressures were measured. Blood samples were obtained from the radial artery, cardiac output (Q) was calculated by the direct Fick method and I-P shunt was determined by administering agitated saline during continuous 2-D echocardiography. A-aDO 2 progressively increased with exercise and was related toQ (r = 0.86) and PAP (r = 0.75). No evidence of I-P shunt was found at rest in the upright position; however, 7 of 8 subjects developed I-P shunts during exercise. In these subjects, point bi-serial correlations indicated that I-P shunts were related to the increased A-aDO 2 (r = 0.68),Q (r = 0.76) and PAP (r = 0.73). During exercise, intra-pulmonary shunt always occurred when A-aDO 2 exceeded 12 mmHg andQ was greater than 24 l min −1 . These results indicate that anatomical I-P shunts develop during exercise and we suggest that shunt recruitment may contribute to the widened A-aDO 2 during exercise.

Non-technical summary Heat stress reduces brain blood flow and impairs orthostatic tolerance. Brain blood flow is largely controlled by the partial pressure of arterial P CO 2 . Indeed, hyperthermia-induced over-breathing and related reductions in arterial P CO 2 account for ∼50% of the reduction in brain blood flow. This investigation tested the unique hypothesis that the distribution of cardiac output during heat stress (challenged by thermoregulatory increases in skin blood flow and sweat loss) contributes to the remaining 50%. We show that cardiac output is not related to brain blood flow, but rather arterial P CO 2 plays a much larger role than previously suggested. These findings help us understand the mechanisms relating heat stress with an increased likelihood of fainting, and are also relevant to pathological conditions that are accompanied by elevations in body temperature.Abstract Cerebral blood flow (CBF) is reduced during passive heat stress, with 50% of this reduction associated with hyperventilatory-induced hypocapnia and subsequent cerebral vasoconstriction. It remains unknown, however, what other factors may contribute to the remaining 50%. We tested the hypothesis that the distribution of cardiac output plays an important role in maintaining cerebral perfusion during mild and severe heat stress. Middle cerebral artery and posterior cerebral artery blood flow velocity (MCAv and PCAv; transcranial Doppler) and left ventricular end-diastolic and end-systolic volumes (2-D echocardiography) were measured under conditions of normothermia and mild and severe passive heat stress (core temperature +0.8 ± 0.1• C (Protocol I; n = 10) and 1.8 ± 0.1 • C (Protocol II; n = 8) above baseline). Venous return was manipulated by passive tilt table positioning (30 deg head-down tilt (HDT) and 30 deg head-up tilt (HUT)). Measurements were made under poikilocapnic and isocapnic conditions. Protocol I consisted of mild heat stress which resulted in small reductions in end-tidal CO 2 (−5.6 ± 3.5%), MCAv/PCAv (−7.3 ± 2.3% and −10.3 ± 2.9%, respectively) and stroke volume (−8.5 ± 4.2%); while end-diastolic volume was significantly reduced (−16.9 ± 4.0%) and cardiac output augmented (17.2 ± 7.4%). During mild heat stress, CBF was related to left ventricular end-diastolic volume (MCAv, r 2 = 0.81; PCAv, r 2 = 0.83; P < 0.05) and stroke volume (MCAv, r 2 = 0.38; PCAv, r 2 = 0.43), but not with cardiac output. Protocol II consisted of severe heat stress which resulted in much greater reductions in end-tidal CO 2 (−87.5 ± 31.5%) and CBF (MCAv, −36.4 ± 6.1%; PCAv, −30.1 ± 4.8%; P < 0.01 for all variables), while end-diastolic volume and stroke volume decreased to a similar extent as for mild heat stress. Importantly, isocapnia restored MCAv and PCAv back to normothermic baseline. This investigation therefore produced two novel findings: first, that venous return and stroke volume are related to CBF during mild heat stress; and second, that hyperventilatory hypocapnia has a major influence on CBF during severe passive heat str...

Combining helium and hyperoxia delays dynamic hyperinflation and improves respiratory mechanics, which translates into added improvements in exercise tolerance for patients with COPD.

Subjects with greater aerobic fitness demonstrate better diastolic compliance at rest, but whether fitness modulates exercise cardiac compliance and cardiac filling pressures remains to be determined. On the basis of maximal oxygen consumption (VO2max), healthy male subjects were categorized into either low (LO: VO2max=43+/-6 ml.kg-1.min-1; n=3) or high (HI: VO2max=60+/-3 ml.kg-1.min-1; n=5) aerobic power. Subjects performed incremental cycle exercise to 90% Vo(2max). Right atrial (RAP) and pulmonary artery wedge (PAWP) pressures were measured, and left ventricular (LV) transmural filling pressure (TMFP=PAWP-RAP) was calculated. Cardiac output (CO) and stroke volume (SV) were determined by direct Fick, and LV end-diastolic volume (EDV) was estimated from echocardiographic fractional area change and Fick SV. There were no between-group differences for any measure at rest. At a submaximal workload of 150 W, PAWP and TMFP were higher (P<0.05) in LO compared with HI (12 vs. 8 mmHg, and 9 vs. 4 mmHg, respectively). At peak exercise, CO, SV, and EDV were lower in LO (P<0.05). RAP was not different at peak exercise, but PAWP (23 vs. 15 mmHg) and TMFP (12 vs. 6 mmHg) were higher in LO (P<0.05). Compared with less fit subjects, subjects with greater aerobic fitness demonstrated lower LV filling pressures during exercise, whereas SV and EDV were either similar (submaximal exercise) or higher (peak exercise), suggesting superior diastolic function and compliance.

Abstract:The focus of this review is on the physiological considerations necessary for developing employment standards within occupations that have a heavy reliance on load carriage. Employees within military, fire fighting, law enforcement, and search and rescue occupations regularly work with heavy loads. For example, soldiers often carry loads >50 kg, whilst structural firefighters wear 20-25 kg of protective clothing and equipment, in addition to carrying external loads. It has long been known that heavy loads modify gait, mobility, metabolic rate, and efficiency, while concurrently elevating the risk of muscle fatigue and injury. In addition, load carriage often occurs within environmentally stressful conditions, with protective ensembles adding to the thermal burden of the workplace. Indeed, physiological strain relates not just to the mass and dimensions of carried objects, but to how those loads are positioned on and around the body. Yet heavy loads must be borne by men and women of varying body size, and with the expectation that operational capability will not be impinged. This presents a recruitment conundrum. How do employers identify capable and injury-resistant individuals while simultaneously avoiding discriminatory selection practices? In this communication, the relevant metabolic, cardiopulmonary, and thermoregulatory consequences of loaded work are reviewed, along with concomitant impediments to physical endurance and mobility. Also emphasised is the importance of including occupation-specific clothing, protective equipment, and loads during work-performance testing. Finally, recommendations are presented for how to address these issues when evaluating readiness for duty.Key words: backpacks, firefighter, load carriage, oxygen cost, military, employment standards, ventilation, work of breathing.Résumé : Cette analyse documentaire traite principalement des aspects physiologiques essentiels à l'élaboration de normes d'emploi pour des postes dont la fonction majeure est le transport de charges. Les employés dans l'armée, les pompiers, la police, les membres de recherche et sauvetage travaillent régulièrement avec de lourdes charges. Par exemple, les soldats transportent souvent des charges de >50 kg et les pompiers de bâtiments portent un vêtement de protection pesant de 20 à 25 kg et déplacent aussi des charges externes. On sait depuis longtemps que le port de charges lourdes modifie la démarche, la mobilité, le taux métabolique et le rendement tout en augmentant le risque de fatigue musculaire et de blessure. De plus, le déplacement des charges est effectué fréquemment dans des conditions environnementales stressantes et les vêtements de protection accroissent la charge thermique dans cet endroit. En outre, la contrainte physiologique ne dépend pas seulement de la masse et des dimensions des objets à déplacer, mais aussi de leur positionnement sur et autour du corps. Pourtant, il faut que ces charges soient déplacées par des hommes et des femmes de gabarit divers dont les capacités opérationnell...