Richard L. Clancy

Pulmonary gas exchange during hypoxic exercise in the rat

Pulmonary gas exchange and O2 transport were studied at rest and during maximal treadmill exercise in rats in acute hypoxia (PIO2 approximately 71 Torr), and in littermates acclimatized to PB = 380 Torr (PIO2 approximately 71 Torr) for 3 weeks (chronic hypoxia). To obtain valid estimates of blood gas partial pressures, particularly during exercise, the temperature coefficients of blood pH, PO2 and PCO2 were determined (Appendix). In both acute and chronic hypoxia, the following changes were observed: alveolar and arterial PO2 increased considerably, but the difference, A-aPO2, did not change significantly; arterial O2 concentration (CaO2) decreased, and apparent pulmonary diffusing capacity for O2, Dapp, increased. The increase in Dapp, together with hyperventilation, may prevent further drop in CaO2 due to a large rightward shift in the blood-O2 equilibrium curve caused by lactic acidosis in conjunction with a large Bohr coefficient characteristic of this species. Comparison with corresponding results obtained in man reveals that during hypoxic exercise, the rat shows a larger increase in PAO2, an increase, instead of a decrease, in PaO2, and a larger increase in Dapp.

Effect of respiratory and metabolic acidosis on coronary vascular resistance (38528).

An increase in coronary flow has been observed during acute hypercapnia (1-4). On the other hand, metabolic acidosis has been shown to produce an increase (5), or a decrease (2, 6) in coronary vascular resistance. Similarly inconsistent results have been observed when either respiratory or metabolic alkalosis was produced (2, 4-6). The apparent contradiction between different studies may be due to one or more of the following. In the first place, changes in the coronary circulation were observed in the presence of changes in the systemic circulation, notably arterial blood pressure and heart rate, making it difficult to determine to what extent coronary changes are a reflection of systemic circulatory changes (1, 5, 6). Secondly, acid-base changes are capable of altering the O2 requirements of the heart which in turn strongly affect the coronary circulation. This variable has not always been taken into account in the interpretation of the results (2, 4, 6). Finally, in some cases simultaneous changes in pH, PCO2 and HCO3 concentration have thwarted attempts to ascertain which of these parameters is responsible for the changes observed (5, 6). The present experiments were undertaken to study the effect of metabolic and respiratory acidosis on coronary vascular resistance. A preparation where hemodynamic variables and O2 requirements could either be controlled or their influence accounted for was employed. Methods. The experiments were performed using isolated perfused rabbit hearts. Rabbits were anesthetized with a mixture of chlora-lose and urethane. The thorax was opened under artificial ventilation and a metal cannula was introduced into the aorta, with its tip oriented towards the ventricle and close to the valve. Perfusion of the coronary circulation with Ringers solution, equilibrated at 37° with 95% O2; 5% CO2 was started immediately with a roller pump.

Role of β-adrenergic and cholinergic systems in acclimatization to hypoxia in the rat

To role of beta-adrenergic and muscarinic cholinergic systems on maximal treadmill exercise performance and systemic O2 transport during hypoxic exercise (PIO2 approximately 70 Torr) was studied in rats acclimatized to hypobaric hypoxia (PIO2 approximately 70 Torr for 3 weeks, A rats) and in non-acclimatized littermates (NA rats). Untreated A rats had lower resting (fH) and maximal heart rate (fHmax) and cardiac output (Q), and higher maximal O2 uptake (VO2max) than NA. The only effect of cholinergic receptor blockade with atropine (Atp) was an increase in pre-exercise fH to comparable levels in A and in NA. beta 1-adrenergic receptor blockade with atenolol (Aten) lowered pre-exercise fH and (fHmax) to comparable values in A and in NA rats. However, since both pre-exercise fH and fHmax were lower in untreated A, the effect of Aten was relatively smaller in A. Aten reduced maximal exercise cardiac output (Qmax) in NA; however, tissue O2 extraction increased such that VO2max was not affected. Aten did not influence Qmax or any other parameter of systemic O2 transport in A. In conclusion the increased cholinergic tone may be responsible for the lower resting fH but not the lower fHmax of A; the integrity of the beta-adrenergic system is not necessary to attain VO2max in hypoxia either in A or in NA; the decreased response to beta-adrenergic stimulation in A limits the efficacy of this system on the mechanisms of systemic O2 transport and reduces the effect of its blockade on these mechanisms.

Understanding the effects of oxygen administration in haemorrhagic shock

AIMS AND OBJECTIVES the aim of this article is to provide a review of the literature regarding oxygen administration and the use of oxygen in patients experiencing haemorrhagic shock (HS). RESULTS oxygen is administered to patients to assist them in maintaining oxygenation. The administration of oxygen is complex and varies significantly among patients. In order to optimize patient care, clinicians need to be aware of the potential effects, both beneficial and harmful, that oxygen can have on the body. INCLUSION AND EXCLUSION CRITERIA literature inclusion criteria for this article was any article (1995 to present) pertaining to oxygen administration and HS. Also included were articles related to tissue injury caused by an overabundance of free radicals with the administration of oxygen. Articles related to oxygen and wound healing, pollution, aerospace, food and industrial uses were excluded. CONCLUSIONS this review of the literature provides an overview of the use of oxygen in clinical practice and HS. The harmful effects of oxygen are highlighted to alert the clinician to this potential when there is an overabundance of oxygen. RELEVANCE TO CLINICAL PRACTICE oxygen is one of the most common drugs used in the medical community; however, the effects of oxygen on the body are not well understood. The use of oxygen if not prescribed correctly can cause cellular damage and death. Clinicians need to be more aware of the effects of oxygen and the damage it may cause if not administered properly.

Treatment and prevention of diaphragm fatigue using low-dose dopamine.

There is increasing evidence that diaphragm fatigue is a major cause of failure in weaning patients frommechanical ventilation. Patients in intensive care units are often administered dopamine to improve renal blood flow without regard to its effect on diaphragmblood flow. The aimof this study was to investigate if intravenous low-dose dopamine, equivalent to the dose used in intensive care units, can treat and prevent diaphragmfatigue. Diaphragmfatigue was produced in anesthetized rats by inspiratory resistance loading (IRL). The effect on diaphragmshortening, diaphragmblood flow, and aortic blood flow was determined. When diaphragm fatigue was attained, group I was given saline for 30 min while maintaining IRL. At the time of diaphragm fatigue, group II was given low-dose dopamine (2 μg/kg/min) for 30 min while maintaining IRL. In group III, dopamine administration was started before and continued throughout the period of IRL. Administering dopamine after the development of diaphragm fatigue (group II) increased diaphragm performance as measured by increased diaphragmshortening and was accompanied by an increased diaphragmblood flow. Administering dopamine prior to and throughout IRL (group III) prevented diaphragmfatigue. Low-dose dopamine can prevent and/or reverse diaphragmfatigue in rats without a significant change in aortic blood flow. This effect of dopamine may be due to increased oxygen delivery associated with the increased diaphragm blood flow, resulting in less free radical formation and thus less muscle damage.

Intracellular pH regulation during prolonged hypoxia in rats

Conscious rats maintained for three weeks at PB 370-380 Torr were studied in a chamber where PIO2 was maintained at 68-70 Torr at ambient barometric pressure (740-750 Torr). Controls were pair-fed rats maintained at ambient barometric pressure and studied at ambient PIO2 for 4 h. Steady-state intracellular pH (pHi) of left and right ventricle, and of tibialis anterior, quadriceps and diaphragm was determined from the distribution of 5,5-dimethyl-2,4-oxazolidinedione (DMO). Apparent non-bicarbonate buffer value (beta app) was calculated as the ratio of the change in HCO3- concentration to the change in pH elicited by the increase in PCO2. beta app of plasma, tibialis anterior, quadriceps and diaphragm was approximately 2, 3, 6 and 12 times higher, respectively, in hypoxic than in normoxic rats. Neither left nor right ventricular beta app was significantly changed by prolonged hypoxia. In the hypoxic animals, bilateral nephrectomy abolished the increase in beta app of plasma, tibialis anterior and quadriceps, and moderated the increase in beta app of diaphragm. No significant effect of nephrectomy was observed in beta app of either left or right ventricle. The results indicate that in the skeletal muscles studied under conditions of an acid load in the form of increased PCO2, intracellular pH is better regulated in hypoxic than in normoxic rats. The effects of nephrectomy suggest that this is due, at least in part, to a more effective renal compensation in hypoxic than in normoxic rats. Prolonged hypoxia, on the other hand, does not affect the cell pH regulation of right or left ventricle.

Acid-base regulation in prolonged hypoxia: Effect of increased PCO2

Conscious rats maintained for 3 wk at PB 370-380 Torr were studied in a chamber where PIO2 was kept at 68-70 Torr at ambient barometric pressure (740-750 Torr). Blood samples were obtained through an arterial catheter. Controls were pair-fed rats maintained at ambient barometric pressure and studied at PIO2 68-70 Torr for 4 h (acute hypoxia) or at ambient PIO2 (normoxia). Arterial blood pH of 3-wk hypoxic rats was not different from that of normoxic rats. Hypercapnia was produced by increasing PICO2 for 4 h. The 3-wk hypoxic rats showed the highest apparent non-bicarbonate buffer value of arterial blood (beta app): 77 mmol/(pH X kg), compared to 38 in normoxia and 43 mmol/(pH X kg) in acute hypoxia. Comparison of beta app at different times of hypercapnia in intact and in nephrectomized rats suggests that the high beta app of prolonged hypoxia is largely due to an increased renal compensation, and, to a smaller extent, to increased chemical buffering. While the extracellular fluid of normoxic and acute hypoxic rats showed a net gain of base of non-renal origin during hypercapnia, the 3-wk hypoxic rats showed a net non-renal base loss, which may be masked by the increased renal compensation.

Effect of dopamine on rat diaphragm apoptosis and muscle performance

The purpose of this study was to determine whether dopamine (DA) decreases diaphragm apoptosis and attenuates the decline in diaphragmatic contractile performance associated with repetitive isometric contraction using an in vitro diaphragm preparation. Strenuous diaphragm contractions produce free radicals and muscle apoptosis. Dopamine is a free radical scavenger and, at higher concentrations, increases muscle contractility by simulating β2‐adrenoreceptors. A total of 47 male Sprague–Dawley rats weighing 330–450 g were used in a prospective, randomized, controlled in vitro study. Following animal anaesthetization, diaphragms were excised, and muscle strips prepared and placed in a temperature‐controlled isolated tissue bath containing Krebs–Ringer solution (KR) or KR plus 100 μm DA. The solutions were equilibrated with oxygen (O2) at 10, 21 or 95% and 5% carbon dioxide, with the balance being nitrogen. Diaphragm isometric twitch and subtetanic contractions were measured intermittently over 65 min. The diaphragms were then removed and, using a nuclear differential dye uptake method, the percentages of normal, apoptotic and necrotic nuclei were determined using fluorescent microscopy. There were significantly fewer apoptotic nuclei in the DA group diaphragms than in the KR‐only group diaphragms in 10 and 21% O2 following either twitch or subtetanic contractions. Dopamine at 100 μm produced only modest increases in muscle performance in both 10 and 21% O2. The attenuation of apoptosis by DA was markedly greater than the effect of DA on muscle performance. Dopamine decreased diaphragmatic apoptosis, perhaps by preventing the activation of intricate apoptotic pathways, stimulating antiapoptotic mechanisms and/or scavenging free radicals.

Dopamine alleviation of diaphragm contractile dysfunction and reduction of deoxyribonucleic acid damage in rats.

BACKGROUND Weaning difficulties from mechanical ventilation are associated with diaphragm fatigue and reduced respiratory muscle endurance capacity. Often the work of breathing is increased during the weaning process as a result of inspiratory resistance loading (IRL). IRL produces increased free radical formation that contributes to deoxyribonucleic acid (DNA) damage. The purpose of this study was to determine whether dopamine reduced nuclei DNA damage when the work of breathing was increased. We hypothesized that the administration of low-dose dopamine (2 microg/kg/min) during IRL decreases myonuclei DNA damage associated with free radical formation. METHODS In this in vivo study, 30 male Sprague-Dawley rats were divided into three groups: (1) the sham group receiving no IRL or no intravenous fluids, (2) IRL with administration intravenous saline, and (3) IRL with intravenous low-dose dopamine (2 microg/kg/min). All rats from the same breed and similar colonies were purchased from one laboratory facility to ensure homogeneity. The animals were anesthetized and tracheotomized, and an ultrasonic sensor was attached to the right hemidiaphragm to measure diaphragm shortening. Diaphragm fatigue was produced by IRL. Dopamine (2 microg/kg/min) was infused intravenously before and during loading. The diaphragms were excised, and myonuclei DNA damage was measured using the fluorescent dyes ethidium bromide and acridine orange and comet analyses as indices of free radical injury. RESULTS In rats receiving saline, diaphragm shortening decreased by 37% after 45 minutes of IRL (P = .002) compared with baseline. In contrast, rats infused with dopamine exhibited a 31% increase in diaphragm shortening after 45 minutes of IRL (P = .037). With the use of differential dye uptake, in the saline group 59% of the nuclei were apoptotic, and 18% were necrotic. However, in the dopamine group there was significantly less apoptotic nuclei (16%, P < .001) and necrotic nuclei (7%, P = .005). Myonuclei DNA damage, measured by comet analyses, was associated with tail length and tail olive moment, which were 37% and 60% greater, respectively, in the saline group than in the dopamine group (P < .05). CONCLUSION These data support the hypothesis that low-dose dopamine during IRL reduced myonuclei DNA damage as measured by the fluorescent dyes and comet analysis. In addition, diaphragm fatigue was prevented by the administration of dopamine during IRL.

Renal compensation of hypercapnia in prolonged hypoxia

We previously showed that rats made hypoxic for three weeks were able to regulate their plasma pH better than normoxic rats during acute hypercapnia. This improved pH regulation was abolished by nephrectomy, suggesting that it was due, at least in part, to a more effective renal compensation of hypercapnia in hypoxic rats. To test this possibility renal acid excretion was measured in conscious rats that had been kept at PB 370-380 Torr for three weeks. The rats were studied in a chamber where PIO2 was kept at 68-70 Torr at ambient PB (740-750 Torr). Controls were pair-fed normoxic rats. After a 2 h control period, inspired PCO2 was increased for 4 h. The apparent non-bicarbonate buffer value of arterial blood plasma was twice as high in the hypoxic than in the normoxic rats. Renal excretion of ammonium increased to a similar extent during hypercapnia in both normoxic and hypoxic rats. Titratable acid excretion of normoxic rats did not change significantly during hypercapnia. In the hypoxic rats, on the other hand, total excretion of titratable acid in the 2 h control period was 90.9 +/- 16.4 mumol/rat; and increased to 150.0 +/- 13.4 mumol/rat in the first 2 h and to 232.9 +/- 26.0 mumol/rat in the last 2 h of hypercapnia. In spite of this large increase in acid excretion, urine pH of hypoxic rats did not change significantly, indicating a higher buffer value of the urine of hypoxic rats. These results confirm our previous observations and support the idea that the improved pH regulation of hypoxic rats is due in part to a more effective renal compensation of hypercapnia.