Changes In Arterial Oxygen Tension Research Articles

CARDIAC ARRHYTHMIA AND HEMODYNAMIC EFFECTS DURING FIBEROPTIC BRONCHOSCOPY

Background: Arterial hypoxaemia is the commonest complication, since respiratory depressant drugs are used for premedication, and the airway is also partially occluded by the bronchoscope. The study is therefore designed to nd out the changes in arterial oxygen tension during FOB along with effect on central hemodynamics, electrocardiographically. Objectives: · To nd out the changes in arterial oxygen tension during FOB. · To study the effects of beroptic bronchoscopy on central hemodynamics, electrocardiographically. Methods: 100 patients aged 20 years and above undergoing FOB were evaluated for the arterial oxygen tension and cardiac rhythm changes at KNCH, Jodhpur. Results: All patients developed a fall in PaO2 following FOB which was highly signicant but hypoxaemia was noted only in 18 cases. There was a uniform rise in heart rate in all the patients after FOB which was statistically signicant. There was slight sinus tachycardia in most of the cases although E.C.G. indicated no major arrhythmias. There was no signicant change in pH level. Mean of SBP and mean of DBP was statistically signicant. Conclusion: All patients showed a decline in arterial PaO2 after FOB which was highly signicant. There was slight sinus tachycardia in most of cases. FOB itself does not endanger the patients with any signicant or serious complications.

PEAK EXPIRATORY FLOW RATE CHANGES AND HYPOXEMIA DURING FIBEROPTIC BRONCHOSCOPY

Background: Arterial hypoxaemia is the common complication, since respiratory depressant drugs are used for premedication, and the airway is also partially occluded by the bronchoscope. The study is therefore designed to nd out the changes in arterial oxygen tension and PEFR during FOB. Objectives: Ÿ To nd out the changes in arterial oxygen tension and in peak expiratory ow rate during FOB. Methods: 100 patients aged 20 years above undergoing FOB were evaluated for the arterial oxygen tension and peak expiratory ow rate changes at KNCH, Jodhpur. Results: All patients developed a fall in PaO2 following FOB but hypoxemia was noted only in 18 cases. The change in PEFR after FOB, no signicant statistical difference could be observed on statistically comparison. Conclusion: All patients showed a decline in arterial PaO2 after FOB which was highly signicant. FOB itself does not causes any signicant or serious complications.

HYPOXEMIA AND LUNG MECHANICS DURING FIBEROPTIC BRONCHOSCOPY

Background: Arterial hypoxaemia is the common complication, since respiratory depressant drugs are used for premedication, and the airway is also partially occluded by the bronchoscope. The study is therefore designed to nd out the changes in arterial oxygen tension and PFTduring FOB. Objectives: To nd out the changes in arterial oxygen tension and pulmonary function tests during FOB. Methods: One hundred patients aged 20 years above undergoing FOB were evaluated for the arterial oxygen tension and pulmonary function tests changes at KNCH, Jodhpur. Results: All patients developed a fall in PaO2 following FOB but hypoxemia was noted only in 18 cases. Bronchial washing, punch biopsy and brush biopsy did contribute to hypoxemia signicantly. The pulmonary functions which include change in V.C, FVC, FEV1 and PEFR after FOB, no statistical difference could be observed on statistically comparison. Conclusion: All patients showed a decline in arterial PaO2 after FOB which was highly signicant. FOB itself does not cause any signicant or serious complications.

An observational study to find out changes in arterial oxygen tension during fiberoptic bronchoscopy

Background: Arterial hypoxaemia is the commonest complication, since respiratory depressant drugs are used for premedication, and the airway is also partially occluded by the bronchoscope. The study is therefore designed to find out the changes in arterial oxygen tension during (FOB) along with effect on central hemodynamics, pre and post procedure ECG and PFT. Objectives were to find out the changes in arterial oxygen tension during FOB, to study the effects of FOB on central hemodynamics, electrocardiographically and pulmonary function tests.Methods: One hundred patients aged 20 years undergoing FOB were evaluated for the arterial oxygen tension and cardiac rhythm changes at KNCH, Jodhpur during 12 months study period from 2019 to 2020.Results: All patients developed a fall in PaO2 following FOB but hypoxaemia was noted only in 18 cases. Bronchial washing, punch biopsy and brush biopsy did contribute to hypoxaemia significantly. There was a uniform rise in heart rate in all the patients after FOB which was statistically significant. There was slight sinus tachycardia in most of the cases although ECG indicated no major arrhythmias. The pulmonary functions which include change in VC, FVC, FEV1 and PEFR after FOB, on statistically comparison no statistical difference could be observed. There was no significant change in PaCO2 level and pH level. Mean of SBP and mean of DBP was statistically significant.Conclusions: All patients showed a decline in arterial PaO2 after FOB which was highly significant. There was slight sinus tachycardia in most of cases. FOB itself does not endanger the patients with any significant or serious complications.

The middle cerebral artery blood velocity response to acute normobaric hypoxia occurs independently of changes in ventilation in humans

What is the central question of this study? Does the ventilatory response to moderate acute hypoxia increase cerebral perfusion independently of changes in arterial oxygen tension in humans? What is the main finding and its importance? The ventilatory response does not increase middle cerebral artery mean blood velocity during moderate isocapnic acute hypoxia beyond that elicited by reduced oxygen saturation. Hypoxia induces ventilatory, cardiovascular and cerebrovascular adjustments to defend against reductions in systemic oxygen delivery. We aimed to determine whether the ventilatory response to moderate acute hypoxia increases cerebral perfusion independently of changes in arterial oxygenation. Eleven young healthy individuals were exposed to four 15min experimental conditions: (1) normoxia (partial pressure of end-tidal oxygen, =100mmHg), (2) hypoxia ( =50mmHg), (3) normoxia with breathing volitionally matched to levels observed during hypoxia (hyperpnoea; =100mmHg) and (4) hypoxia ( =50mmHg) with respiratory frequency and tidal volume volitionally matched to levels observed during normoxia (i.e., restricted breathing (RB)). Isocapnia was maintained in all conditions. Middle cerebral artery mean blood velocity (MCA Vmean ), assessed by transcranial Doppler ultrasound, was increased during hypoxia (58±12cm/s, P=0.04) and hypoxia + RB (61±14cm/s, P<0.001) compared to normoxia (55±11cm/s), while it was unchanged during hyperpnoea (52±13cm/s, P=0.08). MCA Vmean was not different between hypoxia and hypoxia + RB (P>0.05). These findings suggest that the hypoxic ventilatory response does not increase cerebral perfusion, indexed using MCA Vmean , during moderate isocapnic acute hypoxia beyond that elicited by reduced oxygen saturation.

The Effect of Equal Ratio Ventilation on Oxygenation, Respiratory Mechanics, and Cerebral Perfusion Pressure During Laparoscopy in the Trendelenburg Position

The aim of this study was to investigate the effects of equal ratio ventilation (ERV) on oxygenation, respiratory mechanics, and the cerebral perfusion pressure during pneumoperitoneum in the Trendelenburg position. Thirty patients undergoing laparoscopic low anterior resection (25 to 65 y) were enrolled. Mechanical ventilator was set to volume-controlled mode at an inspiratory to expiratory (I:E) ratio of 1:2 with a tidal volume of 8 mL/kg of ideal body weight with a 5 cm H2O positive end-expiratory pressure. Twenty minutes after pneumoperitoneum in the Trendelenburg position, the I:E ratio was changed to 1:1 for 20 minutes and then restored to 1:2. No significant changes in arterial oxygen tension and respiratory compliance after adopting ERV. Mean arterial pressure and cerebral perfusion pressure decreased significantly over time after adopting the Trendelenburg position during pneumoperitoneum (P=0.014 and 0.005, respectively). In conclusion, there was no improvement in oxygenation or respiratory mechanics with ERV.

Changes in arterial oxygen tension and physiological status in resting, unrestrained Arctic charr Salvelinus alpinus (L.) exposed to mild hypoxia and hyperoxia

In arctic charr Salvelinus alpinus, arterial blood partial pressures of oxygen (PaO2) and carbon dioxide increased with increasing water oxygen tension (PwO2), while the water to arterial PO2 difference (PwO2-PaO2) did not change in relation to PwO2.

Carotid body oxygen sensing

The carotid body (CB) is a neural crest-derived organ whose major function is to sense changes in arterial oxygen tension to elicit hyperventilation in hypoxia. The CB is composed of clusters of neuron-like glomus, or type-I, cells enveloped by glia-like sustentacular, or type-II, cells. Responsiveness of CB to acute hypoxia relies on the inhibition of O(2)-sensitive K(+) channels in glomus cells, which leads to cell depolarisation, Ca(2+) entry and release of transmitters that activate afferent nerve fibres. Although this model of O(2) sensing is generally accepted, the molecular mechanisms underlying K(+) channel modulation by O(2) tension are unknown. Among the putative hypoxia-sensing mechanisms there are: the production of oxygen radicals, either in mitochondria or reduced nicotinamide adenine dinucleotide phosphate oxidases; metabolic mitochondrial inhibition and decrease of intracellular ATP; disruption of the prolylhydroxylase/hypoxia inducible factor pathway; or decrease of carbon monoxide production by haemoxygenase-2. In chronic hypoxia, the CB grows with increasing glomus cell number. The current authors have identified, in the CB, neural stem cells, which can differentiate into glomus cells. Cell fate experiments suggest that the CB progenitors are the glia-like sustentacular cells. The CB appears to be involved in the pathophysiology of several prevalent human diseases.

Changes in Arterial Oxygen Tension Correlate with Changes in End-expiratory Carbon Monoxide Level

ObjectiveCarbon monoxide (CO) and oxygen compete for haemoglobin binding sites. While the effects of increased inspiratory oxygen fractions on exhaled carbon monoxide concentrations have been studied previously, the relationships between intravascular oxygen tension, blood carboxyhaemoglobin levels and expiratory CO concentrations remain unclear. We therefore studied the effects of increases in arterial oxygen tension as crucial determinant for the displacement of carbon monoxide from its haemoglobin bond during lung passage.MethodsMeasurements of end-expiratory CO concentrations (eCO), arterial oxygen tensions and carboxyhaemoglobin concentrations were performed in 19 patients while breathing air and oxygen.ResultsWith increasing PaO2 (from 11.5 ± 1.9 to 35.2 ± 10.3 kPa) end-expiratory CO concentrations increased from 8.6 ± 4.9 to 16.7 ± 9.4 ppm, p < 0.001, with a mean increase of 0.36 ppm CO per kPa increase in PaO2 (ΔeCO [ppm] = 0.36 *␣ΔPaO2 [kPa]). Increases of arterial oxygen tension correlated with increases of end-expiratory CO concentration (r2 = 0.33). Arterial carboxyhaemoglobin concentrations decreased from 1.06 ± 0.37 during air breathing to 0.92 ± 0.35 % after 5 minutes of oxygen inhalation (p < 0.001).ConclusionsOxygen-induced increases in exhaled CO correlate with increases in arterial oxygen tensions. Furthermore, oxygen inhalation reduces carboxyhaemoglobin levels, supporting the concept of accelerated CO elimination by oxygen via the lungs.

Regional dynamics of the fMRI-BOLD signal response to hypoxia-hypercapnia in the rat brain.

To examine the regional blood oxygenation level-dependent (BOLD) signal response to rapid changes in arterial oxygen tension. Functional MR imaging (fMRI) was carried out in five male Sprague-Dawley rats anesthetized with Sodium Pentobarbital. Rats were subjected to different durations of apnea as a rapid, graded, and reversible hypoxic-hypercapnic stimulus. Dynamics of the BOLD signal response were studied on a pixel-by-pixel basis in the cerebral cortex, hippocampus, third ventricle, and thalamus in the rat brain. Apnea induced a BOLD signal drop in all the brain regions studied, the magnitude of which increased with longer durations of the stimulus. The signal recovered to preapnic baseline levels after resumption of normal ventilation. Regional variation in the BOLD signal dynamics was observed with the magnitude of the BOLD signal change in the hippocampus being the least, followed by a relatively larger change in the thalamus, cerebral cortex, and third ventricle. The time (t(0)) for the signal change after the onset of the stimulus was estimated for every pixel. Time delay maps generated show the highest onset time values in the hippocampus followed by the thalamus, cerebral cortex, and third ventricle. The regional dynamics of the BOLD signal in the brain in response to apnea may vary depending on the rate of oxygen metabolism in addition to cerebral blood flow (CBF).

Changes in arterial oxygen tension when weaning neonates from inhaled nitric oxide*†

We set out to evaluate changes in arterial oxygen tension (PaO(2)) when weaning neonates from inhaled nitric oxide (INO). We reviewed the records of 505 prospectively collected INO weaning attempts on 84 neonates with hypoxic respiratory failure. PaO(2) values before and 30 min after weaning attempts were recorded. Relationships between change in PaO(2) and decreases in INO concentrations were investigated using regression analysis and ANOVA. PaO(2) decreased (-18.7 +/- 1.8 torr; P < 0.001); when weaning INO. A stepwise decline in PaO(2) was observed weaning INO from 40 ppm. The greatest decline occurred when INO was discontinued (-42.1 +/- 4.1 torr). Forward stepwise multiple regression using variables with significant relationships to the decline in PaO(2) identified the specific dose reduction 7(P < 0.001), the prewean PaO(2) (P < 0.001), and surfactant therapy (P = 0.018) as the variables best describing the change in PaO(2)(P = 0.004, r = 0.51). In conclusion, a graded decline in PaO(2) occurs when reducing INO. INO should be weaned to less than 1 ppm before discontinuing its use. Prior surfactant treatment appears to enhance the oxygenation reserve when weaning INO.

Inhaled Nitric Oxide to Newborns and Infants after Congenital Heart Surgery on Cardiopulmonary Bypass: A Dose-response Study

Twelve patients (median age 3.8 months) with pulmonary hypertension in the postoperative period after congenital heart surgery on cardiopulmonary bypass were given inhaled nitric oxide. Effects on cardiovascular and respiratory systems were measured. Mean pulmonary artery pressure decreased from 33+/-2 to 28+/-2 mmHg (p < 0.001) and arterial oxygen tension increased from 13.3+/-2.3 to 16.7+/-2.7 kPa (p < 0.05). The mean change in arterial oxygen tension in percent was 29.8+/-6.3% (p < 0.05). The response was significant only in the first step from 0 to 3 or 5 ppm with no further significant changes in mean pulmonary artery pressure or oxygenation at higher doses. The decrease in mean pulmonary artery pressure was concomitant with a significant increase in arterial oxygen tension. No dose-response relationship was found with increasing doses to 80 ppm.

Akuter Effekt von inhalativem Stickstoffmonoxid auf die Oxygenation bei Kindern mit Virus-induzierter, hypoxämischer Ateminsuffizienz

Zusammenfassung Fragestellung: Inhalatives Stickstoffmonoxid (iNO) wird seit einigen Jahren zur Verbesserung der Oxygenation bei Kindern und Erwachsenen mit ARDS (acute respiratory distress syndrome) eingesetzt. Wir berichten über den Soforteffekt von iNO bei 4 Kindern mit einem Virus-induzierten, hypoxämischen Atemversagen. Methode: Wir verabreichten iNO während der 1. Stunde in einer Dosis von 20 ppm und reduzierten anschließend auf 5 ppm. Die Wirkung auf die Oxygenation wurde mittels des arteriellen Sauerstoffpartialdrucks und des Oxygenationsindex beurteilt. Ergebnisse: Die Inhalation von NO führte bei 3 Kindern zu einer akuten Verbesserung der Oxygenation und zu einer Reduktion des Oxygenationsindex um mehr als 30%. Bei 1 Kind bewirkte iNO eine deutliche Abnahme des pulmonal-arteriellen Widerstands, ohne die Oxygenation zu verbessern. Diskussion: Obwohl nach bisherigen Studien die Anwendung von iNO die Mortalität des ARDS nicht senkt, halten wir dessen Einsatz bei Kindern mit akutem, hypoxämischem Atemversagen für gerechtfertigt, wenn die erzielte Verbesserung der Oxygenation zu einer Stabilisierung des Krankheitsbilds beiträgt.

Effect of exercise on concentrations of immunoreactive endothelin in bronchoalveolar lavage fluid of normal horses and horses with chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) represents a major cause of loss of performance in the horse. The role of endothelin (ET), a potent bronchoconstrictive and vasoactive peptide, is currently being investigated in asthma and other obstructive respiratory diseases in man. We have previously found elevated systemic and pulmonary endothelin levels in horses during exacerbation of COPD. In the present study, our aim was to examine possible variations in ET concentrations occurring during exercise in COPD horses. We compared the effects of intense treadmill exercise on the recovery of endothelin (ET) in the bronchoalveolar lavage fluid (BALF) as well as in arterial and venous blood, in a group of 5 healthy horses and a group of 5 COPD horses studied alternately in remission and while symptomatic. We also investigated the possible correlations between ET levels and pulmonary function tests during the study. While exercise did not affect the ET levels recovered in BALF among controls, it caused a significant increase (P = 0.02) among symptomatic COPD horses. During remission, wide variations of ET levels among horses, at rest and during exercise, made any significant interpretation difficult. No correlation could be found between exercise-induced changes in ET concentrations and pulmonary function tests or changes in arterial oxygen tension with exercise. We conclude that exercise appears to affect the release of ET by the airways in COPD horses, in contrast to healthy horses. It is still unclear, however, whether these differences relate to adjustments of lung function during exercise.

Comparison of tissue oxygen-tension measurements by different devices. An experimental study in pigs

The aim of this study was to evaluate measurements of subcutaneous oxygen tension (PscO2) generated by three different monitors and to compare measurements obtained with and without the tonometer technique in a standardized animal model. Seven domestic pigs weighing 20-28 kg were anaesthetized and mechanically ventilated. A pulmonary artery catheter was inserted for haemodynamic monitoring and temperature measuring. An external carotid artery was cannulated for blood pressure monitoring and blood sampling. A peripheral venous line was used for fluid and drug infusion. The animals were then subjected to hyperoxia and hypoxia by varying the inspiratory oxygen fractions (FiO2) between 0.1 and 0.8. Arterial and central venous blood gases were analysed at each step of FiO2. Cardiac output was determined using a thermo­dilution technique. Central, subcutaneous and skin surface temperatures were measured. Three different devices for subcutaneous tissue oxygen-tension (PscO2) measurements were compared: (1) Biogenesis, (2) Continucath and (3) Paratrend 7. Continucath was used both with and without tonometer simultaneously. Inter Class Coefficient of Correlation between PscO2 values obtained by the two Continucath sensors was 76% and between Paratrend 7 and Biogenesis 88%. Absolute values generated by Biogenesis and Paratrend 7 were not different. PscO2 values obtained with the Continucath devices were on average 50% higher than those generated by Biogenesis and Paratrend 7 (p<0.05). Conclusions: The new Paratrend 7 and the old Biogenesis sensors generate almost equal tissue oxygen-tension values in response to changes in arterial oxygen tension. The Continucath sensor generates almost 50% higher values compared with the Paratrend 7 and Biogenesis sensors, both with and without tonometer.

Ventilation-perfusion matching in chronic heart failure

Background: The exercise limitation of patients with chronic heart failure is associated with an increased ventilatory response during exercise. This is thought to be due, at least in part, to excessive dead space ventilation. Methods: To assess ventilation perfusion matching, 20 patients with chronic heart failure and eight controls with asymptomatic left ventricular dysfunction underwent symptom limited treadmill exercise with arterial blood sampling. Metabolic gas exchange was determined by expired gas analysis. Fractional dead space ventilation and the alveolar arterial oxygen difference were derived. Results: There was a fall in fractional dead space ventilation (0.43 to 0.28; P < 0.001), more marked in the controls (peak dead space fraction 0.19 (controls), 0.32 (patients); P = 0.002). There was a rise in alveolar arterial difference in all patients (1.59 to 2.55 kPa; P = 0.006) with no difference between patients and controls. Arterial carbon dioxide tension fell during exercise (4.89 to 4.63 kPa; P < 0.001), with no difference between patients and controls. There was no significant change in arterial oxygen tension. Conclusions: The fall in arterial carbon dioxide was the same in both patients and controls. The modest increase in alveolar-arterial oxygen difference tension was the same in both groups, which, coupled with the stable arterial oxygen tension makes it unlikely that a primary change in ventilation-perfusion matching is the cause of increased ventilatory response to exercise in chronic heart failure.

Cerebral blood volume changes during exchange transfusions in infants born at or near term

We investigated the effects on cerebral hemodynamics of blood pressure changes during exchange transfusions in infants born at or near term, using near infrared spectroscopy in eight stable infants (mean gestational age, 36.2 ± 1.3 weeks) who underwent a total of 21 exchange transfusions for erythroblastosis fetalis (rhesus hemolytic disease). Changes in mean arterial blood pressure derived from an indwelling umbilical arterial catheter, transcutaneous arterial oxygen and carbon dioxide tension, as well as changes in cerebral oxygenated hemoglobin (HbO 2), deoxygenated hemoglobin (HbR), and total hemoglobin (Hb tot =HbO 2 + HbR), were recorded continuously from 15 minutes before until the completion of the exchange transfusion. Relative change(s) in cerebral blood volume (dCBV) were calculated as follows: dCBV =change in Hb tot × 0.89/Venous hemoglobin. Changes in mean arterial blood pressure and dCBV were observed during all exchange transfusions; a decrease was found during the withdrawal period and an increase during the infusion period. The mean response of dCBV to a change in mean arterial blood pressure was 0.011 ml · 100 gm - 1 · mm Hg. Multivariate analysis showed that dCBV were primarily associated with changes in mean arterial blood pressure, followed by changes in arterial oxygen tension and in exchange cycle duration. We conclude that in stable term and near-term infants, hemorrhagically induced blood pressure changes provoke dCBV. (J P EDIATR 1994;125:617-21)

The effects of five years of nocturnal cuirass-assisted ventilation in chest wall disease

We investigated the long-term effectiveness of cuirass-assisted ventilation, and examined whether mortality and morbidity could have been predicted at the time of admittance. Twenty five patients were commenced on nocturnal cuirass-assisted ventilation between 1983 and 1985, 10 with scoliosis or kyphosis, 8 with a thoracoplasty and 7 with neuromuscular disease. Mean pretreatment vital capacity was 30% of predicted, and arterial carbon dioxide tension (Paco2) was 8.2 kPa (62 mmHg). Fifteen patients were alive 5 yrs later. Two had discontinued assisted ventilation, both dying soon afterwards, and three had been changed to intermittent positive pressure ventilation. Survival could not have been predicted from age, severity of disease, lung volumes or arterial blood gases at presentation. Paco2 in the survivors had risen from a mean of 6.1 kPa (46 mmHg) after one year to 6.8 kPa (52 mmHg) after 5 yrs (p < 0.05), but remained significantly less than at presentation. There were no significant change in arterial oxygen tension (Pao2), lung volumes, respiratory muscle strength, haemoglobin, right heart failure, exercise tolerance, mental function and symptom scores after 5 yrs, compared to after 1 yr. The median amount of time spent in hospital declined from 15 days per patient in the first year after initial discharge with cuirass-assisted ventilation, to between 3-5.5 days per patient in subsequent years. We conclude that nocturnal cuirass-assisted ventilation has a role in long-term management of patients with neuromuscular and skeletal chest wall disorders. A randomized comparison with nasal intermittent positive pressure ventilation is now indicated.

Hypoxemia During Altitude Exposure: A Meta-Analysis of Chronic Obstructive Pulmonary Disease

A previous study identified spirometric testing as a useful adjunct for estimating PaO2 during altitude exposure in patients with chronic obstructive pulmonary disease (COPD). We sought to examine the validity of this finding by quantitative analysis of recent published reports. We analyzed acute hypoxic exposures from five prior studies involving 71 patients. Across all studies, the change in arterial oxygen tension per unit change in inspired oxygen partial pressure (linear slope, dPaO2/dP1O2) correlated with the preexposure forced expiratory volume in 1 s (FEV1, p < 0.01). The correlation with FEV1 held for values weighted or unweighted by sample size, with rotating deletion of each study from analysis one at a time, and with semilog slope as the dependent variable. A formula derived from the semilog slope relationship with FEV1 gave accurate description of the mean hypoxic response in each prior study and individual responses from one study (n = 18): ln (PaO2alt/PaO2g) = Kn.(PIO2alt-PIO2g). We found that FEV1 modulated the values of kn in this study. We conclude, based on analysis of prior studies, that preexposure arterial oxygen tension and FEV1 both influence the prediction of PaO2 during hypoxic exposures in patients with COPD.

Development of an in vivo perfusion system for bovine fetal small intestine.

Previous work at this institution has focused on the role of oxygen availability on the development of bovine small intestine during the perinatal period. In particular, the change in arterial oxygen tension associated with the conversion from placental to pulmonary respiration at birth was hypothesized to initiate an alteration in intestinal macromolecular permeability during the first 24 hours of life that is characteristic of the bovine neonate. The first model used to test this hypothesis was the hypoxic postnatal calf (Tyler and Ramsey, 1989). By providing the newborn calf with a 90:10 mixture of N2:O2, arterial PO2 was maintained at a level similar to that of the fetal calf. The results of this study were inconclusive, however, which may have been due to other changes occurring at birth that influence intestinal development.