Predictors of increased PaCO2 during immersed prone exercise at 4.7 ATA

Evaluation of the SIMULRESP: A simulation software of child and teenager cardiorespiratory physiology

BACKGROUND

Mathematical models based on the physiology when programmed as a software can be used to teach cardiorespiratory physiology and to forecast the effect of various ventilatory support strategies. We developed a cardiorespiratory simulator for children called "SimulResp." The purpose of this study was to evaluate the quality of SimulResp.

METHODS

SimulResp quality was evaluated on accuracy, robustness, repeatability, and reproducibility. Blood gas values (pH, PaCO2 , PaO2,  and SaO2 ) were simulated for several subjects with different characteristics and in different situations and compared to expected values available as reference. The correlation between reference and simulated data was evaluated by the coefficient of determination and Intraclass correlation coefficient. The agreement was evaluated with the Bland & Altman analysis.

RESULTS

SimulResp produced healthy child physiological values within normal range (pH 7.40 ± 0.5; PaCO2 40 ± 5 mmHg; PaO2 90 ± 10 mmHg; SaO2 97 ± 3%) starting from a weight of 25-35 kg, regardless of ventilator support. SimulResp failed to simulate accurate values for subjects under 25 kg and/or affected with pulmonary disease and mechanically ventilated. Based on the repeatability was considered as excellent and the reproducibility as mild to good. SimulResp's prediction remains stable within time.

CONCLUSIONS

The cardiorespiratory simulator SimulResp requires further development before future integration into a clinical decision support system.

Relative hypoxemia at depth during breath-hold diving investigated through arterial blood gas analysis and lung ultrasound

Pulmonary gas exchange in breath-hold diving (BHD) consists of a progressive increase in arterial partial pressures of oxygen ([Formula: see text]) and carbon dioxide ([Formula: see text]) during descent. However, recent findings have demonstrated that [Formula: see text] does not consistently rise in all subjects. This study aimed at verifying and explaining [Formula: see text] derangements during BHD analyzing arterial blood gases and searching for pulmonary alterations with lung ultrasound. After ethical approval, 14 fit breath-hold divers were included. Experiments were performed in warm water (temperature: 31°C). We analyzed arterial blood gases immediately before, at depth, and immediately after a breath-hold dive to -15 m of fresh water (mfw) and -42 mfw. Signs of lung interstitial edema and atelectasis were searched simultaneously with a marinized lung ultrasound. In five subjects (-15 mfw) and four subjects (-42 mfw), the [Formula: see text] at depth seems to decrease instead of increasing. [Formula: see text] and lactate showed slight variations. At depth, no lung ultrasound alterations were seen except in one subject (hypoxemia and B-lines at -15 mfw; B-lines at the surface). Lung interstitial edema was detected in 3 and 12 subjects after resurfacing from -15 to -42 mfw, respectively. Two subjects developed hypoxemia at depth and a small lung atelectasis (a focal pleural irregularity of triangular shape, surrounded by thickened B-lines) after resurfacing from -42 mfw. Current experiments confirmed that some BH divers can experience hypoxemia at depth. The hypothesized explanation for such a discrepancy is lung atelectasis, which could not be detected in all subjects probably due to limited time available at depth.NEW & NOTEWORTHY During breath-hold diving, arterial partial pressure of oxygen ([Formula: see text]) and arterial partial pressure of carbon dioxide ([Formula: see text]) are believed to increase progressively during descent, as explained by theory, previous end-tidal alveolar gas measurements, and arterial blood gas analysis in hyperbaric chambers. Recent experiments in real underwater environment found a paradoxical [Formula: see text] drop at depth in some divers. This work confirms that some breath-hold divers can experience hypoxemia at depth. The hypothesized explanation for such a discrepancy is lung atelectasis, as suggested by lung ultrasound findings.

Blood Gas Analyses in Hyperbaric and Underwater Environments: A Systematic Review

BACKGROUND

Pulmonary gas exchange during diving or in a dry hyperbaric environment is affected by increased breathing gas density and possibly water immersion. During free diving there is also the effect of apnea. Few studies have published blood gas data in underwater or hyperbaric environments: this review summarizes the available literature and was used to test the hypothesis that arterial PO2 under hyperbaric conditions can be predicted from blood gas measurement at 1 atmosphere assuming a constant arterial/alveolar PO2 ratio (a:A).

METHODS

A systematic search was performed on traditional sources including arterial blood gases obtained on humans in hyperbaric or underwater environments. The a:A was calculated at 1 atmosphere absolute (ATA). For each condition, predicted PaO2 at pressure was calculated using the 1 ATA a:A, and the measured PaO2 was plotted against the predicted value with Spearman correlation coefficients.

RESULTS

Of 3640 records reviewed, 30 studies were included: 25 were reports describing values obtained in hyperbaric chambers, and the remaining were collected while underwater. Increased inspired O2 at pressure resulted in increased PaO2, although underlying lung disease in patients treated with hyperbaric oxygen attenuated the rise. PaCO2 generally increased only slightly. In breath-hold divers, hyperoxemia generally occurred at maximum depth, with hypoxemia after surfacing. The a:A adequately predicted the PaO2 under various conditions: dry (r=0.993, p< 0.0001); rest vs. exercise (r=0.999, p< 0.0001); and breathing mixtures (r=0.995, p< 0.0001).

CONCLUSION

Pulmonary oxygenation under hyperbaric conditions can be reliably and accurately predicted from 1 ATA a:A measurements.

Carotid body chemosensitivity is not attenuated during cold water diving

Tonic carotid body (CB) activity is reduced during exposure to cold and hyperoxia. We tested the hypotheses that cold water diving lowers CB chemosensitivity and augments CO₂ retention more than thermoneutral diving. Thirteen subjects [age: 26 ± 4 yr; body mass index (BMI): 26 ± 2 kg/m²) completed two 4-h head-out water immersion protocols in a hyperbaric chamber (1.6 ATA) in cold (15°C) and thermoneutral (25°C) water. CB chemosensitivity was assessed with brief hypercapnic ventilatory response ([Formula: see text]) and hypoxic ventilatory response ([Formula: see text]) tests before dive, 80 and 160 min into the dive (D80 and D160, respectively), and immediately after and 60 min after dive. Data are reported as an absolute mean (SD) change from predive. End-tidal CO₂ pressure increased during both the thermoneutral water dive [D160: +2 (3) mmHg; P = 0.02] and the cold water dive [D160: +1 (2) mmHg; P = 0.03]. Ventilation increased during the cold water dive [D80: 4.13 (4.38) and D160: 7.75 (5.23) L·min^-1; both P < 0.01] and was greater than the thermoneutral water dive at both time points (both P < 0.01). [Formula: see text] was unchanged during the dive (P = 0.24) and was not different between conditions (P = 0.23). [Formula: see text] decreased during the thermoneutral water dive [D80: -3.45 (3.61) and D160: -2.76 (4.04) L·min·mmHg^-1; P < 0.01 and P = 0.03, respectively] but not the cold water dive. However, [Formula: see text] was not different between conditions (P = 0.17). In conclusion, CB chemosensitivity was not attenuated during the cold stress diving condition and does not appear to contribute to changes in ventilation or CO₂ retention.

Carotid body chemosensitivity at 1.6 ATA breathing air versus 100% oxygen

Hyperoxia reduces the ventilatory response to hypercapnia by suppressing carotid body (CB) activation. This effect may contribute to CO₂ retention during underwater diving due to the high arterial O₂ content associated with hyperbaria. We tested the hypothesis that CB chemosensitivity to hypercapnia and hypoxia is attenuated during hyperbaria. Ten subjects completed two, 4-h dry dives at 1.6 atmosphere absolute (ATA) breathing either 21% O₂ (Air) or 100% O₂ (100% O₂). CB chemosensitivity was assessed using brief hypercapnic ventilatory response ([Formula: see text]) and hypoxic ventilatory response ([Formula: see text]) tests predive, 75 and 155 min into the dives, and 15 and 55 min postdive. End-tidal CO₂ pressure increased during the dive at 75 and 155 min [Air: +9 (SD 4) mmHg and +8 (SD 4) mmHg versus 100% O₂: +6 (SD 4) mmHg and +5 (SD 3) mmHg; all P < 0.01] and was higher while breathing Air (P < 0.01). [Formula: see text] was unchanged during the dive (P = 0.73) and was not different between conditions (P = 0.47). However, [Formula: see text] was attenuated from predive during the dive at 155 min breathing Air [-0.035 (SD 0.037) L·min·mmHg^-1; P = 0.02] and at both time points while breathing 100% O₂ [-0.035 (SD 0.052) L·min·mmHg^-1 and -0.034 (SD 0.064) L·min·mmHg^-1; P = 0.02 and P = 0.02, respectively]. These data indicate that the CB chemoreceptors do not appear to contribute to CO₂ retention in hyperbaria.NEW & NOTEWORTHY We demonstrate that carotid body chemosensitivity to brief exposures of hypercapnia was unchanged during a 4-h dive in a dry hyperbaric chamber at 1.6 ATA regardless of breathing gas condition [i.e., air (21% O₂) versus 100% oxygen]. Therefore, it appears that an attenuation of carotid body chemosensitivity to hypercapnia does not contribute to CO₂ retention in hyperbaria.

Arterial Blood Gas Analysis in Breath-Hold Divers at Depth

The present study aimed to evaluate the partial pressure of arterial blood gases in breath-hold divers performing a submersion at 40 m. Eight breath-hold divers were enrolled for the trials held at "Y-40 THE DEEP JOY" pool (Montegrotto Terme, Padova, Italy). Prior to submersion, an arterial cannula in the radial artery of the non-dominant limb was positioned. All divers performed a sled-assisted breath-hold dive to 40 m. Three blood samplings occurred: at 10 min prior to submersion, at 40 m depth, and within 2 min after diver's surfacing and after resuming normal ventilation. Blood samples were analyzed immediately on site. Six subjects completed the experiment, without diving-related problems. The theoretically predicted hyperoxia at the bottom was observed in 4 divers out of 6, while the other 2 experienced a reduction in the partial pressure of oxygen (paO2) at the bottom. There were no significant increases in arterial partial pressure of carbon dioxide (paCO2) at the end of descent in 4 of 6 divers, while in 2 divers paCO2 decreased. Arterial mean pH and mean bicarbonate (HCO 3 - ) levels exhibited minor changes. There was a statistically significant increase in mean arterial lactate level after the exercise. Ours was the first attempt to verify real changes in blood gases at a depth of 40 m during a breath-hold descent in free-divers. We demonstrated that, at depth, relative hypoxemia can occur, presumably caused by lung compression. Also, hypercapnia exists at depth, to a lesser degree than would be expected from calculations, presumably because of pre-dive hyperventilation and carbon dioxide distribution in blood and tissues.

The effect of water immersion and acute hypercapnia on ventilatory sensitivity and cerebrovascular reactivity

The partial pressure of end tidal carbon dioxide (PETCO2 ), ventilatory sensitivity to CO2 , and cerebral perfusion are augmented during thermoneutral head out water immersion (HOWI). We tested the hypotheses that HOWI and acute hypercapnia augments minute ventilation, ventilatory sensitivity to CO2 , cerebral perfusion, and cerebrovascular reactivity to CO2 . Twelve subjects (age: 24 ± 3 years, BMI: 25.3 ± 2.9 kg/m2 , 6 women) participated in two experimental visits: a HOWI visit (HOWI) and a matched hypercapnia visit (Dry + CO2 ). A rebreathing test was conducted at baseline, 10, 30, 60 min, and post HOWI and Dry + CO2 . PETCO2 , minute ventilation, expired gases, blood pressure, heart rate, and middle cerebral artery blood velocity were recorded continuously. PETCO2 increased throughout HOWI (baseline: 42 ± 2 mmHg; maximum at 10 min: 44 ± 2 mmHg, P ≤ 0.013) and Dry + CO2 (baseline: 42 ± 2 mmHg; maximum at 10 min: 44 ± 2 mmHg, P ≤ 0.013) and was matched between conditions (condition main effect: P = 0.494). Minute ventilation was lower during HOWI versus Dry + CO2 (maximum difference at 60 min: 13.2 ± 1.9 vs. 16.2 ± 2.7 L/min, P < 0.001). Ventilatory sensitivity to CO2 and middle cerebral artery blood velocity were greater during HOWI versus Dry + CO2 (maximum difference at 10 min: 2.60 ± 1.09 vs. 2.20 ± 1.05 L/min/mmHg, P < 0.001, and 63 ± 18 vs. 53 ± 14 cm/sec, P < 0.001 respectively). Cerebrovascular reactivity to CO2 decreased throughout HOWI and Dry + CO2 and was not different between conditions (condition main effect: P = 0.777). These data indicate that acute hypercapnia, matched to what occurs during HOWI, augments minute ventilation but not ventilatory sensitivity to CO2 or middle cerebral artery blood velocity despite an attenuated cerebrovascular reactivity to CO2 .

Peripheral chemosensitivity is not blunted during 2 h of thermoneutral head out water immersion in healthy men and women

Carbon dioxide (CO₂) retention occurs during water immersion, but it is not known if peripheral chemosensitivity is altered during water immersion, which could contribute to CO₂ retention. We tested the hypothesis that peripheral chemosensitivity to hypercapnia and hypoxia is blunted during 2 h of thermoneutral head out water immersion (HOWI) in healthy young adults. Peripheral chemosensitivity was assessed by the ventilatory, heart rate, and blood pressure responses to hypercapnia and hypoxia at baseline, 10, 60, 120 min, and post HOWI and a time‐control visit (control). Subjects inhaled 1 breath of 13% CO₂, 21% O₂, and 66% N₂ to test peripheral chemosensitivity to hypercapnia and 2–6 breaths of 100% N₂ to test peripheral chemosensitivity to hypoxia. Each gas was administered four separate times at each time point. Partial pressure of end‐tidal CO₂ (PETCO₂), arterial oxygen saturation (SpO₂), ventilation, heart rate, and blood pressure were recorded continuously. Ventilation was higher during HOWI versus control at post (P = 0.037). PETCO₂ was higher during HOWI versus control at 10 min (46 ± 2 vs. 44 ± 2 mmHg), 60 min (46 ± 2 vs. 44 ± 2 mmHg), and 120 min (46 ± 3 vs. 43 ± 3 mmHg) (all P < 0.001). Ventilatory (P = 0.898), heart rate (P = 0.760), and blood pressure (P = 0.092) responses to hypercapnia were not different during HOWI versus control at any time point. Ventilatory (P = 0.714), heart rate (P = 0.258), and blood pressure (P = 0.051) responses to hypoxia were not different during HOWI versus control at any time point. These data indicate that CO₂ retention occurs during thermoneutral HOWI despite no changes in peripheral chemosensitivity.

Assessment of the interaction of hyperbaric N2, CO2, and O2 on psychomotor performance in divers

Diving narcosis results from the complex interaction of gases, activities, and environmental conditions. We hypothesized that these interactions could be separated into their component parts. Where previous studies have tested single cognitive tasks sequentially, we varied inspired partial pressures of CO₂, N₂, and O₂ in immersed, exercising subjects while assessing multitasking performance with the Multi-Attribute Task Battery II (MATB-II) flight simulator. Cognitive performance was tested under 20 conditions of gas partial pressure and exercise in 42 male subjects meeting U.S. Navy age and fitness profiles. Inspired nitrogen (N₂) and oxygen (O₂) partial pressures were 0, 4.5, and 5.6 ATA and 0.21, 1.0, and 1.22 ATA, respectively, at rest and during 100-W immersed exercise with and without 0.075-ATA CO₂ Linear regression modeled the association of gas partial pressure with task performance while controlling for exercise, hypercapnic ventilatory response, dive training, video game frequency, and age. Subjects served as their own controls. Impairment of memory, attention, and planning, but not motor tasks, was associated with N₂ partial pressures >4.5 ATA. Sea level O₂ at 0.925 ATA partially rescued motor and memory reaction time impaired by 0.075-ATA CO₂; however, at hyperbaric pressures an unexpectedly strong interaction between CO₂, N₂, and exercise caused incapacitating narcosis with amnesia, which was augmented by O₂ Perception of narcosis was not correlated with actual scores. The relative contributions of factors associated with diving narcosis will be useful to predict the effects of gas mixtures and exercise conditions on the cognitive performance of divers. The O₂ effects are consistent with O₂ narcosis or enhanced O₂ toxicity.

Swimming-Induced Pulmonary Edema: Pathophysiology and Risk Reduction With Sildenafil

BACKGROUND

Swimming-induced pulmonary edema (SIPE) occurs during swimming or scuba diving, often in young individuals with no predisposing conditions, and its pathophysiology is poorly understood. This study tested the hypothesis that pulmonary artery and pulmonary artery wedge pressures are higher in SIPE-susceptible individuals during submerged exercise than in the general population and are reduced by sildenafil.

METHODS AND RESULTS

Ten study subjects with a history of SIPE (mean age, 41.6 years) and 20 control subjects (mean age, 36.2 years) were instrumented with radial artery and pulmonary artery catheters and performed moderate cycle ergometer exercise for 6 to 7 minutes while submersed in 20°C water. SIPE-susceptible subjects repeated the exercise 150 minutes after oral administration of 50 mg sildenafil. Work rate and mean arterial pressure during exercise were similar in controls and SIPE-susceptible subjects. Average o2 and cardiac output in controls and SIPE-susceptible subjects were: o2 2.42 L·min(-1) versus 1.95 L·min(-1), P=0.2; and cardiac output 17.9 L·min(-1) versus 13.8 L·min(-1), P=0.01. Accounting for differences in cardiac output between groups, mean pulmonary artery pressure at cardiac output=13.8 L·min(-1) was 22.5 mm Hg in controls versus 34.0 mm Hg in SIPE-susceptible subjects (P=0.004), and the corresponding pulmonary artery wedge pressure was 11.0 mm Hg versus 18.8 mm Hg (P=0.028). After sildenafil, there were no statistically significant differences in mean pulmonary artery pressure or pulmonary artery wedge pressure between SIPE-susceptible subjects and controls.

CONCLUSIONS

These observations confirm that SIPE is a form of hemodynamic pulmonary edema. The reduction in pulmonary vascular pressures after sildenafil with no adverse effect on exercise hemodynamics suggests that it may be useful in SIPE prevention.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00815646.

Human Physiology in an Aquatic Environment

Water covers over 70% of the earth, has varying depths and temperatures and contains much of the earth's resources. Head-out water immersion (HOWI) or submersion at various depths (diving) in water of thermoneutral (TN) temperature elicits profound cardiorespiratory, endocrine, and renal responses. The translocation of blood into the thorax and elevation of plasma volume by autotransfusion of fluid from cells to the vascular compartment lead to increased cardiac stroke volume and output and there is a hyperperfusion of some tissues. Pulmonary artery and capillary hydrostatic pressures increase causing a decline in vital capacity with the potential for pulmonary edema. Atrial stretch and increased arterial pressure cause reflex autonomic responses which result in endocrine changes that return plasma volume and arterial pressure to preimmersion levels. Plasma volume is regulated via a reflex diuresis and natriuresis. Hydrostatic pressure also leads to elastic loading of the chest, increasing work of breathing, energy cost, and thus blood flow to respiratory muscles. Decreases in water temperature in HOWI do not affect the cardiac output compared to TN; however, they influence heart rate and the distribution of muscle and fat blood flow. The reduced muscle blood flow results in a reduced maximal oxygen consumption. The properties of water determine the mechanical load and the physiological responses during exercise in water (e.g. swimming and water based activities). Increased hydrostatic pressure caused by submersion does not affect stroke volume; however, progressive bradycardia decreases cardiac output. During submersion, compressed gas must be breathed which introduces the potential for oxygen toxicity, narcosis due to nitrogen, and tissue and vascular gas bubbles during decompression and after may cause pain in joints and the nervous system.

Effects of elevated oxygen and carbon dioxide partial pressures on respiratory function and cognitive performance

Hyperoxia during diving has been suggested to exacerbate hypercapnic narcosis and promote unconsciousness. We tested this hypothesis in male volunteers (12 at rest, 10 at 75 W cycle ergometer exercise) breathing each of four gases in a hyperbaric chamber. Inspired Po2 (PiO2) was 0.21 and 1.3 atmospheres (atm) without or with an individual subject's maximum tolerable inspired CO2 (PiO2 = 0.055–0.085 atm). Measurements included end-tidal CO2 partial pressure (PetCO2), rating of perceived discomfort (RPD), expired minute ventilation (V̇e), and cognitive function assessed by auditory n-back test. The most prominent finding was, irrespective of PetCO2, that minute ventilation was 8–9 l/min greater for rest or exercise with a PiO2 of 1.3 atm compared with 0.21 atm ( P < 0.0001). For hyperoxic gases, PetCO2 was consistently less than for normoxic gases ( P < 0.01). For hyperoxic hypercapnic gases, n-back scores were higher than for normoxic gases ( P < 0.01), and RPD was lower for exercise but not rest ( P < 0.02). Subjects completed 66 hyperoxic hypercapnic trials without incident, but five stopped prematurely because of serious symptoms (tunnel vision, vision loss, dizziness, panic, exhaustion, or near syncope) during 69 normoxic hypercapnic trials ( P = 0.0582). Serious symptoms during hypercapnic trials occurred only during normoxia. We conclude serious symptoms with hyperoxic hypercapnia were absent because of decreased PetCO2 consequent to increased ventilation.

Hyperbaric conditions

Exposure to elevated ambient pressure (hyperbaric conditions) occurs most commonly in underwater diving, during which respired gas density and partial pressures, work of breathing, and physiological dead space are all increased. There is a tendency toward hypercapnia during diving, with several potential causes. Most importantly, there may be reduced responsiveness of the respiratory controller to rising arterial CO₂, leading to hypoventilation and CO₂ retention. Contributory factors may include elevated arterial PO₂, inert gas narcosis and an innate (but variable) tendency of the respiratory controller to sacrifice tight control of arterial CO₂ when work of breathing increases. Oxygen is usually breathed at elevated partial pressure under hyperbaric conditions. Oxygen breathing at modest hyperbaric pressure is used therapeutically in hyperbaric chambers to increase arterial carriage of oxygen and diffusion into tissues. However, to avoid cerebral and pulmonary oxygen toxicity during underwater diving, both the magnitude and duration of oxygen exposure must be managed. Therefore, most underwater diving is conducted breathing mixtures of oxygen and inert gases such as nitrogen or helium, often simply air. At hyperbaric pressure, tissues equilibrate over time with high inspired inert gas partial pressure. Subsequent decompression may reduce ambient pressure below the sum of tissue gas partial pressures (supersaturation) which can result in tissue gas bubble formation and potential injury (decompression sickness). Risk of decompression sickness is minimized by scheduling time at depth and decompression rate to limit tissue supersaturation or size and profusion of bubbles in accord with models of tissue gas kinetics and bubble formation and growth.

Simulations for mechanical ventilation in children: review and future prospects

Mechanical ventilation is a very effective therapy, but with many complications. Simulators are used in many fields, including medicine, to enhance safety issues. In the intensive care unit, they are used for teaching cardiorespiratory physiology and ventilation, for testing ventilator performance, for forecasting the effect of ventilatory support, and to determine optimal ventilatory management. They are also used in research and development of clinical decision support systems (CDSSs) and explicit computerized protocols in closed loop. For all those reasons, cardiorespiratory simulators are one of the tools that help to decrease mechanical ventilation duration and complications. This paper describes the different types of simulators described in the literature for physiologic simulation and modeling of the respiratory system, including a new simulator (SimulResp), and proposes a validation process for these simulators.

Relative effects of submersion and increased pressure on respiratory mechanics, work, and energy cost of breathing

Submersion and increased pressure (depth) characterize the diving environment and may independently increase demand on the respiratory system. To quantify changes in respiratory mechanics, this study employed a unique protocol and techniques to measure, in a hyperbaric chamber, inspiratory and expiratory alveolar pressures (interrupter technique), inspiratory and expiratory resistance in the airways (RawI and RawE, esophageal balloon technique), nitric oxide elimination (thought to correlate with Raw), inspiratory and expiratory mechanical power of breathing, and the total energy cost of ventilation. Eight healthy adult men underwent experiments at 1, 2.7, and 4.6 atmospheres absolute (ATA) in dry and fully submersed conditions. Subjects rested, cycled on an ergometer at 100 W, and rested while voluntarily matching their ventilation to their own exercise hyperpnea (isocapnic simulated exercise ventilation). During isocapnic simulated exercise ventilation, increased O2 uptake (above rest values) resulted from increased expired ventilation. RawI decreased with submersion (mean 43% during rest and 20% during exercise) but increased from 1 to 4.6 ATA (19% during rest and 75% during exercise), as did RawE (53% decrease with submersion during rest and 10% during exercise; 9% increase from 1 to 4.6 ATA during rest and 66% during exercise). Nitric oxide elimination did not correlate with Raw. Depth increased inspiratory mechanical power of breathing during rest (40%) and exercise (20%). Expiratory mechanical power of breathing was largely unchanged. These results suggest that the diving environment affects ventilatory mechanics primarily by increasing Raw, secondary to increased gas density. This necessitates increased alveolar pressure and increases the work and energy cost of breathing as the diver descends. These findings can inform physician assessment of diver fitness and the pulmonary risks of hyperbaric O2 therapy.

Vasoconstrictor responsiveness during hyperbaric hyperoxia in contracting human muscle

Large increases in systemic oxygen content cause substantial reductions in exercising forearm blood flow (FBF) due to increased vascular resistance. We hypothesized that 1) functional sympatholysis (blunting of sympathetic α-adrenergic vasoconstriction) would be attenuated during hyperoxic exercise and 2) α-adrenergic blockade would limit vasoconstriction during hyperoxia and increase FBF to levels observed under normoxic conditions. Nine male subjects (age 28 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Studies were performed in a hyperbaric chamber at 1 atmosphere absolute (ATA; sea level) while breathing 21% O(2) and at 2.82 ATA while breathing 100% O(2) (estimated change in arterial O(2) content ∼6 ml O(2)/100 ml). FBF (ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated from FBF and blood pressure (arterial catheter). Vasoconstrictor responsiveness was determined using intra-arterial tyramine. FBF and FVC were substantially lower during hyperoxic exercise than normoxic exercise (∼20-25%; P < 0.01). At rest, vasoconstriction to tyramine (% decrease from pretyramine values) did not differ between normoxia and hyperoxia (P > 0.05). During exercise, vasoconstrictor responsiveness was slightly greater during hyperoxia than normoxia (-22 ± 3 vs. -17 ± 2%; P < 0.05). However, during α-adrenergic blockade, hyperoxic exercise FBF and FVC remained lower than during normoxia (P < 0.01). Therefore, our data suggest that although the vasoconstrictor responsiveness during hyperoxic exercise was slightly greater, it likely does not explain the majority of the large reductions in FBF and FVC (∼20-25%) during hyperbaric hyperoxic exercise.

Hyperbaric hyperoxia reduces exercising forearm blood flow in humans

Hypoxia during exercise augments blood flow in active muscles to maintain the delivery of O(2) at normoxic levels. However, the impact of hyperoxia on skeletal muscle blood flow during exercise is not completely understood. Therefore, we tested the hypothesis that the hyperemic response to forearm exercise during hyperbaric hyperoxia would be blunted compared with exercise during normoxia. Seven subjects (6 men/1 woman; 25 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Forearm blood flow (FBF; in ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC; in ml·min(-1)·100 mmHg(-1)) was calculated from FBF and blood pressure (in mmHg; brachial arterial catheter). Studies were performed in a hyperbaric chamber with the subjects supine at 1 atmospheres absolute (ATA) (sea level) while breathing normoxic gas [21% O(2), 1 ATA; inspired Po(2) (Pi(O(2))) ≈ 150 mmHg] and at 2.82 ATA while breathing hyperbaric normoxic (7.4% O(2), 2.82 ATA, Pi(O(2)) ≈ 150 mmHg) and hyperoxic (100% O(2), 2.82 ATA, Pi(O(2)) ≈ 2,100 mmHg) gas. Resting FBF and FVC were less during hyperbaric hyperoxia compared with hyperbaric normoxia (P < 0.05). The change in FBF and FVC (Δ from rest) during exercise under normoxia (204 ± 29 ml/min and 229 ± 37 ml·min(-1)·100 mmHg(-1), respectively) and hyperbaric normoxia (203 ± 28 ml/min and 217 ± 35 ml·min(-1)·100 mmHg(-1), respectively) did not differ (P = 0.66-0.99). However, the ΔFBF (166 ± 21 ml/min) and ΔFVC (163 ± 23 ml·min(-1)·100 mmHg(-1)) during hyperbaric hyperoxia were substantially attenuated compared with other conditions (P < 0.01). Our data suggest that exercise hyperemia in skeletal muscle is highly dependent on oxygen availability during hyperoxia.

Risk factors for immersion pulmonary edema: hyperoxia does not attenuate pulmonary hypertension associated with cold water-immersed prone exercise at 4.7 ATA

Hyperoxia has been shown to attenuate the increase in pulmonary artery (PA) pressure associated with immersed exercise in thermoneutral water, which could serve as a possible preventive strategy for the development of immersion pulmonary edema (IPE). We tested the hypothesis that the same is true during exercise in cold water. Six healthy volunteers instrumented with arterial and PA catheters were studied during two 16-min exercise trials during prone immersion in cold water (19.9-20.9°C) in normoxia [0.21 atmospheres absolute (ATA)] and hyperoxia (1.75 ATA) at 4.7 ATA. Heart rate (HR), Fick cardiac output (CO), mean arterial pressure (MAP), pulmonary artery pressure (PAP), pulmonary artery wedge pressure (PAWP), central venous pressure (CVP), arterial and venous blood gases, and ventilatory parameters were measured both early (E, 5-6 min) and late (L, 15-16 min) in exercise. During exercise at an average oxygen consumption rate (Vo(2)) of 2.38 l/min, [corrected] CO, CVP, and pulmonary vascular resistance were not affected by inspired (Vo(2)) [corrected] or exercise duration. Minute ventilation (Ve), alveolar ventilation (Va), and ventilation frequency (f) were significantly lower in hyperoxia compared with normoxia (mean ± SD: Ve 58.8 ± 8.0 vs. 65.1 ± 9.2, P = 0.003; Va 40.2 ± 5.4 vs. 44.2 ± 9.0, P = 0.01; f 25.4 ± 5.4 vs. 27.2 ± 4.2, P = 0.04). Mixed venous pH was lower in hyperoxia compared with normoxia (7.17 ± 0.07 vs. 7.20 ± 0.07), and this result was significant early in exercise (P = 0.002). There was no difference in mean PAP (MPAP: 28.28 ± 8.1 and 29.09 ± 14.3 mmHg) or PAWP (18.0 ± 7.6 and 18.7 ± 8.7 mmHg) between normoxia and hyperoxia, respectively. PAWP decreased from early to late exercise in hyperoxia (P = 0.002). These results suggest that the increase in pulmonary vascular pressures associated with cold water immersion is not attenuated with hyperoxia.

Effects of hyperoxia on ventilation and pulmonary hemodynamics during immersed prone exercise at 4.7 ATA: possible implications for immersion pulmonary edema

Immersion pulmonary edema (IPE) can occur in otherwise healthy swimmers and divers, likely because of stress failure of pulmonary capillaries secondary to increased pulmonary vascular pressures. Prior studies have revealed progressive increase in ventilation [minute ventilation (Ve)] during prolonged immersed exercise. We hypothesized that this increase occurs because of development of metabolic acidosis with concomitant rise in mean pulmonary artery pressure (MPAP) and that hyperoxia attenuates this increase. Ten subjects were studied at rest and during 16 min of exercise submersed at 1 atm absolute (ATA) breathing air and at 4.7 ATA in normoxia and hyperoxia [inspired P(O(2)) (Pi(O(2))) 1.75 ATA]. Ve increased from early (E, 6th minute) to late (L, 16th minute) exercise at 1 ATA (64.1 +/- 8.6 to 71.7 +/- 10.9 l/min BTPS; P < 0.001), with no change in arterial pH or Pco(2). MPAP decreased from E to L at 1 ATA (26.7 +/- 5.8 to 22.7 +/- 5.2 mmHg; P = 0.003). Ve and MPAP did not change from E to L at 4.7 ATA. Hyperoxia reduced Ve (62.6 +/- 10.5 to 53.1 +/- 6.1 l/min BTPS; P < 0.0001) and MPAP (29.7 +/- 7.4 to 25.1 +/- 5.7 mmHg, P = 0.002). Variability in MPAP among subjects was wide (range 14.1-42.1 mmHg during surface and depth exercise). Alveolar-arterial Po(2) difference increased from E to L in normoxia, consistent with increased lung water. We conclude that increased Ve at 1 ATA is not due to acidosis and is more consistent with respiratory muscle fatigue and that progressive pulmonary vascular hypertension does not occur during prolonged immersed exercise. Wide variation in MPAP among healthy subjects is consistent with variable individual susceptibility to IPE.

Pulmonary gas exchange in diving

Diving-related pulmonary effects are due mostly to increased gas density, immersion-related increase in pulmonary blood volume, and (usually) a higher inspired Po2. Higher gas density produces an increase in airways resistance and work of breathing, and a reduced maximum breathing capacity. An additional mechanical load is due to immersion, which can impose a static transrespiratory pressure load as well as a decrease in pulmonary compliance. The combination of resistive and elastic loads is largely responsible for the reduction in ventilation during underwater exercise. Additionally, there is a density-related increase in dead space/tidal volume ratio (Vd/Vt), possibly due to impairment of intrapulmonary gas phase diffusion and distribution of ventilation. The net result of relative hypoventilation and increased Vd/Vt is hypercapnia. The effect of high inspired Po2and inert gas narcosis on respiratory drive appear to be minimal. Exchange of oxygen by the lung is not impaired, at least up to a gas density of 25 g/l. There are few effects of pressure per se, other than a reduction in the P50 of hemoglobin, probably due to either a conformational change or an effect of inert gas binding.

Effects of head and body cooling on hemodynamics during immersed prone exercise at 1 ATA

Immersion pulmonary edema (IPE) is a condition with sudden onset in divers and swimmers suspected to be due to pulmonary arterial or venous hypertension induced by exercise in cold water, although it does occur even with adequate thermal protection. We tested the hypothesis that cold head immersion could facilitate IPE via a reflex rise in pulmonary vascular pressure due solely to cooling of the head. Ten volunteers were instrumented with ECG and radial and pulmonary artery catheters and studied at 1 atm absolute (ATA) during dry and immersed rest and exercise in thermoneutral (29-31 degrees C) and cold (18-20 degrees C) water. A head tent varied the temperature of the water surrounding the head independently of the trunk and limbs. Heart rate, Fick cardiac output (CO), mean arterial pressure (MAP), mean pulmonary artery pressure (MPAP), pulmonary artery wedge pressure (PAWP), and central venous pressure (CVP) were measured. MPAP, PAWP, and CO were significantly higher in cold pool water (P < or = 0.004). Resting MPAP and PAWP values (means +/- SD) were 20 +/- 2.9/13 +/- 3.9 (cold body/cold head), 21 +/- 3.1/14 +/- 5.2 (cold/warm), 14 +/- 1.5/10 +/- 2.2 (warm/warm), and 15 +/- 1.6/10 +/- 2.6 mmHg (warm/cold). Exercise values were higher; cold body immersion augmented the rise in MPAP during exercise. MAP increased during immersion, especially in cold water (P < 0.0001). Except for a transient additive effect on MAP and MPAP during rapid head cooling, cold water on the head had no effect on vascular pressures. The results support a hemodynamic cause for IPE mediated in part by cooling of the trunk and extremities. This does not support the use of increased head insulation to prevent IPE.