Inspired O2 Concentration Research Articles

Control of Breathing in Acrochordus javanicus, an Aquatic Snake

Ventilation and oxygen uptake were measured in the elephant trunk snake, Acrochordus javanicus, during normal, undisturbed breathing. Ventilation is periodic, with ventilatory periods (VP) lasting 2.5-3.5 min, interrupted by nonventilatory periods (NVP) averaging 10 times longer at 25 C. Average tidal volumes ranged from 27.8 to 47.7 ml·kg⁻¹, while breathing frequency during VP ranged from 78 h⁻¹ to 155 h⁻¹. Total ventilation (V̇I) ranged from 250 to 543 ml·kg⁻¹·h⁻¹ BTPS (body temperature and pressure, saturated with water vapor). Oxygen uptake (V̇o2) averaged 16.6 mlO₂· kg⁻¹·h⁻¹ STPD (standard temperature and pressure, dry) at 20 C and 29.3 mlO₂· kg⁻¹·h⁻¹ at 30 C. Corresponding ventilations were 225 ml·kg⁻¹·h⁻¹ and 504 ml·2kg⁻¹· h⁻¹ BTPS. Ventilatory requirements, V̇I/V̇o2, were 13.8 and 17.2 for 20 and 30 C. Both ventilation and oxygen uptake are low in Acrochordus, compared with other reptiles. Also, the V̇I/V̇o2 ratio is much lower for Acrochordus than for most reptiles. A well-developed, richly vascularized lung and the long NVP contribute to unusually high extractions of O₂ from the ventilated air. The mode of breathing of Acrochordus allows it to stay submerged more than 90% of total lapsed time. During hypoxic breathing, ventilation is stimulated when inspired O₂ concentration is less than 10%. Large alveolar-to-blood Po2 gradients indicate that blood and lung O₂ stores are nearly exhausted when the hypoxic stimulus breaks the breath holds. The increased ventilation resulted from a shortening of NVP while tidal volumes and the frequency of breathing during VP remained unchanged. Breathing 100% O₂ significantly reduced ventilation. Hypercapnic breathing showed no ventilatory change when breathing 2% and 4% CO₂. Breathing 6% and 8/% caused an actual depression of ventilation. The results indicate an effective elimination of CO₂ by cutaneous exchange, which may account for the lack of a ventilatory response to CO₂ breathing. Ventilation in Acrochordus appears controlled for maximum utilization of oxygen stores and rapid recovery during breathing.

RATED PERCEIVED EXERTION ASSOCIATED WITH BREATHING HYPEROXIC MIXTURES DURING SUBMAXIMAL WORK

The effect of an enriched inspired oxygen concentration on perceived exertion (RPE) was investigated while running at two submaximal treadmill loads. Twelve males (VO2 max = 49.3 ml/kg-min) worked at 50% and 80% VO2 max, breathing either air or 80% O2-20% N2 in random order using a single blind technique. Subjects were evaluated while running for 10 min and during a 20 min recovery. Heart rate (HR), ventilation (VE), respiration rate (RR), tidal volume (VT) and RPE were measured before, during and after work. Blood lactate was measured 1 min after work. Oxygen concentration did not statistically affect HR, VE, RR or VT during exercise or recovery. At both loads, RPE at the end of exercise was significantly reduced breathing the hyperoxic mixture. At 50% VO2 max, mean RPE decreased from 11.2 breathing room air to 9.6 breathing 80% O2 and, 80% VO2 max, from 13.8 to 11.7 (P less than 0.01). Blood lactates were significantly reduced breathing 80% O2; from 23.4 mg to 13.3 at 50% VO2 max and from 55.5 to 36.5 at 80% VO2 max (P less than 0.01). The RPE correlated with lactate (r=0.64) at the end of work. Results indicate that during moderate and heavy work RPE is significantly affected by the inspired O2 concentration and there is a significant relationship between RPE and blood lactate.

Effects of O2 on the ventilatory response to CO2 in preterm infants.

To measure the effects of O2 on the ventilatory response to CO2 in preterm infants, we studied eight babies (birth wt 1-2 kg; gestational age 32-36 wk) 10 times during the first 11 days of life. After breathing 21% O2 for 3 min, they were given 15%, 21%, 40%, or 100% O2 for 4 min and then 2% CO2 plus the various concentrations of O2 for 4 min each. The mean slopes of the CO2 response curves were 0.013, 0.027, 0.034, and 0.056 1/(min-kg-mmHg PACO2) with 15%, 21%, 40%, and 100% inspired O2, respectively. Thus, the more hypoxic the infant, the flatter was the response to CO2. These findings suggest that in preterm infants 1) the response to inhaled CO2 is the reverse of that seen in adult man where the higher the inspired O2 concentration, the flatter the response, and 2) the respiratory center is depressed during hypoxia.

Instability of lung units with low Va/Q ratios during O2 breathing

Using a multiple inert gas infusion method, we have observed the development of shunts during oxygen breathing in lungs which contained areas of low ventilation-perfusion ratios while breathing air. This paper gives a theoretical analysis of the factors involved. When the inspired ventilation-perfusion ratio (VaI/Q) of a lung unit is gradually reduced, a point is reached where the expired ventilation falls to zero. Such a unit will no longer eliminate gas but may continue gas uptake unless it becomes atelectatic. This critical VaI/Q is determined by the net flux of O2, CO2, and N2 from alveolar gas to capillary blood, and its value increases from about 0.001 to 0.1 as the inspired gas is changed from air to 100% O2. The critical VaI/Q at any inspired O2 concentration is raised if the O2 or N2content of mixed venous blood are reduced or if N2 is replaced by a more soluble gas. In distributions of ventilation-perfusion ratios, the amount of shunt which develops during oxygen breathing depends on the degree of dispersion of the VaI ratios. The release of hypoxic vasoconstruction following O2 administration, in general, reduces the amount of shunt.

Hypoxia and extracellular potassium activity in the guinea-pig cortex.

During brief periods of hypoxia caused by the inhalation of pure nitrogen, reversible increases in extracellular potassium activity (aK) were detected in the guinea-pig cortex with K+-selective microelectrodes. The increases in aK produced by varying intensities of hypoxia were inversely related to the availability of O2 measured simultaneously in brain tissue with a [Formula: see text] electrode. They were associated with an increased negativity of the potential recorded between the reference electrode and ground, which is presumed to reflect depolarization of cortical elements such as glia and presynaptic terminals. When the inspired O2 concentration was raised from 21 to 100% small decreases in extracellular aK were sometimes observed.

HYPOXIA IN PRETERM INFANTS: EVIDENCE FOR CENTRAL RESPIRATORY DEPRESSION

Hypoxia depresses ventilation in preterm infants. Because PACO2 does not rise, the decrease in ventilation has been attributed to a decrease in metabolism rather than to the central depressant effect of hypoxia. To determine whether central depression is present, we measured the ventilatory sensitivity to CO2 at varying inspired O2 concentrations in preterm infants. Eight babies (B.W.I to 2 kg;G.A.<36 weeks) were studied 10 times during the first 10 days of life. After breathing 21% O2 for 3 minutes, they were given 15,21,40 or 100% O2 for 4 minutes and then 2% CO2 plus the various concentrations of O2 for 4 minutes each. We determined respiratory minute volume and frequency, PACO2, PAO2 and heart rate. Ventilation was measured with a nosepiece and screen flowmeter, using a constant flow-through to eliminate dead space and valves. We plotted minute ventilation against PACO2 to assess the respiratory sensitivity at varying inspired O2 concentrations. The mean slopes of the CO2 response curves were 0.011, 0.022, 0.039 and 0.168 L/min/kg/mm Hg PACO2 with 15,21,40 and 100% inspired O2 respectively (p<0.05). Therefore, the more hypoxic the infant, the flatter was the response to CO2. These findings suggest: a)the respiratory center is depressed during hypoxia in preterm infants despite a decrease in PACO2; b) the response to inhaled CO2 is the opposite to that seen in male adults, where the higher the inspired O2 concentration, the flatter the response.

The normal alveolar-arterial oxygen-tension gradient in man.

1. The alveolar-arterial oxygen-tension gradient was measured in duplicate, in forty-eight healthy subjects (twenty-four men and twenty-four women) aged from 20 to 74 years, while breathing oxygen concentrations of approximately 14, 21, 40, 60 and 100%. 2. The gradient increased with age and with inspired O2 concentration up to 60%. Above 60%, during normal breathing, there was no significant change in gradient. 3. Breathing 100% O2 in deep breaths decreased the gradient in all age-groups, significantly so in subjects of 50 years and younger. The remaining gradient was due to anatomical venous shunt. 4. When this anatomical shunt was allowed for, the differences in gradient between age-groups were largely abolished at all inspired O2 concentrations. The gradient which was not due to anatomical shunt, when breathing air, remained higher in subjects over 60 years than in the younger subjects. 5. The results can be interpreted to mean that three components contribute to the normal gradient: (a) irreversible anatomical shunt; (b) closed, unventilated alveoli which can only be inflated by deep breaths; and (c) cyclical airway closure during normal breathing in subjects over 60 years. Diffusion disequilibrium may contribute a gradient of up to 3 mmHg during the breathing of low O2 concentrations.

Plasma catecholamine concentration during dynamic exercise involving different muscle groups.

The change in plasma catecholamine concentration (ΔC) has been studied in four healthy male subjects during work, involving different muscle groups, whilst breathing air and 45% oxygen. The results show that during arm and (1- and 2-) leg(s) work ΔC was more closely associated with relative (expressed as a % of $$\dot V_{{\text{O}}_{{\text{2max}}} }$$ ) than absolute work load; a rise in C occurred at ∼ 60% $$\dot V_{{\text{O}}_{{\text{2max}}} }$$ in all 3 forms of exercise. However, in arm and 1-leg work the curves relating ΔC to % $$\dot V_{{\text{O}}_{{\text{2max}}} }$$ were displaced to the right indicating the independence of the two variables. Further, breathing 45% oxygen reduced ΔC but was without effect on either $$\dot V_{O_2 } $$ at a given work load or $$\dot V_{{\text{O}}_{{\text{2max}}} }$$ . For a given $$\dot V_{O_2 } $$ , ΔC was inversely related to the effective muscle (plus bone) volume used to perform the work and associated with change of blood lactic acid (LA) concentration, but again the use of exercise involving different muscle groups indicated that the changes in C and LA were essentially independent. This was also true of the changes of C with cardiac output but not cardiac frequency (f H ). Plasma C changed as a curvilinear function of f H , the association between the two variables being independent of type of exercise and inspired O2 concentration within the range used in this study. This suggests that the rise in C and f H in exercise may be closely related to circulatory stress and may reflect the degree of vasoconstriction present in ‘non-active’ tissues and efficacy of the body's ability to maintain the integrity of systemic blood pressure in the face of increased demands of the exercising muscles for blood and the transport of oxygen.

Digital Simulation of the Chemical Control of Ventilation

Experimental human data, obtained for the development and evaluation of a digital computer model of the human respiratory system, are presented. The data are from two series of experiments. In the first series the transient responses of tidal volume, respiratory frequency, minute ventilation, alveolar Pco2 and alveolar Po2 were obtained for several inspired CO2 concentrations (3, 5, 6, and 7 percent). In the second series, transient responses of the same variables were obtained for steps of inspired O2 concentration from room air to several lower oxygen levels (9, 8, and 7 percent). An example of the use of the data for the development and evaluation of a model is indicated.

THE EFFECT OF OXYGEN ADMINISTRATION ON GAS EXCHANGE AND CARDIOPULMONARY FUNCTION IN NORMAL SUBJECTS

Abstract The effects of six different inspired oxygen concentrations on arterial (14 subjects) and mixed venous (8 subjects) pH, PCO2 and PO2 were measured. In all 14 subjects, o, CO2, minute ventilation, mean arterial blood pressure and heart rate were measured at each inspired concentration. Inspired O2 concentrations between 21 and 40% produced a linear increase in arterial oxygen pressure and content and a progressive small rise in mixed venous oxygen content, with only minimal increase in mixed venous O2 pressure and without change in O2, pH, PCO2, ventilation or cardiac output. With 100% O2 the further rise in arterial PO2 was accompanied by a significant increase in mixed venous oxygen pressure. The ventilation increased slightly; there was no change in cardiac output or acid-base balance.

Continuous Positive Transpulmonary Pressure in Hyaline Membrane Disease: A Simple Device

A simple modification of the Isolette incubator -respirator such that a constant negative (subatmospheric) pressure (CNP) may be applied to the body chamber is described. A constant subatmospheric pressure around the thorax (constant positive transpulmonary pressure) has been used in the treatment of five newborn infants with severe hyaline membrane disease with favourable results. Arterial Po2 increased significantly while the alveolar-arterial Po2 difference decreased. There was no significant influence on arterial Pco2. This approach appears to be a useful adjunct to the treatment of hyaline membrane disease since it increases arterial Po2, allows more rapid reduction of inspired o2 concentrations to levels not toxic to the lung, and may obviate the need for endotracheal intubation and artificial respiration.

Response of the arterial Po2 to oxygen administration in chronic pulmonary disease. Interpretation of findings in a study of 46 patients and 14 normal subjects.

In 34 patients with emphysema, 12 patients with pulmonary insufficiency from other causes, and 14 normal subjects, arterial Po2, Pco2, and pH were measured when inspired oxygen concentration was 21%, 24%, 28%, 35%, 40%, and 100%, The dead space to tidal volume ratio was measured during ambient air breathing in all, as was venous admixture in 21 patients and 8 normals. An explanation based on variability of the abnormality of alveolar ventilation to blood flow ratio is offered for the known wide variation in the response of arterial Po2 to inspired O2 concentrations among patients with pulmonary disease. Linear regressions are described that allow estimation of response to a given oxygen concentration based on measurement of arterial Po2 during air breathing. These regressions apply to a variety of chronic lung diseases, suggesting similarities in the distribution of low ratios of alveolar ventilation to blood flow in chronic pulmonary disease regardless of etiology.

RULES OF EVIDENCE APPLIED TO TREATMENTS OF NEONATAL HYPOXEMIA AND ACIDOSIS

BIOCHEMICAL MONITORING of the low birth weight infant during the first 24 hours of life frequently reveals abnormalities of acid-base balance and hypoxemia; these anomalies are very striking in infants with clinical signs of respiratory distress. The clinician is then faced with two problems: the first in diagnosis, the second in treatment. In the past few years certain regimes for respiratory distress have been recommended with varying degrees of fact and forcefulness. These include the careful maintenance of thermal balance to keep oxygen consumption at a minimum, correction of acidosis by NaHCO3 or THAM in rapid or slow infusion, adequate oxygenation sometimes requiring an inspired O2 concentration above 40%, assisted ventilation (either through a tracheal tube or by a negative pressure tank), and, finally, administration of agents acting on vasomotor tone.