Rational Use of Oxygen in COVID-19 Pandemic – Are We Doing Enough?

During the episodes of large case surge of COVID-19, the health care system of many nations have struggled, more so in nations with resource limitations. Recently, Nepal and the neighboring nation India are being hit hard by the pandemic. Management of patients with moderate and severe COVID-19 remains largely supportive, with oxygen therapy being the cornerstone of the management. Procurement, maintenance of oxygen supply system, coupled with avoiding misuse and wastage of oxygen is of paramount importance to better utilize the scarce resources amidst the peaks of a pandemic. Nepal needs to adopt policies to make best use of its oxygen stores and supplies with a collective effort from all stakeholders to save additional lives.

NOTA SUI LIMITI FISIOLOGICI DELLE IMMERSIONI PROFONDE IN APNEA

This article discusses the limits of deep breath-hold diving in humans. After a short historical introduction and a discussion of the evolution of depth records, the classical theories of breath-hold diving limits are presented and discussed, namely that of the ratio between total lung capacity and residual volume and that of blood shift, implying an increase in central blood volume. Then the current vision is introduced, based on the principles of the energetics of muscular exercise. The new vision has turned the classical vision upside down, moving the discussion to a different level. A direct consequence of the new theory is the importance of having large lung volumes at the start of a dive, in order to increase body oxygen stores. I finally discuss the role of anaerobic lactic metabolism as a possible mechanism of oxygen preservation, thus prolonging breath-hold duration.

In Vivo Measurements of Lung Volumes in Ringed Seals: Insights from Biomedical Imaging

Marine mammals rely on oxygen stored in blood, muscle, and lungs to support breath-hold diving and foraging at sea. Here, we used biomedical imaging to examine lung oxygen stores and other key respiratory parameters in living ringed seals (Pusa hispida). Three-dimensional models created from computed tomography (CT) images were used to quantify total lung capacity (TLC), respiratory dead space, minimum air volume, and total body volume to improve assessments of lung oxygen storage capacity, scaling relationships, and buoyant force estimates. Results suggest that lung oxygen stores determined in vivo are smaller than those derived from postmortem measurements. We also demonstrate that—while established allometric relationships hold well for most pinnipeds—these relationships consistently overestimate TLC for the smallest phocid seal. Finally, measures of total body volume reveal differences in body density and net vertical forces in the water column that influence costs associated with diving and foraging in free-ranging seals.

Safety proposals for freediving time limits should consider the metabolic-rate dependence of oxygen stores depletion

(Sadler C, Brett K, Heerboth A, Swisher AR, Mehregani N, Touriel R, Cannon DT. Safety proposals for freediving time limits should consider the metabolic-rate dependence of oxygen stores depletion. Diving and Hyperbaric Medicine. 2020 December 20;50(4):356–362. doi: 10.28920/dhm50.4.356-362. PMID: 33325016.) Introduction: There is no required training for breath-hold diving, making dissemination of safety protocols difficult. A recommended breath-hold dive time limit of 60 s was proposed for amateur divers. However, this does not consider the metabolic-rate dependence of oxygen stores depletion. We aimed to measure the effect of apnoea time and metabolic rate on arterial and tissue oxygenation. Methods: Fifty healthy participants (23 (SD 3) y, 22 women) completed four periods of apnoea for 60 s (or to tolerable limit) during rest and cycle ergometry at 20, 40, and 60 W. Apnoea was initiated after hyperventilation to achieve PETCO2 of approximately 25 mmHg. Pulse oximetry, frontal lobe oxygenation, and pulmonary gas exchange were measured throughout. We defined hypoxia as SpO2 < 88%. Results: Static and exercise (20, 40, 60 W) breath-hold break times were 57 (SD 7), 50 (11), 48 (11), and 46 (11) s (F [2.432, 119.2] = 32.0, P < 0.01). The rise in PETCO2 from initiation to breaking of apnoea was dependent on metabolic rate (time × metabolic rate interaction; F [3,147] = 38.6, P < 0.0001). The same was true for the fall in SpO2 (F [3,147] = 2.9, P = 0.03). SpO2 fell to < 88% on 14 occasions in eight participants, all of whom were asymptomatic. Conclusions: Independent of the added complexities of a fall in ambient pressure on ascent, the effect of apnoea time on hypoxia depends on the metabolic rate and is highly variable among individuals. Therefore, we contend that a universally recommended time limit for breath-hold diving or swimming is not useful to guarantee safety.

Postnatal development of diving physiology: implications of anthropogenic disturbance for immature marine mammals

ABSTRACTMarine mammals endure extended breath-holds while performing active behaviors, which has fascinated scientists for over a century. It is now known that these animals have large onboard oxygen stores and utilize oxygen-conserving mechanisms to prolong aerobically supported dives to great depths, while typically avoiding (or tolerating) hypoxia, hypercarbia, acidosis and decompression sickness (DCS). Over the last few decades, research has revealed that diving physiology is underdeveloped at birth. Here, I review the postnatal development of the body's oxygen stores, cardiorespiratory system and other attributes of diving physiology for pinnipeds and cetaceans to assess how physiological immaturity makes young marine mammals vulnerable to disturbance. Generally, the duration required for body oxygen stores to mature varies across species in accordance with the maternal dependency period, which can be over 2 years long in some species. However, some Arctic and deep-diving species achieve mature oxygen stores comparatively early in life (prior to weaning). Accelerated development in these species supports survival during prolonged hypoxic periods when calves accompany their mothers under sea ice and to the bathypelagic zone, respectively. Studies on oxygen utilization patterns and heart rates while diving are limited, but the data indicate that immature marine mammals have a limited capacity to regulate heart rate (and hence oxygen utilization) during breath-hold. Underdeveloped diving physiology, in combination with small body size, limits diving and swimming performance. This makes immature marine mammals particularly vulnerable to mortality during periods of food limitation, habitat alterations associated with global climate change, fishery interactions and other anthropogenic disturbances, such as exposure to sonar.

Heart rate and its relationship with activity in free-ranging Cheloniidae sea turtles

The primary oxygen stores in Cheloniidae sea turtles are in the lungs. Therefore, management of blood oxygen transportation to peripheral tissues by cardiovascular adjustments while diving is crucial to maximize benefits from dives. However, heart rate, particularly cardiac response to exercise in free-ranging dives, has rarely been examined for sea turtles. In this study, heart rate and its relationship with the amount of activity were determined in six free-ranging green turtles using bio-logging techniques. Our results demonstrated that resting heart rate took 7–11 h to reduce to steady levels after turtles were released in the tank, indicating that turtles may not present normal physiological rates right after release. After heart rate reduction, resting heart rate of green turtles in free-ranging dives was generally low (10.9 ± 2.5 bpm), but they often presented arrhythmia (4–54 bpm) even in resting states. The amount of activity during a dive linearly increased heart rate, but maximum heart rates (39.0–69.8 bpm) were recorded during ventilation at surface. These results indicate that turtles have the capability of cardiac response to increased metabolic demands in their muscles while submerged, and also of cardiovascular adjustment for a rapid renewal of oxygen stores and removal of CO2 during ventilation. Such well-organized cardiac adjustments may be because of characteristics of Cheloniidae sea turtles such as ectothermy and oxygen storage in lungs while submerged.Summary statementGreen sea turtles in free-ranging dive had generally lower heart rate compared to other air-breathing divers and it varied with the amount of exercise. Turtles often showed extreme arrhythmia.

Swimming mechanics and propulsive efficiency in the chambered nautilus

The chambered nautilus ( Nautilus pompilius ) encounters severe environmental hypoxia during diurnal vertical movements in the ocean. The metabolic cost of locomotion ( C met ) and swimming performance depend on how efficiently momentum is imparted to the water and how long on-board oxygen stores last. While propulsive efficiency is generally thought to be relatively low in jet propelled animals, the low C met in Nautilus indicates that this is not the case. We measured the wake structure in Nautilus during jet propulsion swimming, to determine their propulsive efficiency. Animals swam with either an anterior-first or posterior-first orientation. With increasing swimming speed, whole cycle propulsive efficiency increased during posterior-first swimming but decreased during anterior-first swimming, reaching a maximum of 0.76. The highest propulsive efficiencies were achieved by using an asymmetrical contractile cycle in which the fluid ejection phase was relatively longer than the refilling phase, reducing the volume flow rate of the ejected fluid. Our results demonstrate that a relatively high whole cycle propulsive efficiency underlies the low C met in Nautilus , representing a strategy to reduce the metabolic demands in an animal that spends a significant part of its daily life in a hypoxic environment.