Neal W. Pollock

Microparticle production, neutrophil activation, and intravascular bubbles following open-water SCUBA diving

The goal of this study was to evaluate annexin V-positive microparticles (MPs) and neutrophil activation in humans following decompression from open-water SCUBA diving with the hypothesis that changes are related to intravascular bubble formation. Sixteen male volunteer divers followed a uniform profile of four daily SCUBA dives to 18 m of sea water for 47 min. Blood was obtained prior to and at 80 min following the first and fourth dives to evaluate the impact of repetitive diving, and intravascular bubbles were quantified by trans-thoracic echocardiography carried out at 20-min intervals for 2 h after each dive. MPs increased by 3.4-fold after each dive, neutrophil activation occurred as assessed by surface expression of myeloperoxidase and the CD18 component of β(2)-integrins, and there was an increased presence of the platelet-derived CD41 protein on the neutrophil surface indicating interactions with platelet membranes. Intravascular bubbles were detected in all divers. Surprisingly, significant inverse correlations were found among postdiving bubble scores and MPs, most consistently at 80 min or more after the dive on the fourth day. There were significant positive correlations between MPs and platelet-neutrophil interactions after the first dive and between platelet-neutrophil interactions and neutrophil activation documented as an elevation in β(2)-integrin expression after the fourth dive. We conclude that MPs- and neutrophil-related events in humans are consistent with findings in an animal decompression model. Whether there are causal relationships among bubbles, MPs, platelet-neutrophil interactions, and neutrophil activation remains obscure and requires additional study.

Bubbles, microparticles, and neutrophil activation: changes with exercise level and breathing gas during open-water SCUBA diving

The study goal was to evaluate responses in humans following decompression from open-water SCUBA diving with the hypothesis that exertion underwater and use of a breathing mixture containing more oxygen and less nitrogen (enriched air nitrox) would alter annexin V-positive microparticle (MP) production and size changes and neutrophil activation, as well as their relationships to intravascular bubble formation. Twenty-four divers followed a uniform dive profile to 18 m of sea water breathing air or 22.5 m breathing 32% oxygen/68% nitrogen for 47 min, either swimming with moderately heavy exertion underwater or remaining stationary at depth. Blood was obtained pre- and at 15 and 120 min postdive. Intravascular bubbles were quantified by transthoracic echocardiography postdive at 20-min intervals for 2 h. There were no significant differences in maximum bubble scores among the dives. MP number increased 2.7-fold, on average, within 15 min after each dive; only the air-exertion dive resulted in a significant further increase to 5-fold over baseline at 2 h postdive. Neutrophil activation occurred after all dives. For the enriched air nitrox stationary at depth dive, but not for other conditions, the numbers of postdive annexin V-positive particles above 1 μm in diameter were correlated with intravascular bubble scores (correlation coefficients ∼0.9, P < 0.05). We conclude that postdecompression relationships among bubbles, MPs, platelet-neutrophil interactions, and neutrophil activation appear to exist, but more study is required to improve confidence in the associations.

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.

Predictors of increased PaCO2 during immersed prone exercise at 4.7 ATA.

During diving, arterial Pco(2) (Pa(CO(2))) levels can increase and contribute to psychomotor impairment and unconsciousness. This study was designed to investigate the effects of the hypercapnic ventilatory response (HCVR), exercise, inspired Po(2), and externally applied transrespiratory pressure (P(tr)) on Pa(CO(2)) during immersed prone exercise in subjects breathing oxygen-nitrogen mixes at 4.7 ATA. Twenty-five subjects were studied at rest and during 6 min of exercise while dry and submersed at 1 ATA and during exercise submersed at 4.7 ATA. At 4.7 ATA, subsets of the 25 subjects (9-10 for each condition) exercised as P(tr) was varied between +10, 0, and -10 cmH(2)O; breathing gas Po(2) was 0.7, 1.0, and 1.3 ATA; and inspiratory and expiratory breathing resistances were varied using 14.9-, 11.6-, and 10.2-mm-diameter-aperture disks. During exercise, Pa(CO(2)) (Torr) increased from 31.5 +/- 4.1 (mean +/- SD for all subjects) dry to 34.2 +/- 4.8 (P = 0.02) submersed, to 46.1 +/- 5.9 (P < 0.001) at 4.7 ATA during air breathing and to 49.9 +/- 5.4 (P < 0.001 vs. 1 ATA) during breathing with high external resistance. There was no significant effect of inspired Po(2) or P(tr) on Pa(CO(2)) or minute ventilation (Ve). Ve (l/min) decreased from 89.2 +/- 22.9 dry to 76.3 +/- 20.5 (P = 0.02) submersed, to 61.6 +/- 13.9 (P < 0.001) at 4.7 ATA during air breathing and to 49.2 +/- 7.3 (P < 0.001) during breathing with resistance. We conclude that the major contributors to increased Pa(CO(2)) during exercise at 4.7 ATA are increased depth and external respiratory resistance. HCVR and maximal O(2) consumption were also weakly predictive. The effects of P(tr), inspired Po(2), and O(2) consumption during short-term exercise were not significant.

Influence of Repeated Daily Diving on Decompression Stress

Acclimatization (an adaptive change in response to repeated environmental exposure) to diving could reduce decompression stress. A decrease in post-dive circulating venous gas emboli (VGE or bubbles) would represent positive acclimatization. The purpose of this study was to determine whether four days of daily diving alter post-dive bubble grades. 16 male divers performed identical no-decompression air dives on 4 consecutive days to 18 meters of sea water for 47 min bottom times. VGE monitoring was performed with transthoracic echocardiography every 20 min for 120 min post-dive. Completion of identical daily dives resulted in progressively decreasing odds (or logit risk) of having relatively higher grade bubbles on consecutive days. The odds on Day 4 were half that of Day 1 (OR 0.50, 95% CI: 0.34, 0.73). The odds ratio for a >III bubble grade on Day 4 was 0.37 (95% CI: 0.20, 0.70) when compared to Day 1. The current study indicates that repetitive daily diving may reduce bubble formation, representing a positive (protective) acclimatization to diving. Further work is required to evaluate the impact of additional days of diving and multiple dive days and to determine if the effect is sufficient to alter the absolute risk of decompression sickness.

Evaluation of the System O2 Inc portable nonpressurized oxygen delivery system

OBJECTIVE To evaluate the performance of the System O2 portable non-pressurized delivery system (SysO2). This device produces oxygen through chemical reaction and might have utility for emergency/field use. METHODS Performance was evaluated with 10 unmanned trials conducted under standard laboratory conditions. Measures included oxygen flow (mean and peak), total oxygen yield, and system weight-indexed yield. RESULTS Oxygen flow peaked at 5.74 +/- 0.28 L x min(-1) (mean +/- SD) at 16.9 +/- 1.5 minutes before rapidly falling to zero. Mean flow was 2.98 +/- 1.52 L x min(-1) with a total yield of 62.9 +/- 6.6 L. Mean oxygen fraction was 0.96 +/- 0.15. The weight per unit of oxygen is substantially higher than for commercially available pressurized cylinders; e.g., 47.7 vs. 10.2 g x L(-1) for the small 246 L M9 cylinder. CONCLUSIONS Given the limited flow rate and supply duration, we believe the SysO2 system does not offer significant advantage over the available pressurized oxygen systems as a source for emergency oxygen.

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 subjects 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.

Chemical Oxygen Generation: Evaluation of the Green Dot Systems, Inc Portable, Nonpressurized emOx Device

OBJECTIVE To evaluate the performance of the emOx emergency powdered oxygen portable nonpressurized delivery system. This device produces oxygen through chemical reaction and is marketed for emergency first aid use until professional medical assistance is available. METHODS Seven unmanned trials were conducted under standard laboratory conditions. Measures included oxygen flow, reaction canister external wall temperature, delivered gas temperature, and delivered gas relative humidity. RESULTS The mean oxygen flow was 1.75 ± 1.58 L x min(-1) (mean ± SD) with a total yield of 40.4 ± 2.6 L. Oxygen flow increased slowly and with substantial variability between reactant groups, exceeding 2.0 L x min(-1) after 15.7 ± 6.4 minutes of operation. Oxygen flow briefly peaked at 5.93 ± 0.56 L x min(-1) at 17.8 ± 7.9 minutes before rapidly falling to zero. The mean oxygen fraction was 0.81 ± 0.28, exceeding 0.96 in 10.7 ± 2.9 minutes. The reaction canister external wall temperature reached 54.7 ± 7.4 °C. Delivered gas temperature varied little from ambient. Delivered gas relative humidity surpassed 75% in 8 ± 3 minutes and 90% in 15 ± 5 minutes of operation. CONCLUSIONS A readily available, high concentration oxygen supply could have utility to manage many conditions in advance of the arrival of professional emergency medical services (EMS). Unfortunately, the highly variable activation time and low average oxygen flow rate make the rapid deployment value of the emOx equivocal. The limited total oxygen yield makes it inappropriate for conditions demanding significant oxygen resources. Advancement in oxygen concentrator systems likely holds far more promise than powdered chemical oxygen generation for first aid and emergency medical applications.

Performance Characteristics of the Second-Generation Remote Emergency Medical Oxygen Closed-Circuit Rebreather

Abstract Objective.—Closed-circuit oxygen rebreathers may provide high concentrations of oxygen at extremely low flow rates appropriate for field use with limited oxygen supplies. The performance of the preproduction, second-generation remote emergency medical oxygen (REMO2) system developed for Divers Alert Network was evaluated. Methods.—The unidirectional circuit was made up of a solid, prepackaged CO2 scrubber canister (984 ± 14 [SD] g scrubber mass), standard 22-mm-inside-diameter anesthesia circuit hoses, 5-L breathing bag, 5-cm H2O positive end-expiratory pressure valve, and oronasal mask. Oxygen flow, inspired oxygen, expired CO2, peak inspired and expired mask pressures, time to reach scrubber canister saturation or “breakthrough” (postscrubber CO2 concentration reaching 3.8 mm Hg), and subject tolerance were measured under standard laboratory conditions. Results.—Six trials were completed using healthy volunteers (94.7 ± 19.6 kg). Five of the 6 completed trials did not reach breakthrough at the planned trial limit of 8 hours. Mean average oxygen flow rate was 1.00 ± 0.17 L·min−1. Mean peak inspired and expired mask pressures were −5.0 ± 1.9 and 6.5 ± 1.9 cm H2O, respectively. Subjects generally reported good tolerance to circuit breathing. Conclusions.—The second-generation REMO2 was well tolerated by healthy subjects during 8-hour laboratory evaluation trials. The device provided high mean inspired oxygen fractions at low mean oxygen flow rates, relatively modest mean maximal inspired and expired pressures, and excellent scrubber canister duration. Further evaluation of field performance with a patient population is warranted.

What is wilderness medicine

Defining the field of wilderness medicine can be challenging, particularly for those who do not participate. The founders had a vision of a specialty that incorporates the essentials of practicing medicine in the outdoors without the “luxuries” of a hospital or medical clinic. Rumors abound of the early naming debates— should this group of researchers and practitioners be named the “Wilderness Medical Society,” the “Mountain Medicine Society,” or another, more specific title? Mountain medicine includes high altitude medicine, hypothermia, frostbite, and avalanche injuries, to name a few. Wilderness medicine encompasses mountain medicine, but its scope reaches far beyond. Dive and marine medicine, plant toxinology, animal attacks, and search and rescue all fall within this broad field. Difficult access to patients and environmental extremes are common elements that produce an array of challenges to medical practice. This issue is a tribute to the expansive range of topics that comprise wilderness medicine. Scanning the Table of