Hypoxia inhibits the peroxisome proliferator-activated receptor alpha/retinoid X receptor gene regulatory pathway in cardiac myocytes: a mechanism for O2-dependent modulation of mitochondrial fatty acid oxidation.

Hypoxia triggers a cascade of cellular energy metabolic responses including a decrease in mitochondrial oxidative flux. To characterize gene regulatory mechanisms by which mitochondrial fatty acid oxidative capacity is diminished in response to hypoxia, cardiac myocytes in culture were exposed to long-chain fatty acids (LCFA) under normoxic or hypoxic conditions. Hypoxia prevented the known LCFA-induced accumulation of mRNA encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme that catalyzes the rate-limiting step in mitochondrial fatty acid oxidation (FAO). Under hypoxic conditions, myocytes exhibited significant accumulation of intracellular neutral lipid consistent with reduced CPT I activity and diminished FAO capacity. Transient transfection experiments demonstrated that the hypoxia-mediated blunting of M-CPT I gene expression occurs at the transcriptional level, is localized to an LCFA/peroxisome proliferator-activated receptor alpha (PPARalpha)/retinoid X receptor (RXR) response element within the M-CPT I gene promoter, and is PPARalpha-dependent. DNA-protein binding studies demonstrated that exposure to hypoxia reduces PPARalpha/RXR binding activity. Immunoblotting studies demonstrated that whereas hypoxia had no effect on nuclear levels of PPARalpha protein, nuclear and cellular RXRalpha levels were reduced. Hypoxia also diminished the 9-cis-retinoic acid-mediated activation of a reporter containing an RXR homodimer response element. These results demonstrate that hypoxia deactivates PPARalpha by reducing the availability of its obligate partner RXR.

Effect of extracorporeal membrane oxygenation on left ventricular function of swine.

BACKGROUND: Previous clinical and experimental investigations have produced inconsistent data describing the effects of veno-arterial extracorporeal membrane oxygenation (VA ECMO) on intrinsic left ventricular (LV) function. We report an animal model that allows investigation of the effects of VA ECMO on the mechanics of the LV using two load-insensitive indices: end-systolic pressure-minor axis dimension relationship (ESPDR) and preload recruitable dimensional stroke work (PRDSW). METHODS: Eight piglets (5 to 11 kg) were anesthetized, instrumented, and placed on VA ECMO. Throughout the experiment, systemic and left atrial partial pressure of oxygen were maintained between 100 to 200 mm Hg. At ECMO flow rate of 50% of baseline cardiac output, data were collected prior to ECMO, at 4 and 6 hours during ECMO, and after weaning from ECMO. Data measured or calculated for each time point included heart rate, LV pressures and minor axis dimensions at different pre-loads, first derivative of LV pressure with respect to time, velocity of circumferential fiber length shortening (VCF), LV shortening fraction (LVSF), ESPDR, and PRDSW. RESULTS: A significant (p < 0.05) decrease in LVSF and VCF was seen at 4 and 6 hours during ECMO when compared to baseline, but the ESPDR and PRDSW did not change during ECMO. CONCLUSIONS: VA ECMO alone changes some of the load-dependent parameters of contractility, but intrinsic function of the heart is not significantly affected as measured by load-insensitive indices of LV performance.

Left ventricular dysfunction during extracorporeal membrane oxygenation in a hypoxemic swine model.

BACKGROUND: Perfusion of the coronary circulation with hypoxemic blood from the left ventricle has been postulated to cause myocardial dysfunction during venoarterial extracorporeal membrane oxygenation for respiratory support. METHODS: We investigated this hypothesis in 10 anesthetized open-chest piglets (7 to 9 kg) undergoing venoarterial extracorporeal membrane oxygenation after placement of minor-axis sonomicrometry crystals and left ventricular micromanometer. The left atrial partial pressure of oxygen was made hypoxemic (25 to 40 mm Hg) after initiation of extracorporeal membrane oxygenation by ventilation with a hypoxic gas mixture. Left ventricular contractile function, including peak LV pressure, shortening fraction, maximum rate of increase of left ventricular pressure, velocity of circumferential fiber shortening, end-systolic pressure-minor axis dimension relationship, and preload recruitable dimensional stroke work, was measured or calculated on extracorporeal membrane oxygenation before (baseline) and at 4 and 6 hours after rendering the left atrial blood hypoxemic. RESULTS: Left ventricular shortening fraction and velocity of circumferential fiber shortening were significantly lower (p < 0.05) at 4 and 6 hours when compared with baseline. The slope of the end-systolic pressure-minor axis dimension relationship decreased but was not significantly different at 4 and 6 hours when compared with baseline owing to poor linear correlation (r = 0.30 to 0.93). The preload recruitable dimensional stroke work was more linear (r = 0.87 to 0.99), and the slope was significantly lower (p < 0.01) at 4 and 6 hours when compared with baseline. CONCLUSIONS: Hypoxemic cardiac output from the left ventricle during venoarterial extracorporeal membrane oxygenation is associated with depression of left ventricular systolic function in this animal model. Current use of venoarterial extracorporeal membrane oxygenation for respiratory support may not provide adequate oxygen supply to the myocardium.

A comparison of oral transmucosal fentanyl citrate and oral hydromorphone for inpatient pediatric burn wound care analgesia.

The ideal oral wound care analgesic for children should be palatable, provide potent analgesia of rapid onset and short duration, and require minimal, yet appropriate, monitoring. With use of a double-blinded crossover design, we compared the efficacy and safety of oral transmucosal fentanyl citrate (OTFC) (approximately 10 micrograms/kg) with the efficacy and safety of oral hydromorphone (60 micrograms/kg) in 14 pediatric inpatients (ages 4 to 17 years) undergoing daily burn wound care in a ward setting. Pulse oximetry, vital signs, side effects, patient pain scores, and observer scores for cooperation, anxiety, and sedation were recorded. Pulse oximetry, vital signs, cooperation, sedation, incidence of nausea or vomiting, and the amount of time it took to resume normal activities were similar in both treatment groups. OTFC resulted in improved pain scores before wound care and improved anxiolysis during wound care, but at other points it was similar in effect to hydromorphone. We conclude that OTFC is a safe and effective analgesic, that it may provide minor improvements in analgesia and anxiolysis compared with hydromorphone, and that it offers a palatable alternative route of opioid administration without intravenous access for wound care procedures in children.

Morphine pharmacokinetics during continuous infusion of morphine sulfate for infants receiving extracorporeal membrane oxygenation.

OBJECTIVES: To determine a) if serum morphine concentration changes during the first 3 hrs of extracorporeal membrane oxygenation (ECMO); and b) if absorption of morphine onto the membrane oxygenator is responsible for these changes. Also, morphine clearance during the first 5 days of ECMO was studied. DESIGN: Prospective, open-label study with consecutive patient enrollment. SETTING: Neonatal intensive care unit at a university-affiliated, children's hospital. SUBJECTS: Eleven neonates with severe persistent pulmonary hypertension of the newborn receiving continuous intravenous infusions of morphine sulfate and requiring ECMO. INTERVENTIONS: Blood samples were obtained from the subjects and ECMO circuits at predetermined time intervals. MEASUREMENTS AND MAIN RESULTS: Serum morphine concentration was determined using high-performance liquid chromatography. Morphine concentrations were no different from baseline at 5 mins, 1 hr, or 3 hrs after beginning ECMO. There was no significant difference in morphine concentration from samples taken immediately proximal and distal to the membrane oxygenator at 5 mins, 1 hr, and 3 hrs after the start of ECMO. Morphine clearance was calculated on days 1, 3, and 5 of ECMO. The mean value for morphine clearance was 11.7 +/- 9.3 (SD) ml/min/kg (range 2.6 to 34.5). CONCLUSIONS: The initiation of ECMO does not lead to a significant decrease in serum morphine concentration and there is no uptake of morphine onto the membrane oxygenator of the ECMO circuit. Morphine clearance for infants receiving ECMO is variable.

Regulation of ATP synthase subunit e gene expression by hypoxia: cell differentiation stage-specific control.

Using the technique of differential display, we identified genes that are expressed differentially under normoxic and hypoxic conditions. One regulated gene encoded subunit e of mitochondrial F1F0-ATP synthase (subunit e). The hypoxia-mediated regulation of subunit e expression in C2C12 cells was influenced by the stage of cellular differentiation. Under normoxic conditions, subunit e expression was markedly upregulated during the transition from myoblast to myotube. After exposure to hypoxia for 24 h, subunit e mRNA expression markedly decreased (>70%) in C2C12 myotubes. In contrast, subunit e mRNA levels increased slightly in response to hypoxia in C2C12 myoblasts. Studies performed with primary rat cardiocytes demonstrated that expression of subunit e mRNA and a cardiac-enriched related transcript was downregulated after a hypoxic exposure. We conclude that expression of subunit e is regulated, at the pretranslational level, by oxygen availability via cell differentiation stage-specific mechanisms consistent with the proposed regulatory role of this protein in cellular ATP production.

Routine chest radiographs following repositioning of endotracheal tubes are necessary to assess correct position in pediatric patients.

Attempts to correctly reposition endotracheal tubes (ETTs) are not always successful in pediatric patients, even when chest radiographs (CXRs) are measured to determine the distance that the ETT deviates from the correct position. We determined the frequency of continued ETT malposition after repositioning in a pediatric intensive care unit (PICU). Forty children with malpositioned ETTs were identified during a 4-month period. After repositioning, ten (25 percent) continued to be malpositioned on the next CXR. Of 47 children with correctly positioned ETTs, only one ETT (2 percent) was found to be incorrectly positioned on the next routine CXR obtained 24 h later. The difference in frequency of ETT malposition between these two groups of children is significant (p < 0.0001). The children were similar in weight and age. Despite repositioning based on measurements taken from a CXR, a large percentage of pediatric patients had continued ETT malposition. However, after radiographic documentation of correct position, we demonstrated that significant movement was uncommon. Routine confirmation of ETT position by CXRs should be considered after repositioning ETTs in pediatric patients.

Extracorporeal membrane oxygenation.

Extracorporeal membrane oxygenation is still a relatively new technology that has recently achieved recognition after initial clinical disappointment in the late 1970s. At present, it is considered standard therapy for the full-term infant with PPHN who fails CMV and extraordinary, heroic therapy for older children and adults with ARF or cardiac failure, or both. Currently, the emphasis is on developing new technologies for increasing safety and effectiveness. Areas of interest include heparinless circuits, carotid artery reconstruction, improved monitoring, and expanding applications of VV ECMO. As ECMO becomes safer and more effective, it is believed that new and expanding patient populations will emerge to include premature infants, earlier intervention in term infants, and more liberal application to pediatric and adult populations.