Determination of the optimal dose of ephedrine in the treatment of arterial hypotension due to general anesthesia in neonates and infants below 6 months old: the ephedrine study protocol for a randomized, open-label, controlled, dose escalation trial

BACKGROUND

Arterial hypotension induced by general anesthesia is commonly identified as a risk factor of morbidity, especially neurological, after cardiac or noncardiac surgery in adults and children. Intraoperative hypotension is observed with sevoflurane anesthesia in children, in particular in neonates, infants younger than 6 months, and preterm babies. Ephedrine is commonly used to treat intraoperative hypotension. It is an attractive therapeutic, due to its dual action on receptors alpha and beta and its possible peripheral intravenous infusion. There are few data in the literature on the use of ephedrine in the context of pediatric anesthesia. The actual recommended dose of ephedrine (0.1 to 0.2 mg/Kg) frequently leads to a therapeutic failure in neonates and infants up to 6 months of age. The use of higher doses would probably lead to a better correction of hypotension in this population. The objective of our project is to determine the optimal dose of ephedrine for the treatment of hypotension after induction of general anesthesia with sevoflurane, in neonates and infants up to 6 months of age.

METHODS

The ephedrine study is a prospective, randomized, open-label, controlled, dose-escalation trial. The dose escalation consists of 6 successive cohorts of 20 subjects. The doses studied are 0.6, 0.8, 1, 1.2, and 1.4 mg/kg. The dose chosen as the reference is 0.1 mg/kg, the actual recommended dose. Neonates and infants younger than 6 months, males and females, including preterm babies who undergo a surgery with general anesthesia inducted with sevoflurane were eligible. Parents of the subject were informed. Then, the subjects were randomized if presenting a decrease in mean blood pressure superior to 20% of their initial mean blood pressure (before induction of anesthesia), despite a vascular filling with sodium chloride 0.9%. The primary outcome is the success of the therapy defined as an mBP superior to 80% of the baseline mBP (prior to anesthesia) within 10 min post ephedrine administration. The subjects were followed-up for 3 days postanesthesia.

DISCUSSION

This study is the first randomized, controlled trial intending to determine the optimal dose of ephedrine to treat hypotension in neonates and infants below 6 months old.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02384876 . Registered on March 2015.

Metabolic CO2 Removal by Dialysis: THAM vs NaOH Infusion

New methods of respiratory support are needed to reduce the high mortality rate of acute respiratory failure. To simplify the procedures of extracorporeal CO2 elimination under apneic oxygenation, one approach is to replace the membrane lung by a hemodialyzer and to administer an alkali, since hemodialysis requires a lower blood flow rate than blood-gas exchange. This study compared the effectiveness of trishydroxymethyl aminomethane (THAM) and NaOH in this procedure. Twelve male Anglo-Poitevin dogs (25 to 33 kg) were anesthetized, curarized and mechanically hypoventilated (&Vdot;E = 41% of the control value). After not less than 15 min, a venovenous shunt was used for dialysis with blood flow of 7-10 ml. min.-1kg-1 for at least 8 hours. The dialysate contained no acetate, bicarbonate or lactate, but was alkalinized to a pH of 8-9 by the addition of NaOH. A solution of THAM (0.5 N) was infused into the right heart at the rate of 0.30 ml.min.-1kg-1 in six animals, and NaOH (0.15 N) was infused in the other six at the rate of 0.80 ml.min.-1 kg-1. The injected volumes were compensated for by an equivalent amount of ultrafiltration. Elimination of CO2 (mean †CO2 = 2.3 ml.min.-1kg-1) was the same with both methods and the difference for the electrolytes and acid-base equilibrium was only very small. However, hemolysis was six times greater with NaOH than with THAM. Despite ultrafiltration, a similar marked weight gain was observed from the second hour of the experiment in the NaOH series, but only after 7 hours with THAM. It thus appears that hemodialysis combined with alkalinisation is still too complex a procedure to be safely applied in acute or chronic pulmonary failure

[Radiation-related heart toxicity: Update in women]

Breast cancer is a common diagnosis in women and thus women are at risk of radiation-induced heart disease, in particular during radiotherapy for left breast cancer and when the internal mammary chain is included. Rates of major cardiac events increase with younger age at the time of irradiation, diagnosis before 1990s, higher radiation doses, coexisting cardiovascular risk factors and adjuvant cardiotoxic chemotherapy. Radiation-induced heart disease comprises a spectrum of cardiac pathologies, including pericardial disease, cardiomyopathy, coronary artery disease and valvular disease. The cardiac injury can appear a long time after radiotherapy and can consist of complex lesions with poor prognosis. The disciplines of cardiology and oncology have increasingly recognized the benefits of collaborating in the care of cancer patients with cardiac disease, developing guidelines for the assessment and management of radiation-related cardiovascular disease. We could consider screening patients with previous chest radiation every 5 years with transthoracic echocardiography and functional imaging. However, prevention remains the primary goal, using cardiac sparing doses and avoidance techniques in radiotherapy to improve patient survival.

[French residents' training in instrumental deliveries: A national survey]

OBJECTIVES

To evaluate French residents in Obstetrics and Gynaecology's training in instrumental deliveries in 2015.

PATIENTS AND METHODS

We conducted a national descriptive survey among 758 residents between December 2014 and January 2015. Respondents were invited by email to specify their University Hospital, their current university term, the number of instrumental deliveries performed by vacuum extractor, forceps or spatulas, and whether they made systematic ultrasound exams before performing the extraction.

RESULTS

Response rate was 34.7 % (n=263). There were important differences between regions in terms of type of instruments used. Vacuum extractor was the most commonly used instrument for instrumental deliveries by French residents (56.9 %), more than forceps (25.2 %) and spatulas (17.9 %). At the end of the residency, all the residents had been trained in instrumental deliveries with at least two instruments.

CONCLUSION

The training of difficult techniques as well as their perfect control is required for instrumental deliveries. Yet, we are forced to note that there are substantial differences in the French residents' training in instrumental deliveries depending on their region. So, teaching at least two techniques seems essential as well as improving the training capacities and standardizing practices. A greater systematization of the teaching of the mechanics and obstetric techniques might be a solution to be considered too.

Volume dependence of respiratory system resistance during artificial ventilation in rabbits

The volume dependence of respiratory resistance (Rrs), usually observed during normal breathing, is expected to be accentuated during expiratory flow limitation (EFL). In order to quantify this dependence we studied the pressure, flow, and volume data obtained from eight New Zealand rabbits, artificially ventilated at different levels of applied expiratory pressure (0-10 hPa), before and during histamine i. v. infusion. EFL was provoked by lowering the expiratory pressure and was detected by the application of an additional negative expiratory pressure and by forced oscillations. The analysis of respiratory system mechanics was performed by multiple regression, using the classical linear first-order model and also a nonlinear model, accounting for volume dependence of Rrs. Both models satisfactorily fitted the data in the absence of EFL. The nonlinear model proved to be more appropriate in the presence of EFL. The coefficient expressing the volume dependence of Rrs (Rvd) was significantly more negative during EFL. Rvd values were highly correlated with the fraction of the tidal volume left to be expired at the onset of EFL. A threshold Rvd value of -1,000 (hPa x s x l(-2)) detected EFL with high sensitivity and specificity. We conclude that a strongly negative volume dependence of Rrs is a reliable and noninvasive index of EFL during artificial ventilation.

Role of NO pathway, calcium and potassium channels in the peripheral pulmonary vascular tone in dogs

Because hypoxic pulmonary vasoconstriction occurs mainly in the small pulmonary arteries, the authors investigated the effects of drugs acting on the nitric oxide (NO) pathway and the calcium and potassium channels in the peripheral pulmonary circulation, without interference with the overall pulmonary or systemic circulation. Mixed venous blood was infused in wedged areas to study the pressure/flow relationship and to compute peripheral pulmonary vascular resistance (PPVR). The authors studied the effects of Nomega-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, sodium nitroprusside (SNP, an NO donor), the calcium channel blockers verapamil, nifedipine and nicardipine, and the potassium channel opener levcromakalim, during normoxia and acute mild normocapnic hypoxia. In the peripheral pulmonary circulation, L-NAME caused an increase in PPVR during normoxia (+95%; p<0.001) and hypoxia (+60%; p<0.01). Following the increase by L-NAME, SNP decreased PPVR during normoxia (-24%; p<0.05) and hypoxia (-23%; p<0.05). Verapamil, nifedipine and nicardipine did not modify PPVR during normoxia but during hypoxia they decreased PPVR (-28%, nonsignificant; -27%, p<0.01 and -33%, p<0.05, respectively). Levcromakalim did not modify PPVR during normoxia or hypoxia. In conclusion, the nitric oxide pathway and voltage-dependent calcium channels, and not adenosine triphosphate sensitive potassium channels, play an important role in the control of peripheral pulmonary circulation in dogs.

Changes in peripheral pulmonary vascular resistance after ioxaglate infusion in anesthetized dogs

The pulmonary vascular resistance in a peripheral portion of the pulmonary vascular bed can be determined with a catheter wedged in a peripheral branch of the pulmonary artery, from the pressure increase induced by infusion of mixed venous blood at known flow rates. The volume of the wedged vascular bed can be estimated during fluoroscopy, from the volume of a contrast agent infused until the corresponding pulmonary vein can be seen. The present work was undertaken to determine the possible influence of this maneuver on the peripheral pulmonary vascular resistance (PPVR). In anesthetized dogs, after a control flow run, Hexabrix (meglumine ioxaglate and sodium ioxaglate) was infused, then the catheter was flushed with saline, and another flow run was performed. There was no significant difference between two consecutive control runs in the same site (n = 14). Before Hexabrix (n = 23), PPVR was 162 (+/-24) x 10(3) dyn.s.cm-5 (1 dyn = 10 microN) during the control run; it increased to 339 (+/-33) x 10(3) dyn.s.cm-5 after Hexabrix (p < 0.001). These results show that care should be taken to avoid interference between Hexabrix and the results of physiological or pharmacological interventions with this technique.

Expiratory flow limitation during mechanical ventilation detected by the forced oscillation method

We have previously observed large phasic variations of respiratory mechanical impedance in chronic obstructive pulmonary disease (COPD) patients mechanically-ventilated for acute respiratory failure, and postulated that they were due to expiratory flow limitation (EFL). The aim of this study was to test that assumption experimentally and to assess the value of impedance for automatic and noninvasive detection of EFL during mechanical ventilation. The study was performed: 1) in a mechanical analogue, including a flow-limiting element; and 2) in eight anaesthetized and paralysed rabbits, before and during histamine infusion. In both instances, EFL was obtained by lowering the expiratory pressure, using a computer-controlled ventilator; the absence of flow increase when expiratory pressure was further lowered was taken as evidence of EFL. Impedance was measured by applying 15 Hz oscillations at the airway opening. Its real (Re) and imaginary (Im) parts were measured separately during the inspiratory and the expiratory phases, and their differences were related to the mean inspiratory modulus. With the analogue, EFL was accompanied by large decreases both of Re and Im during the expiratory phase. In the rabbits, phasic variations of Re were variable in sign and were not significantly different with and without EFL. In contrast, EFL systematically and specifically decreased Im during the expiratory phase. A threshold of -50% provided a sensitivity of 96% and a specificity of 100% for detecting EFL. The observed phasic variations may be explained by airway wall shunt properties. The study suggests that a large decrease of the imaginary part of impedance during the expiratory phase is a sensitive and specific index of expiratory flow limitation during artificial ventilation.

Analysis of low-frequency lung impedance in rabbits with nonlinear models

Lung mechanics was studied in six paralyzed tracheotomized rabbits ventilated with a specially devised computer-controlled ventilator. The target flow waveform contained noninteger multiple frequencies ranging from 0.83 to 6-10 Hz and met a neither-sum-nor-difference criterion to minimize the effects of nonlinearity (B. Suki and K. Lutchen. IEEE Trans. Biomed. Eng. 39: 1142-1151, 1992). The actual flow, however, contained harmonics of the two lowest frequencies. Measurements were performed at mean airway pressure (Paw) levels of 8 and 12 hPa and during histamine-induced bronchoconstriction. Smooth impedance curves were observed in unchallenged rabbits at low mean Paw levels. In contrast, unrealistic impedance fluctuations, suggestive of cross talk from the unwanted frequency components in the flow input, were seen at high mean Paw levels and during acute bronchoconstriction. Model analysis was performed by using the actual flow signal as an input to various nonlinear models. The impedance fluctuations observed at high mean Paw levels were well simulated by a model featuring a volume-dependent elastance, and those observed after histamine were almost perfectly reproduced by a model where resistance increased with the reciprocal of lung volume. We conclude that impedance data biased by cross talk may provide useful information on the presence and nature of respiratory system nonlinearities.

Influence of hypoxia and hypercapnia on the kinetics and hypokaliaemic effect of salbutamol in the rabbit

1. The aims were to document the influence of moderate hypoxia or hypercapnia on salbutamol kinetics and its hypokaliaemic effect, following its administration through the intravenous (60 micrograms/kg), intratracheal (60 micrograms/kg), and oral (2400 micrograms/kg) routes (n = 5). In control animals, PaO2 was around 85 mmHg and PaCO2 20 mmHg; in hypoxic animals PaO2 was around 40 mmHg and in the hypercapnic rabbit PaCO2 was 50 mmHg. 2. Following the intravenous administration of salbutamol, the apparent volume of distribution increased two-fold (p < 0.05) in animals with hypoxia and hypercapnia. Consequently, its half life was enhanced (p < 0.05). Given via the trachea, salbutamol bioavailability was decreased by hypoxia. 3. When salbutamol was given orally, hypoxia or hypercapnia increased the area under salbutamol plasma concentration as a function of time (p < 0.05). 4. In control animals, the salbutamol hypokaliaemic effect was greater when administered orally than through the other routes. Compared with control animals, the experimental conditions reduced the hypokaliaemic effect of salbutamol only when given orally. 5. It is concluded that salbutamol kinetics and dynamics can be altered by hypoxia and hypercapnia.

Fourier analysis versus multiple linear regression to analyse pressure-flow data during artificial ventilation

Respiratory resistance (Rrs) and elastance (Ers) are commonly measured in artificially-ventilated patients or animals by multiple linear regression of airway opening pressure (Pao) versus flow (V') and volume (V), according to the first order model: Pao = P0 + Ers.V + Rrs.V', where P0 is the static recoil pressure at end-expiration. An alternative way to obtain Rrs and Ers is to derive them from the Fourier coefficients of Pao and V' at the breathing frequency. A potential advantage of the second approach over the first is that it should be insensitive to a zero offset on V' and to the corresponding volume drift. The two methods were assessed comparatively in six tracheotomized, paralysed and artificially ventilated rabbits with and without adding to V' an offset equal to 5% of the mean unsigned flow. The 5% flow offset did not modify the results of Fourier analysis, but increased Rrs and Ers from linear regression by 15.8 +/- 4.6% and 4.55 +/- 0.64%, respectively. Without additional offset, differences between the two methods averaged 30.2 +/- 14.0% for Rrs and 9.3 +/- 6.2% for Ers. The differences almost completely disappeared (2.47 and 0.61%, respectively) when the flow signal was zero-corrected using the assumption that inspired and expired volumes were the same. After induced bronchoconstriction, however, Ers was still slightly larger by linear regression than by Fourier analysis, which may result from nonlinearities and/or frequency dependence of the parameters. We conclude that the regression method requires zero flow correction and that Fourier analysis is an attractive alternative.

Influence of chronic hypoxia on salbutamol tissular concentrations and on respiratory resistance in anesthetized rabbits

Salbutamol is a potent beta 2-adrenoceptor agonist given to patients with bronchial asthma who are frequently hypoxemic. The aims of this study were to document the influence of chronic hypoxia on salbutamol tissue concentrations and on salbutamol effect on total respiratory resistance. To this purpose, salbutamol (60 micrograms/kg) was administered intravenously to four groups of six rabbits exposed to four experimental conditions: (1) control rabbits breathing air, (2) histamine-induced bronchoconstriction in rabbits breathing air, (3) animals with chronic hypoxia, (4) histamine-induced bronchoconstriction in animals with chronic hypoxia. The area under salbutamol plasma concentration time curve (0 to 45 min) was not affected by these experimental conditions. Compared with control rabbits breathing air, following histamine-induced bronchoconstriction, salbutamol concentrations rose by 40 to 50% in lung and heart (p < 0.05). Hypoxia did not affect salbutamol distribution in these organs; however, in hypoxic animals, histamine-induced bronchoconstriction increased salbutamol concentrations only in the heart (p < 0.05), without affecting those in the lung. Compared with rabbits breathing air and with histamine-induced bronchoconstriction, the effect of salbutamol was reduced in rabbits under chronic hypoxia and histamine-induced bronchoconstriction (p < 0.05). We conclude that chronic hypoxia reduces salbutamol effect on pulmonary resistance, possibly by decreasing salbutamol lung concentrations.

Two-frequency analysis of respiratory mechanics in artificially ventilated rabbits

The frequency dependence of respiratory mechanical properties was studied in 10 paralyzed, artificially ventilated rabbits, by superimposing a single sinusoidal signal with a frequency of 10, 20 or 30 Hz upon the ventilator waveform. The tracheal pressure and flow signals were analyzed both with the usual first order model, which provided total respiratory elastance (Ers) and resistance (Rrs), and by Fourier analysis, which provided respiratory impedance (Zrs) at the breathing frequency (0.85 Hz) and at the superimposed oscillation frequency. The real part of Zrs (Re(Zrs)) decreased by 30% from 0.85 to 10 Hz (P < 0.001), but did not vary significantly from 10 to 30 Hz. This finding is satisfactorily explained by tissue viscoelasticity. Following a histamine aerosol, the frequency dependence of Re(Zrs) changed very little in three out of four rabbits, but increased substantially in the fourth. In that instance, assuming that lung hysteresivity was not markedly modified by histamine, the results suggest inhomogeneous airway obstruction and/or airway wall shunting.

Effect of hypoxia and exercise on the distribution of 65Zn in rat muscles

The aim of this study was to determine the influence of hypoxia and exercise in 65Zn distribution in various rat muscles. A total of 80 male wistar rats weighting 250 +/- 10 g were randomly separated into eight groups: four groups in air, two at rest, and the other two after exercise; four groups in hypoxia (3 days exposure), two at rest, and the other two after exercise. Two groups (n = 10) in air and two other groups under hypoxia were injected with 65Zn. The 65Zn uptake by blood, pH, and PaO2 decreased significantly during hypoxia. The Zn concentration increased significantly in serum, diaphragm, gastrocnemius, and heart Zn after exercise in air. After hypoxia, at res, Zn increased in serum and in diaphragm. After chronic hypoxia, at rest, radioactivity of 65Zn increased in heart, gastrocnemius, and bone, and it was similar in air condition after exercise with respect to rest. In both cases (air and hypoxia), the uptake of 65Zn by the tissues was higher at rest. After exercise in hypoxia, endogenous Zn decreased in heart and in diaphragm.

Small-amplitude pressure oscillations do not modify respiratory mechanics in rabbits

Changes in respiratory mechanics have occasionally been observed during high-frequency ventilation. In this study we investigated whether small pressure oscillations such as those used for respiratory impedance measurements modified total respiratory resistance (Rrs) and total respiratory elastance (Ers). The latter were measured in six paralyzed artificially ventilated rabbits with and without superimposed pressure oscillations at the airway opening. Rrs and Ers were obtained by least square fitting of low-pass filtered tracheal pressure and flow to the usual first-order model. Pressure oscillations of 2-4 hPa peak-to-peak at 10, 20, and 30 Hz applied for periods of 10 min had virtually no effect on Ers (changes ranging from -2.5 to 2.6%) and Rrs (0-8.2%). Analysis of variance did not show a significant difference on the pooled data. Pressure oscillations were also applied every other minute after a histamine aerosol. Ers and Rrs were similarly unchanged. We conclude that the small pressure oscillations used in respiratory impedance measurements do not modify lung mechanical properties and lung response to bronchomotor agents.

Effect of acute and chronic moderate hypoxia on diltiazem kinetics and metabolism in the dog

The aim of this study was to assess whether moderate hypoxia affects the disposition of diltiazem. Six male beagle dogs received diltiazem (0.6 mg/kg) on three occasions: (1) while breathing air, i.e. during normoxia; (2) 1 h after initiating exposure to a FiO2 of 0.08, a FiCO2 of 0.035 and a FiN2 of 0.885, i.e. during acute hypoxia and normocapnia, and (3) during chronic hypoxia, i.e. after 120 h of exposure to a FiO2 of 0.08. Multiple blood samples were withdrawn and urine was collected to assay diltiazem and metabolites [N-desmethyl diltiazem (MA), deacetyl diltiazem (DAD) and N-desmethyl deacetyl diltiazem (M2)]. Breathing air, mean arterial partial pressure of oxygen was 83.2 +/- 3.2; during acute hypoxia 42.2 +/- 0.7; and during chronic hypoxia, 41.9 +/- 0.6 mm Hg. Acute hypoxia did not alter diltiazem disposition. Compared to dogs with normoxia, chronic hypoxia reduced diltiazem metabolic clearance, from 64 +/- 3 to 51 +/- 5 ml/min/kg (p < 0.05), as well as its volume of distribution, from 11.4 +/- 1.2 to 9.1 +/- 0.3 liters/kg (p < 0.05). Chronic hypoxia decreased the fraction of diltiazem metabolic clearance, normalized by the glomerular filtration rate, generating the M2 metabolite, although this experimental condition did not affect the formation of MA or DAD. It is concluded that chronic moderate hypoxia reduced diltiazem systemic clearance because it decreased selected pathways of biotransformation.

Effect of atrial natriuretic peptide on bronchial tone in anesthetized rabbits

The effect of atrial natriuretic peptide (ANP) on histamine-induced bronchoconstriction was studied in vivo (in normoxic and in hypoxic rabbits) and in vitro. Thirty-two anesthetized rabbits, spontaneously breathing room air or 10% O2, received infusions of ANP (20, 40, or 80 ng/min/kg normoxia; 20 ng/min/kg hypoxia) or the vehicle for 100 min. After 75 min of ANP infusion, bronchoconstriction was induced inhaling histamine; respiratory resistance (Rrs) was measured prior to and until 20 min posthistamine. The results show that the histamine-induced increase in Rrs was significantly reduced by ANP 80 ng/kg/min in normoxia, and by ANP 20 ng/kg/min in hypoxia. In vitro, ANP had no effect on tracheal and bronchial smooth muscle precontracted with histamine or acetylcholine. These results show that ANP can decrease a histamine-induced bronchoconstriction in vivo but not in vitro, suggesting an indirect mechanism of action.

Muscarinic effect of atrial natriuretic peptide on rabbit airways

1. The aim of the present work was to investigate under which circumstances atrial natriuretic peptide (ANP) modulates airway resistance. 2. Of the six groups of rabbits (n = 5) studied, three received an infusion of ANP (80 ng min-1 kg-1 i.v.) for a period of 100 min, while the other three were infused with the vehicle. Before receiving the infusion of ANP or the vehicle, the animals were pretreated with atropine (0.5 mg kg-1 i.v.), propranolol (2 mg kg-1 i.v.) or not pretreated. After 75 min of infusion of ANP, bronchoconstriction was induced by inhalation of histamine. Respiratory resistance (Rrs) was measured before and 3, 5, 10, 15 and 20 min post-histamine challenge. 3. Following 75 min of ANP infusion, plasma ANP concentration increased from 153 +/- 52 (mean +/- s.e.mean) to 1441 +/- 203 pg ml-1 (P < 0.05) without affecting baseline Rrs. Control Rrs values (12.5-20.4 cmH2O l-1 s) were significantly increased following the inhalation of histamine (P < 0.001). By themselves, atropine, propranolol or ANP did not modify the histamine-induced increase in Rrs. However, when the animals were pretreated with atropine, ANP infusion significantly reduced the increase in Rrs induced by histamine (30 +/- 2 vs 51 +/- 6 cmH2O l-1 s; P < 0.05). 4. These data suggest that ANP has an indirect modulating effect on the airway smooth muscle and will decrease Rrs when muscarinic receptors are blocked.

Peripheral pulmonary vascular resistance

The pressure-flow relationship has been studied in a peripheral portion of the lung vasculature in anesthetized dogs with use of a double-lumen catheter wedged in a distal pulmonary artery. One lumen was used to infuse mixed venous blood in the wedged area and the other to measure the corresponding perfusion pressure. Flow ranged from 0 to 9.2 ml/min, and the mean volume of the wedged area (n = 59) was 0.75 +/- 0.05 (SE) ml. In the areas where the distal pulmonary artery was in the same direction as the catheter ("coaxial"), the mean pressure-flow curve showed a negligible gamma-intercept and no significant difference between ascending and descending flow. The slope of the initial part of the ascending limb (peripheral pulmonary vascular resistance) varied from site to site and did not show a significant correlation with the overall pulmonary vascular resistance; it was inversely correlated with the volume of the wedged area (r = -0.35, P < 0.05) and directly, as expected, correlated with the y-intercept (r = 0.78, P < 0.001) and hysteresis (r = 0.48, P < 0.001). The results of two consecutive pressure-flow runs in the same site showed similar results, with no difference exceeding the error of measurement. In contrast, the slope increased by 71% during hypoxia (fraction of inspired O2 was 0.10, n = 5). This procedure seems suitable to determine the effects of physiological or pharmacological interventions on the pulmonary vessels, without interference of the systemic circulation.

Pressure/flow relation in the peripheral pulmonary vascular bed in dogs: a preliminary study

In order to test a technique for the determination of the pressure/flow relationship in the peripheral pulmonary vascular bed, the perfusion pressure changes with increasing and then decreasing flow in a small part of the lung (around 1 ml) were studied in anaesthetized supine dogs, after insertion of a specially designed double distal lumen Swan-Ganz catheter. One lumen was used for the pressure measurement, one for infusion of saline by a pump with variable flow, from 0.1 to 1.0 ml s-1. A conventional thermodilution Swan-Ganz catheter was also advanced in the pulmonary artery, to measure pressures in the pulmonary circulation as well as cardiac output. During infusion in the wedged catheter, right atrial, pulmonary arterial and balloon occlusion wedge pressures did not change. The pressure/flow curve of the occluded vascular bed showed a shape similar to that of collapsible tubes, with a pressure plateau at high flow, but this could also be due to vascular recruitment. The curve exhibited hysteresis, with a lower pressure when flow decreased. The slope of the initial part of the curve increased, on average, from 54 +/- 9 during normoxia to 91 +/- 27 mmHg s ml-1 during hypoxia (FIO2 = 0.10); this difference was not significant, but the perfusion pressure at high flow was significantly higher during hypoxia (P less than 0.05). Using blood instead of saline would allow the determination of the peripheral pulmonary vascular resistance under physiological conditions, and further work is needed to estimate the sensitivity and the reproducibility of this technique.

Effect of acute and chronic moderate hypoxia on the kinetics of lidocaine and its metabolites and on regional blood flow

The effect of acute and chronic hypoxia on the disposition of lidocaine and its metabolites, and on regional blood flow has been examined in conscious beagles (n = 5). Each dog received an infusion of lidocaine for 5 h under three experimental conditions: (1) breathing air; (2) following acute exposure to a FIO2 of 8% and a FICO2 of 3.5% to generate a PaO2 of 45 mmHg without hypocapnia; and (3) after 6 days of hypoxemia. Multiple blood samples were drawn to assess the kinetics of lidocaine and its metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX). Three hours after the end of the infusion of lidocaine, the dogs received radioactive microspheres to estimate hepatic, renal and brain blood flow. Neither acute nor chronic moderate hypoxia affected the kinetics of lidocaine, the parent compound. However, both experimental conditions increased plasma concentrations of MEGX and GX and increased the ratio of the area under their plasma concentration curves to the dose of lidocaine received. Acute moderate hypoxia increased brain blood flow, although it did not affect liver or renal perfusion. Chronic moderate hypoxia did not significantly change the blood flow to any of the organs studied. It was concluded that acute and chronic moderate hypoxia decreases the rate of elimination of both active metabolites of lidocaine without modifying the perfusion to the organs responsible for their elimination.

Preserved CO2 response in cerebral and muscular blood vessels during cimetidine treatment

It is known that cimetidine inhibits the hypoxia-induced increase in cerebral blood flow (CFB) in dogs, but the mechanism of this inhibition is not fully understood. Since the accepted mechanisms explaining the increase in CBF during hypercapnia are very different from those active during hypoxia, acute hypercapnia was induced in 12 conscious dogs in order to study the cimetidine effect in this condition. Six dogs were given i.v. saline (control group) and the other six, i.v. cimetidine (4 mg kg-1). After 15 min, CBF and various muscular blood flow measurements were performed, using the microspheres technique under two conditions: (1) breathing air and (2) after 2 h inhalation of a gas mixture with FiCO2 0.10, FiO2 0.21 in nitrogen. The CBF increase was similar in both series with or without cimetidine. The changes in muscular blood flow were unaffected by the H2-blocker. We conclude that cimetidine has no effect on the CBF and on muscular blood flow during acute hypercapnia.

Effects of H2-receptor blockers on response of cerebral blood flow to normocapnic hypoxia

Cimetidine blunts the increase in cerebral blood flow (CBF) normally observed during hypoxia. It is important, therefore, to know whether other H2-blockers also affect the cerebral circulation adaptation to hypoxia. Cerebral blood flow was measured in 24 awake dogs after an intravenous injection of either saline (control) or one of three H2-blockers: 1 mg/kg ranitidine, 0.4 mg/kg famotidine, or 1 mg/kg roxatidine. These doses are equipotent blockers of H2-gastric receptors. Each dog was studied during normoxia and after 2 and 4 h of normocapnic hypoxia (FIO2, 0.10; FICO2, 0.035). During each set of experimental conditions, a bolus of either saline or one of the anti-H2 drugs was administered, and, 15 min later, radiolabeled microspheres (ruthenium 103, scandium 46, and cerium 141) were injected into the left atrium for measurement of regional CBF. After death by an overdose of thiopental, each dog's brain was excised and fixed in 10% formaldehyde; it was then weighed and dissected by region, with the radioactivity measured in each region using a gamma counter. During hypoxia, PaO2 ranged from 45 to 50 mm Hg, and pH, PaCO2, and hematocrit were within the normal limits. In the control group CBF increased 34% above normoxic baseline levels after 2 h and 31% after 4 h of hypoxia. Ranitidine (1 mg/kg) did not prevent the increase in CBF during hypoxia, but famotidine and roxatidine prevented it. When the dose of ranitidine was doubled (2 mg/kg), it too abolished the increase of CBF induced by hypoxia. In conclusion, H2-receptor blockers could interfere with the adaptation of CBF during hypoxia.

Accuracy and precision of fourteen pulse oximeters

Two sets of seven pulse oximeters (Criticare CSI-502; Nellcor-N200; Datex-Satlite; Physio-Control-Lifestat 1600; Critikon-Oxyshuttle; Ohmeda-Biox 3700; Ohmeda-Biox 3740; Radiometer-Oxi; Spectramed-Pulsat; Kontron-7840; Biochem-Ox2000; Invivo-4500; Engstrom-EOS; Novametrix-505) were studied in two groups of eight healthy subjects, aged 26-50 yrs. The transcutaneous oxygen saturation (SpO2) was compared with arterial oxygen saturation (SaO2) measured in simultaneously with drawn blood samples (OSM2 Radiometer) at four 20 min steady-state levels of inspired oxygen fraction (FIO2) (0.21, 0.10, 0.08 and 0.07; SaO2 99-55%) in a conditioned chamber. Both the error in accuracy (mean SpO2-SaO2 difference), and the error in precision (SD of the differences) remained less than 3% for the two highest FIO2 levels (SaO2 greater than 83%) but, during deeper hypoxia, they were increased to 8% and 5%, respectively. An instrumental systematic bias affected accuracy in particular. We concluded that a good agreement between SpO2 and SaO2, as reflected by the Bartko's intraclass coefficient, was observed in nine instruments.

Prevalence of antibodies to HTLV-1 in South American Indians (Mapuches) from Chile

The seroprevalence of HTLV-1 antibodies was investigated in 405 serum samples from healthy South American Indians (Mapuches) from Chile, using enzyme linked immunoassay (ELISA), Western immunoblot (WB) and radioimmuno precipitation assay (RIPA). Six samples were positive by ELISA; 3 of them were confirmed by WB/RIPA. Thus, we observed a seroprevalence of 0.7% for HTLV-1 antibodies in healthy Mapuches.

Effects of cilazapril, a novel angiotensin converting enzyme inhibitor, on the structure of pulmonary arteries of rats exposed to chronic hypoxia

Chronic hypoxia is known to be associated with a thickening of the media of pulmonary arteries. The goal of the present study was to assess if cilazapril, a novel long-acting angiotensin converting enzyme (ACE) inhibitor, could prevent this thickening. For this purpose, three groups of rats were studied. One group was kept in normal room air. Two other groups were exposed to chronic hypoxia (inspired fraction of oxygen equal to 8% during 4 weeks). One group of hypoxic rats was treated with placebo and the other group received cilazapril (as food admixture of approximately 3 mg/kg/day). After 4 weeks, rats were anesthetized and pulmonary artery pressure and hematocrit measured. Then, the lungs were perfused and fixed and morphometry of the pulmonary arteries was performed. Hypoxia induced an increase in pulmonary artery pressure and hematocrit associated with a dramatic increase in the thickness of the media of the pulmonary arteries. Cilazapril completely prevented the thickening of the media of the pulmonary arteries but did not significantly decrease the pulmonary artery pressure or right ventricular weight.

Accuracy and precision of fourteen pulse oximeters

Two sets of seven pulse oximeters (Criticare CSI-502; Nellcor-N200; Datex-Satlite; Physio-Control-Lifestat 1600; Critikon-Oxyshuttle; Ohmeda-Biox 3700; Ohmeda-Biox 3740; Radiometer-Oxi; Spectramed-Pulsat; Kontron-7840; Biochem-Ox2000; Invivo-4500; Engström-EOS; Novametrix-505) were studied in two groups of eight healthy subjects, aged 26-50 yrs. The transcutaneous oxygen saturation (SpO2) was compared with arterial oxygen saturation (SaO2) measured in simultaneously with drawn blood samples (OSM2 Radiometer) at four 20 min steady-state levels of inspired oxygen fraction (FIO2) (0.21, 0.10, 0.08 and 0.07; SaO2 99-55%) in a conditioned chamber. Both the error in accuracy (mean SpO2-SaO2 difference), and the error in precision (SD of the differences) remained less than 3% for the two highest FIO2 levels (SaO2 greater than 83%) but, during deeper hypoxia, they were increased to 8% and 5%, respectively. An instrumental systematic bias affected accuracy in particular. We concluded that a good agreement between SpO2 and SaO2, as reflected by the Bartko's intraclass coefficient, was observed in nine instruments.

[HTLV-1 in a Mapuche population]

The seroprevalence of HTLV-1 in 405 serum samples from South American Indians, Mapuches, from the IXth region of Chile was investigated using enzyme linked immunosorbent assay (ELISA). Six samples were positive and only 3 of them were also positive by western blot and radio immuno precipitation assay. This corresponds to a seroprevalence of 0.74% for HTLV-1 in healthy Mapuches, which differs from that observed in other populations throughout the world. Additional studies are necessary to evaluate the real magnitudes of HTLV-1 infection in Chile.

The inotropic effect of digoxin on an isolated rat heart in hypercapnia and (or) hypoxia

The explanation for the increased frequency of troubles with digoxin therapy in patients with chronic pulmonary diseases is debated. The reported effects of hypoxia in vivo on myocardial levels of digoxin are contradictory, and there have been few studies on the effects of hypercapnia. In the past, it has been shown in rat myocardial tissue at rest in vitro that hypoxia decreased and hypercapnic acidosis increased the digoxin uptake. We performed a new study in vitro in an isolated beating rat heart perfused at constant flow (37 degrees C) and stimulated at a constant frequency (6 Hz). The performances were recorded with an intraventricular balloon equipped with a tip-manometer catheter. The action of digoxin was studied by recording systolic pressure (PS) and diastolic pressure (PD), the left ventricular developed pressure (LVDP = PS - PD), the (dP/dt)max, and the ratio (dP/dt)max/PS. First, the heart was perfused for 30 min with a modified Tyrode's solution perfusate aerated with carbogen (pH = 7.40; PCO2 = 37 mmHg; PO2 = 530 mmHg) (1 mmHg = 133.32 Pa). Various parameters of contractions were recorded (initial control values). Then the heart was perfused for 15 min with Tyrode's solution aerated either with a hypoxic gas mixture (pH = 7.41; PCO2 = 36 mmHg; PO2 = 122 mmHg), a hypercapnic gas mixture (pH = 7.08; PCO2 75 mmHg; PO2 = 485 mmHg), or a hypoxic-hypercapnic gas mixture (pH = 7.09; PCO2 = 73 mmHg; PO2 = 124 mmHg). Control hearts were continuously perfused with Tyrode's solution aerated with carbogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic CO2 removal by dialysis: THAM vs NaOH infusion

New methods of respiratory support are needed to reduce the high mortality rate of acute respiratory failure. To simplify the procedures of extracorporeal CO2 elimination under apneic oxygenation, one approach is to replace the membrane lung by a hemodialyzer and to administer an alkali, since hemodialysis requires a lower blood flow rate than blood-gas exchange. This study compared the effectiveness of trishydroxymethyl aminomethane (THAM) and NaOH in this procedure. Twelve male Anglo-Poitevin dogs (25 to 33 kg) were anesthetized, curarized and mechanically hypoventilated (VE = 41% of the control value). After not less than 15 min, a venovenous shunt was used for dialysis with blood flow of 7-10 ml. min.-1kg-1 for at least 8 hours. The dialysate contained no acetate, bicarbonate or lactate, but was alkalinized to a pH of 8-9 by the addition of NaOH. A solution of THAM (0.5 N) was infused into the right heart at the rate of 0.30 ml.min.-1kg-1 in six animals, and NaOH (0.15 N) was infused in the other six at the rate of 0.80 ml.min-1kg-1. The injected volumes were compensated for by an equivalent amount of ultrafiltration. Elimination of CO2 (mean TCO2 = 2.3 ml.min.-1kg-1) was the same with both methods and the difference for the electrolytes and acid-base equilibrium was only very small. However, hemolysis was six times greater with NaOH than with THAM. Despite ultrafiltration, a similar marked weight gain was observed from the second hour of the experiment in the NaOH series, but only after 7 hours with THAM. It thus appears that hemodialysis combined with alkalinisation is still too complex a procedure to be safely applied in acute or chronic pulmonary failure.

Theophylline disposition in patients with COLD with and without hypoxemia

This study aimed to assess the effect of hypoxemia on theophylline disposition. Ten patients with a mean (+/- SEM) of 58 +/- 3 years with COLD (PaO2 55 +/- 1 mm Hg, PaCO2 46 +/- 2 mm Hg, and pH of 7.39 +/- 0.01) were hospitalized to have oxygen therapy. Before starting O2, they received intravenously, 4 mg/kg of theophylline over a 20-minute period; blood samples and urine were collected for six hours. The results suggested that hypoxia does not influence the disposition of theophylline or its metabolites.

Effects of hypoxia and hypercapnia on atrial natriuretic factor and plasma renin activity in conscious dogs

1. The aim of the present study was to assess the effects of hypercapnia or hypoxia on plasma concentrations of atrial natriuretic factor (ANF) in conscious unrestrained dogs. 2. For this purpose, chronically instrumented dogs were exposed in a random order to either room air, or to an atmosphere containing 21% O2/10% CO2/69% N2 to produce hypercapnia, or 10% O2/3% CO2/87% N2 to produce hypoxia without respiratory alkalosis. 3. Plasma concentrations of ANF did not change significantly during hypoxia. 4. In contrast, during hypercapnia, plasma concentrations of ANF increased by more than 100% and returned to baseline at the end of hypercapnia. 5. Hypercapnia, but not hypoxia, induced an increase in left atrial and central venous pressures. 6. We conclude that hypercapnia increases plasma ANF concentration, and that this increase may be secondary to an increase of the left and right atrial pressures. These phenomena may explain the increase in diuresis and natriuresis which has been described during hypercapnia.

Cardiorespiratory function and pathological findings in heart-lung block reimplanted after hypothermic preservation

A good result from the heart-lung transplantation depends on the quality of the preservation of cardiopulmonary transplants. To determine the functional and pathological status of the heart-lung block after preservation for several hours, we performed 10 heterologous heart-lung transplantations in Beagle dogs (weight 13.5 kg) under extracorporeal circulation. Weight and length compatibility between donor and receiver was ensured. Measurements of hemodynamics, lung mechanics and blood gases were performed in the donor and in the receiver before the transplantation, and in the receiver after heart-lung reimplantation. Histological studies were carried out by biopsy on the heart and on the lung of the donor before removal, at the beginning of the preservation at low temperature, after 3 h of ischemia in cold, and every hour after recirculation in the heart-lung block. Myocardial preservation was conducted with cold cardioplegia at 4 degrees C (Ringer lactate solution with high potassium). Lung preservation was achieved by injecting a Euro-Collins solution at 4 degrees C, with addition of dog plasma, into the pulmonary artery; during the whole ischemic phase, the lung parenchyma was maintained at 0 degrees C, and inflated at a 10 cm H2O pressure. After transplantation, we observed that cardiac output was low in all cases, with normal or subnormal pulmonary arterial pressure. Dynamic lung compliance was very low immediately after transplantation, and increased when restarting the circulation, but deteriorated again after several hours. At the same time alveolo-arterial O2 pressure difference and arterio-alveolar CO2 pressure difference progressively increased, due to the extensive gas exchange impairment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of moderate hypoxemia on atrial natriuretic factor and arginine vasopressin in normal man

The object of this study was to assess the effect of moderate acute hypoxemia on plasma concentrations of atrial natriuretic factor (ANF), arginine vasopressin (AVP), plasma renin activity (PRA) and urinary excretion of prostaglandin E2 (UPGE2V). Eight volunteers were exposed for 2 hours to a gas mixture containing 10% O2, 4.5% CO2 and 85.5% N2. Hypoxia increased diastolic blood pressure and free water clearance. Hypoxia did not change the AVP, PRA or UPG2V, although increased ANF from 17.7 +/- 3.4 pg/mL to 27.2 +/- 1.7 pg/mL (p less than 0.005) at 120 minutes. ANF changes were closely associated with the rise in blood pressure.

Cimetidine inhibits cerebral and hepatic mitochondrial respiration in rat

The presence of cimetidine in the incubation medium of rat brain mitochondria caused decreased oxygen uptake, especially during oxidative phosphorylation (state 3). This inhibition of the respiratory control and of ATP synthesis was dose-dependent. The same observations were made for hepatic mitochondria. The significance of these results is discussed in terms of both the neurological side-effects of cimetidine and its effect on regulatory mechanisms of cerebral or hepatic blood flow.

Resting muscle glucose metabolites and related compounds in hypercapnia

Glucose metabolites (lactate, pyruvate, citrate, malate), alanine, glutamate and adenosine triphosphate (ATP) were determined in the resting anterior tibial muscle of dogs. The muscle was sampled in anesthetized animals first breathing air, and secondly after an hour of breathing a hypercapnic mixture, FICO2 = 0.10 (experimental subjects n = 6) or air (control subjects n = 6). A decrease in concentration of glucose metabolites (lactate: -34%; pyruvate: -24%; Citrate: -34%; malate: -54%), glutamate (-43%), alanine (-35%) and ATP (-8%) was observed in the resting muscle during acute hypercapnic acidosis. This was not the case in control animals breathing air.

[Tracheal rupture in closed cervicothoracic injuries]

Two cases are reported of tracheal fracture occurring during blunt cervico-thoracic trauma. The first case emphasized the impossibility of passing a tracheal tube below the fracture preoperatively, without the fibreoptic or rigid tracheoscope which has the added advantage of making an accurate diagnosis of the fracture. The clinical and paraclinical signs could only lead to a suspicion of the diagnosis, but not its confirmation. The second case showed the use of different ventilatory means during and after surgery (separate lung ventilation, jet ventilation, high frequency ventilation) in case of proximal fractures. The monitoring of intratracheal pressure should be used so as to have the best ventilation for the lowest pressures in the suture zone.

Cimetidine inhibits the hypoxia-induced increase in cerebral blood flow in dogs

Cimetidine is an H2 receptor-blocking drug frequently given to ICU patients for the prevention of stress ulcers. However, histamine causes potent cerebral vasodilation through the H2 receptors. This study tested the hypothesis that cimetidine, by blocking the H2 receptors, could blunt the increase of cerebral blood flow induced by hypoxia. We induced isocapnic hypoxia in 12 conscious dogs that were randomly divided into two groups. Six dogs received no treatment (control group), and the other six received iv cimetidine (4 mg/kg) to block the H2 receptors. Cerebral blood flow (CBF) was measured with the radioactive microsphere technique before, and 2 and 4 h after hypoxia was induced. In the control group, CBF significantly increased with hypoxia in all the regions of the brain. Cimetidine blunted this increase in all the regions of the brain except the pons and bulb. As a result of the reduced flow, cimetidine significantly decreased the oxygen supply to the brain compared to the control group. We conclude that cimetidine blunts the increase in CBF during hypoxia and might reduce oxygen supply to the brain in hypoxic patients.

The influence of hypoxemia on tritiated digoxin plasma kinetics and tissue distribution in the conscious dog

The goals of this study were to document the effect of hypoxemia on the distribution of digoxin in conscious dogs. For this purpose, 6 beagles were exposed to air and 6 others to an atmosphere containing 10% O2, to generate a PaO2 equal to 46.3 +/- 0.3 mmHg (mean +/- SEM). The animals received 25 micrograms/kg of digoxin containing 2.17 micrograms/kg of 3H-digoxin, and then blood and urine samples were collected over the next 48 h, at which time they were killed to determine digoxin concentrations in several tissues. Five additional beagles were used to assess the influence of hypoxemia on the blood perfusion to these tissues using radioactive microspheres. The results, expressed as digoxin equivalents, indicated that hypoxemia increased the digoxin apparent volume of distribution (2.85 +/- 0.10 versus 2.01 +/- 0.11 L/kg; p less than 0.001) and the time required to achieve this distribution (9.7 +/- 1.4 versus 2.6 +/- 0.3 h; p less than 0.01). As digoxin clearance was not influenced by hypoxemia, the half-life was increased from 25.2 +/- 1.5 to 33.4 +/- 1.3 h (p less than 0.01). With hypoxemia, digoxin concentrations increased significantly in the brain and diaphragmatic muscle, but only marginally in other organs, including the heart, the latter despite a significant increase in blood flow. It is concluded that hypoxia does change digoxin disposition but does not increase digoxin heart concentrations. Therefore, factors other than changes in digoxin plasma kinetics and heart distribution may be responsible for the decrease in digitalis tolerance during hypoxemia.

In vitro effects of hypoxia and (or) hypercapnic acidosis on the myocardial uptake of digoxin

A recent study has shown in the conscious dog that hypoxia associated with respiratory acidosis could increase the in vivo distribution of digoxin in the myocardium. The aim of the present study was to evaluate in vitro the effects of hypoxia and (or) hypercapnic acidosis on the digoxin uptake. For this purpose, rat myocardium was incubated for 180 min with radiolabelled [3H]digoxin. The uptake of digoxin which was expressed in nanograms of digoxin bound per 100 mg of myocardium was decreased by hypoxia and increased by hypercapnic acidosis. The association of hypoxia and hypercapnic acidosis had no effect on the digoxin uptake, suggesting that in vitro hypoxia acts in an opposite way to hypercapnia.

Influence of hypoxemia and respiratory acidosis on the plasma kinetics and tissue distribution of digoxin in the conscious dog

The aim of the present study was to investigate the influence of hypoxemia combined with respiratory acidosis on the kinetics of digoxin in conscious dogs. One group of three beagles was exposed to air and 7 days later to 10% O2, 10% CO2, and 80% N2. In a second group of three dogs, the order of exposure to the two atmospheric conditions was reversed. The dogs received 25 micrograms/kg digoxin and blood and urine samples were collected over the next 29 h. At the conclusion of the second treatment, the dogs were sacrificed to determine digoxin concentrations in the left ventricle, liver, renal cortex, and skeletal muscle. Digoxin total body clearance increased from 6.2 +/- 0.9 in control to 9.0 +/- 1.0 mL X min-1 X kg-1 in hypoxemic and hypercapnic dogs (p less than 0.05). The digoxin apparent volume of distribution at steady state (Vss) was increased in the dogs with hypoxemia and hypercapnia (11.63 +/- 1.11 vs. 8.62 +/- 0.41 L/kg in the controls, p less than 0.05). As a consequence the digoxin plasma half-life remained unchanged (18.6 +/- 1.5 h in hypoxemic and hypercapnic dogs versus 20.1 +/- 2.8 h in the controls). In dogs with hypoxemia and hypercapnia, the ratio of tissue to plasma digoxin concentrations tended to increase in the liver, in the renal cortex, and in the left ventricle and remained unchanged in the left hind leg muscle. In vitro studies showed that the digoxin total binding to erythrocyte membranes was slightly increased in the dogs with hypoxemia and hypercapnia, resulting from an increase in the apparent intrinsic association constant for digoxin (p less than 0.003). It is concluded that hypoxemia combined with respiratory acidosis changes digoxin disposition in the conscious dog and is the cause of a digoxin redistribution into the tissues.