Björn A Feet

Effects of hypoxemia and reoxygenation with 21% or 100% oxygen in newborn piglets: extracellular hypoxanthine in cerebral cortex and femoral muscle.

OBJECTIVE To determine whether reoxygenation with an FIO2 of 0.21 (21% oxygen) is preferable to an FIO2 of 1.0 (100% oxygen) in normalizing brain and muscle hypoxia in the newborn. DESIGN Prospective, randomized, animal study. SETTING Hospital surgical research laboratory. SUBJECTS Twenty-six anesthetized, mechanically ventilated, domestic piglets, 2 to 5 days of age. INTERVENTIONS The piglets were randomized to control or hypoxemia groups. Hypoxemia was induced by ventilating the piglets with 8% oxygen in nitrogen, which was continued until mean arterial pressure decreased to <20 mm Hg. After hypoxemia, the piglets were further randomized to receive reoxygenation with an FIO2 of 0.21 (21% oxygen group, n = 9) or an FIO2 of 1.0 for 30 mins followed by an FIO2 of 0.21 (100% oxygen group, n = 9), and followed for 5 hrs. The piglets in the control group were mechanically ventilated with 21% oxygen (n = 8). MEASUREMENTS AND MAIN RESULTS We measured extracellular concentrations of hypoxanthine in the cerebral cortex and femoral muscle (in vivo microdialysis), plasma hypoxanthine concentrations, cerebral arterial-venous differences for hypoxanthine, acid base balances, arterial and venous (sagittal sinus) blood gases, and mean arterial pressures. The lowest pH values of 6.91 +/- 0.11 (21% oxygen group, mean +/- SD) and 6.90 +/- 0.07 (100% oxygen group) were reached at the end of hypoxemia and then normalized during the reoxygenation period. Plasma hypoxanthine increased during hypoxemia from 28.1 +/- 9.3 to 119.1 +/- 31.9 micromol/L in the 21% oxygen group (p < .001) and from 32.6 +/0- 14.5 to 135.0 +/- 31.4 micromol/L in the 100% oxygen group (p <.001). Plasma hypoxanthine concentrations then normalized over the next 2 hrs in both groups. In the cerebral cortex, extracellular concentrations of hypoxanthine increased during hypoxemia from 3.9 +/- 2.8 to 20.2 +/- 7.4 micromol/L in the 21% oxygen group (p < .001) and from 5.9 +/- 5.0 to 25.1 +/- 7.1 micromol/L in the 100% oxygen group (p < .001). In contrast to plasma hypoxanthine, extracellular hypoxanthine in the cerebral cortex increased significantly further during early reoxygenation, and, within the first 30 mins, reached maximum values of 24.9 +/- 6.3 micromol/L in the 21% oxygen group (p < .01) and 34.8 +/- 10.9 micromol/L in the 100% oxygen group (p < .001). This increase was significantly larger in the 100% oxygen group than in the 21% oxygen group (9.7 +/- 4.7 vs. 4.7 +/- 2.6 micromol/L, p < .05). There were no significant differences between the two reoxygenated groups in duration of hypoxemia, hypoxanthine concentrations in femoral muscle, plasma hypoxanthine concentrations, pH, or mean arterial pressure. The cerebral arterial-venous difference for hypoxanthine was positive both at baseline, at the end of hypoxemia, and after 30 mins and 300 mins of reoxygenation, and no differences were found between the two reoxygenated groups. CONCLUSIONS Significantly higher extracellular concentrations of hypoxanthine were found in the cerebral cortex during the initial period of reoxygenation with 100% oxygen compared with 21% oxygen. Hypoxanthine is a marker of hypoxia, and reflects the intracellular energy status. These results therefore suggest a possibly more severe impairment of energy metabolism in the cerebral cortex or an increased blood-brain barrier damage during reoxygenation with 100% oxygen compared with 21% oxygen in this newborn piglet hypoxia model.

Nitric Oxide Contributes to Surfactant-Induced Vasodilatation in Surfactant-Depleted Newborn Piglets

To investigate whether nitric oxide (NO) is involved in surfactant-induced systemic and pulmonary vasodilatation in newborn piglets with surfactant deficiency, 2-6-d-old piglets were subjected to repeated saline lung lavages. They were then randomly assigned to one of two groups (seven in each group): the Nω-nitro-L-arginine methyl ester (L-NAME) group received 3 mg/kg L-NAME i.v. 45 min before endotracheal instillation of 200 mg/kg porcine surfactant; the saline group received saline i.v. at the same time point, and instillation of 200 mg/kg surfactant. Mean arterial blood pressure, systemic vascular resistance, pulmonary arterial pressure, and pulmonary vascular resistance increased significantly after injection of L-NAME (all p < 0.01), whereas the cardiac index decreased significantly (p < 0.05). Saline injection did not change any variable. Significant decreases in mean arterial blood pressure (from a mean± SD of 66 ± 10 to 53 ± 9 mm Hg, p < 0.01), pulmonary arterial pressure (from 29 ± 6 to 23 ± 6 mm Hg,p < 0.01), and systemic vascular resistance (from 0.40 ± 0.13 to 0.33 ± 0.12 mm Hg/mL/min/kg, p < 0.05) were observed only in the saline group after surfactant instillation, whereas the decrease in pulmonary vascular resistance was not significant after surfactant instillation (p = 0.06). In contrast to the saline group, these variables were not modified in the L-NAME group after surfactant instillation. We conclude that the vasodilatory effect of porcine surfactant instillation in newborn piglets with surfactant deficiency is associated with activation of NO synthase.

Cerebral excitatory amino acids and Na+,K+-ATPase activity during resuscitation of severely hypoxic newborn piglets

We tested the hypothesis that early brain recovery in hypoxic newborn piglets is improved by resuscitating with an O2 supply close to the minimum level required by the newborn piglet brain. Severely hypoxic 2‐5‐d‐old anaesthetized piglets were randomly divided into three resuscitation groups: hypoxaemic (n= 8), 21% O2 (n= 8), and 100% O2 groups (n= 8). The hypoxaemic group was mechanically ventilated with 12‐18% O2 adjusted to achieve a cerebral venous O2 saturation of 17‐23% (baseline; 45 ± 1%, mean ± SEM). During the 2h resuscitation period, extracellular aspartate and glutamate concentrations in the cerebral striatum were higher during hypoxaemic resuscitation (p= 0:044 and p= 0:055, respectively) than during resuscitation with 21% O2 or 100% O2, suggesting an unfavourable accumulation of potent excitotoxins during hypoxaemic resuscitation. The cell membrane Na+, K+‐ATPase activity of cerebral cortical tissue after 2h resuscitation was similar in the three groups (p= 0:30). In conclusion, hypoxaemic resuscitation did not normalize early cerebral metabolic recovery as efficiently as resuscitation with 21% O2 or 100% O2. Resuscitation with 21% O2 was as efficient as resuscitation with 100% O2 in this newborn piglet hypoxia model.

Hypoxemic Resuscitation in Newborn Piglets: Recovery of Somatosensory Evoked Potentials, Hypoxanthine, and Acid-Base Balance

We tested the hypothesis that hypoxic newborn piglets can be successfully resuscitated with lower O2 concentrations than 21%. Severely hypoxic, 2-4-d-old, anesthetized piglets were randomly divided into five resuscitation groups: 21% O2 (n = 10), 18% O2 (n = 9), 15% O2 (n = 9), 12% O2 (n = 8), all normoventilated, and a hypoventilated 21% O2 group (PaCO2; 7.0-8.0 kPa, n = 9). Base excess (BE) reached -20 ± 1 mmol/L at the end of hypoxia. After 3 h of resuscitation, BE had risen to -4 ± 1 mmol/L in the 21% O2, 18% O2, and hypoventilated groups, but was -10 ± 2 mmol/L in the 15% O2 group (p < 0.05versus 21% O2 group) and -22 ± 2 mmol/L in the 12% O2 group (p < 0.05 versus 21% O2 group). Four animals died during resuscitation, all allocated to the 12% O2 group (p < 0.05 versus 21% O2 group). Somatosensory evoked potentials (SEPs) recovered in 39 of 45 piglets, and remained present during resuscitation in all except the 12% O2 group. SEP recovered initially even in six of eight animals in the 12% O2 group, but disappeared again in all later during resuscitation. The SEP amplitude recovered to levels not significantly different from the 21% O2 group in all groups except the 12% O2 group. Plasma hypoxanthine concentrations and extracellular hypoxanthine concentrations in the striatum decreased during resuscitation to levels not significantly different from the 21% O2 group in all but the 12% O2 group(p < 0.05 versus 21% O2 group). In conclusion, severely hypoxic newborn piglets were resuscitated as efficiently with both hypoventilation and 18% O2 as with 21% O2.

Oxygen delivery and consumption in surfactant-depleted newborn piglets

Objective: To study the relationship between oxygen (O2) delivery (DO2) and O2 consumption (VO2) in surfactant-depleted newborn piglets.Design: Prospective animal study.Setting: Hospital surgical research laboratory.Subjects: Twenty-six anesthetized and ventilated newborn piglets.Interventions: Twenty of the animals were subjected to repeated saline lung lavages, and then assigned to either the saline group or the L-NAME group. The other six animals without lavage were studied as the control group. Piglets in the L-NAME group and the control group received 3 mg/kg of Nω-nitro-L-arginine methyl ester (L-NAME, an inhibitor of NO synthase) i.v.; and those in the saline group received the same volume of saline i. v.Measurements and results: Cardiac output (CO) was measured and arterial and mixed venous blood gases were analyzed. DO2, O2 extraction ratio (O2ER) and VO2 were calculated. Plasma hypoxanthine was analyzed. In the lung lavaged groups, cardiac index, DO2 and VO2 decreased significantly after L-NAME i. v. but not after saline i. v. Further, the decrease in VO2 in the L-NAME group correlated with the decrease in DO2 (r=0.83, p < 0.001). In the control group, cardiac index and DO2, but not VO2, decreased significantly after L-NAME i.v. Simultaneously, O2ER increased significantly. Plasma hypoxanthine was not modified by lung lavage but increased after L-NAME i. v. in both the L-NAME and control groups.Conclusion: These data suggest that O2 supply dependency is present in surfactant-depleted newborn piglets.

EFFECTS OF HYPOXIA AND RESUSCITATION WITH 21% and 100% O 2 IN NEWBORN PIGLETS: EXTRACELLULAR HYPOXANTHINE (HX) IN BRAIN CORTEX AND FEMORAL MUSCLE. • 1232

EFFECTS OF HYPOXIA AND RESUSCITATION WITH 21% and 100% O 2 IN NEWBORN PIGLETS: EXTRACELLULAR HYPOXANTHINE (HX) IN BRAIN CORTEX AND FEMORAL MUSCLE. • 1232

RESUSCITATION WITH LOW OXYGEN CONCENTRATIONS IN NEWBORN PIGLETS: RECOVERY OF SOMATOSENSORY EVOKED POTENTIALS (SEP) AND ACID BASE BALANCE† 872

We tested the hypothesis that hypoxic newborn piglets can be successfully resuscitated with lower O2 concentrations than 21%. Methods: Hypoxia was induced in 45 anesthetized and instrumented newborn piglets by ventilating them with 8% O2 in N2. When SEP was abolished and mean arterial blood pressure fell below 20 mm Hg, a 3 hour resuscitation period was started. The piglets were randomly divided into five groups: 21% O2 (n=10), 18% O2 (n=9), 15% O2 (n=9), 12% O2(n=8), all normoventilated (PaCO2 = 34 to 45 mm Hg) and a hypoventilated 21% O2 group (PaCO2 = 52 to 60 mm Hg, n=9). Results: Arterial oxygen tensions at 30 min of resuscitation were(mean ± SEM) 84±2, 67±3, 46±2, 42±2, and 62±3 mm Hg in the 21%, 18%, 15%, 12%, and hypoventilated groups, respectively. Mean base excess (BE) was -19 to -22 mmol/L at the end of hypoxia (p=0.90 between all groups). After 3 hours BE had risen to-4 ± 1 mmol/L both in the 21%, 18%, and the hypoventilated groups, but was -10±2 (p<0.05 vs. 21% O2 group) and-22±2 mmol/L (p<0.05 vs. 21% O2 group) in the 15% and 12% O2 groups. SEP recovered in 39 of 45 animals during resuscitation (p=0.61 between all groups), and remained present during resuscitation in all groups except the 12% O2 group. SEP recovered even in 6 of 8 animals in the 12% O2 group, but disappeared in all later during resuscitation, and four of these 8 animals died. The SEP amplitude at the end of resuscitation was 63±17, 62±17, 67±12, and 47±15% of baseline in the 21%, 18%, 15%, and hypoventilated groups, respectively (p=0.71). The SEP latency did not show any differences between the groups. Conclusion: Resuscitation with 18% O2 and hypoventilation with 21% O2 normalizes both the metabolic acidosis and SEP as efficiently as resuscitation with 21% O2. Resuscitation with 15% O2, however, normalizes SEP, but not the metabolic acidosis as efficiently as 21% O2.

RESUSCITATION WITH LOW OXYGEN CONCENTRATIONS IN NEWBORN PIGLETS DOES NOT IMPROVE CEREBRAL METABOLIC RECOVERY OR EEG RECOVERY |[dagger]| 871

We investigated whether resuscitation with lower O2 concentrations than 21% reduces reoxygenation injury and thereby improves cerebral metabolic recovery and EEG recovery. Methods: Hypoxia was induced by ventilating 24 anesthetized and instrumented newborn piglets with 6% O2 in N2. CO2 was added to the inspiratory gas to achieve PaCO2 between 52 and 60 mm Hg. When EEG became isoelectric, and either mean arterial blood pressure fell below 25 mmHg or base excess (BE) was lower than -25 mmol/L, a 2 hour resuscitation period was started. The piglets were randomly divided into three groups: Hypoxemic (n=8), 21% O2 (n=8), and 100% O2 group (n=8). The hypoxemic group was ventilated with 12 to 18% O2 to achieve a cerebral venous O2 saturation of 17 to 23%(baseline; 42-46%), levels which in a previous study was shown not to give any changes of EEG or hypoxanthine (Hx) in cerebral cortex in healthy piglets. PaCO2 was kept in normal range during resuscitation. Extracellular Hx was sampled by in vivo microdialysis from cerebral cortex and striatum. EEG and Oxygenation Index (Near Infrared Spectrophotometry, NIRS) were continuously recorded. Results: Arterial oxygen tensions at 30 mins of resuscitation were (mean±SEM) 45±2, 74±4, and 431±20 mmHg in the hypoxemic, 21% O2 and 100% O2 groups. There were no significant differences between the groups in recovery of EEG(p=0.22) or Hx concentrations in cerebral cortex, striatum, and plasma. Mean BE was -20 to -23 mmol/L at the end of hypoxia (p=0.67) and rose during resuscitation to -13±2, -5±2, and -5±1 mmol/L in the hypoxemic, 21% O2 and 100% O2 groups(p<0.05, hypoxemic vs. 21% O2 and 100% O2 groups). Resuscitation with 21% O2 and 100% O2 reestablished the O2 support to the brain measured by NIRS within the first mins of resuscitation. Conclusion: Hypoxemic resuscitation of newborn piglets does not improve cerebral metabolic recovery or EEG recovery, and delays the correction of the metabolic acidosis.

EFFECT OF NITRIC OXIDE (NO) SYNTHASE INHIBITION ON PLASMA HYPOXANTHINE (Hx) IN NEWBORN PIGLETS WITH SURFACTANT DEFICIENCY: A POSSIBLE RELATIONSHIP BETWEEN PLASMA Hx AND PULMONARY HYPERTENSION |[dagger]| 1627

The Hx-xanthine oxidase system is an important generator of free radical which are reported to induce pulmonary hypertension. In vitro studies have shown that NO is an important free radical scavenger, and inhibition of NO synthase enhances production of superoxide anion in adult rats. The purpose of this study was to investigate the effect of NO synthase inhibition on plasma Hx in newborn piglets with surfactant deficiency, and to study a possible relationship between plasma Hx and pulmonary arterial pressure (Ppa). Methods: Nineteen anesthetized and instrumented newborn piglets were subjected to repeated lung lavages, and then randomly assigned to two groups: the L-NAME group (n = 12) received 3 mg/kg of L-NAME i.v; and the control group (n = 7) received same volume of saline i.v. Ppa was continuously recorded. Plasma Hx was analyzed with HPLC. Results: Plasma Hx was not modified by the repeated lung lavages, but increased significantly 45 minutes after L-NAME i.v. (p < 0.01)(fig). Saline injection, however, did not modify plasma Hx. Furthermore, the differences in Δ-plasma Hx between the two groups after L-NAME and saline i.v. were also significant (p < 0.05). Ppa was not changed after saline i.v., but increased significantly after L-NAME i.v. The change in Ppa after L-NAME i.v. was significantly correlated to plasma Hx (n = 24, r = 0.43, p < 0.05). Conclusion: These data show that inhibition of NO synthase may augments plasma Hx. We speculate that inhibition of NO synthase may potentiate production of free radicals during reperfusion which may contribute to pulmonary hypertension.

Controlled Hypoxemic Resuscitation Does Not Improve Early Cerebral Metabolic or Electrophysiological Recovery in Piglets

Background: We tested the hypothesis that controlled hypoxemic resuscitation of hypoxic newborn piglets improves early cerebral metabolic and electrophysiological recovery. Subjects: Twenty-four 2-5 days old piglets. Interventions: Hypoxia was induced by ventilating the piglets with 6% O2 in N2. When EEG became isoelectric, and either mean arterial blood pressure fell below 25 mm Hg or base excess (BE) was lower than -25 mmol/L, a 2 h resuscitation period was started. The piglets were randomly divided into three resuscitation groups: Hypoxemic (n=8), 21% O2 (n=8), and 100% O2 group (n=8). The hypoxemic group was ventilated with 12-18% O2 to achieve a cerebral venous oxygen saturation of 17-23% (baseline; 44±6%). Results: BE reached-22±6 mmol/L (mean±SE) at the end of hypoxia. PaO2 at 30 min of resuscitation were 6.0±0.4, 9.9±0.5, and 57.5±2.7 kPa in the hypoxemic, 21% O2, and 100% O2 group. No significant differences in time to recovery of EEG (p=0.17), quality of EEG at recovery (p=0.22), or extracellular hypoxanthine concentrations in cerebral cortex and striatum were found between the groups. Brain cell membrane Na+,K+-ATPase activity at the end of resuscitation was not different between the groups (p=0.30). BE and plasma hypoxanthine, however, normalized during hypoxemic resuscitation significantly slower than during resuscitation with 21% or 100% O2.Conclusion: Controlled hypoxemic resuscitation did not improve early brain recovery compared with resuscitation with 21% and 100% O2, and showed a delayed metabolic correction of the total body hypoxia. In contrast, resuscitation with 21% O2 was as efficient as resuscitation with 100% O2 in this newborn piglet hypoxia model.