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Effect of Theophylline on Metabolic and Ventilatory Response to Hypoxemia During Quiet Sleep in Piglets

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Effect of Theophylline on Metabolic and Ventilatory Response to Hypoxemia During Quiet Sleep in Piglets 0031-3998/95/3806-0926$03.00/0 PEDIATRIC RESEARCH Vol. 38, No. 6, 1995 Copyright O 1995 International Pediatric Research Foundation, Inc Printed in U.S.A. Effect of Theophylline on Metabolic and Ventilatory Response to Hypoxemia during Quiet Sleep in Piglets Division of Respiratory Medicine and The Jeremy Rill Centre for SIDS and Respiratory Control Disorders, Department of Pediatrics, Montreal Children's Hospital-Research Institute, McGill University, Montrial, Canada Theophylline, a drug frequently used in newborns, stimulates significant increase in respiratory exchange ratio (R)in response respiration and increases the metabolic rate in a sustained fash- to hypoxemia (from 0.87 -C 0.05 to 0.97 2 0.04, p < 0.001). ion; hypoxemia, on the other hand, decreases metabolic rate and However, after the administration of theophylline, additional increases ventilation slightly and, at times, only transiently. This exposure to hypoxemia did not result in a change in R. In study looked at the effect of theophylline on the metabolic and summary, our results show that, in sleeping newborn piglets, ventilatory response to hypoxemia in piglets. We studied two theophylline does not abolish the decrease in oxygen consump- groups of piglets during normoxia and hypoxemia: first during a tion observed in response to hypoxemia; nor does it enhance the baseline period; and second, after the infusion of either theophyl- ventilatory response to a moderate degree of hypoxemia. line or a placebo. All studies were done in quiet sleep, 2 d after (Pediatr Res 38: 926-931, 1995) instrumentation was performed to place vascular catheters and electroencephalographic electrodes. 0, consumption (Vo,) and Abbreviations CO, production (Vco,) were measured in a metabolic chamber, Fio,, fractional concentration of inspired oxygen and alveolar ventilation (VA)was then derived from Vco, and Fico,, fractional concentration of inspired CO, Paco,. We found that theophylline did not abolish the small Paco,, partial pressure of arterial CO, decrease in oxygen consumption brought about by hypoxemia. PAco,, partial pressure of alveolar CO, Nor did theophylline augment the ventilatory response to hypox- VO,, oxygen consumption emia. In fact, the percent change in alveolar ventilation decreased ~co,,CO, production slightly: going from 17 2 8% during the baseline period to 9 i- VA, alveolar ventilation 6% (p < 0.005) after theophylline administration. We found a EEG, electroencephalograph Compared with adults, newborns have high demands for newborn piglets in which Paco, was measured along with oxygen. They also, however, are prone to diseases in which minute ventilation (4), the use of theophylline, although stim- hypoxemia occurs. These include pulmonary involvement sec- ulating the ventilatory response to hypoxemia, did not yield the ondary mostly to prematurity but also infections and apnea of expected diminution of Paco,. Therefore, even though meta- infancy. These conditions are often treated with theophylline, a bolic rate was not measured, the results suggest that the respiratory stimulant (1, 2). metabolic rate (C02 production) had increased, thereby in- The effect of theophylline on the response to hypoxemia in creasing ventilation after theophylline administration. newborns has been studied largely from the point of view of Clearly, it is important to know the metabolic rate when the ventilation. The studies, however, have shown conflicting re- effect of theophylline during hypoxemia is being assessed in sults: both a stimulation of the ventilatory response to hypox- newborns, for at that age theophylline and hypoxemia have emia (3, 4) and no effect at all (5). In fact, in one study of different effects on ventilation and metabolic rate. Indeed, although hypoxemia increases ventilation slightly in new- Received September 21, 1994; accepted June 20, 1995. born-sometimes only transiently-(reviewed in Refs. 6 and Correspondence and reprints requests: Aurore CBtt, Division of Respiratory Medicine 7) and also depresses metabolic rate (8, 9), theophylline in- (D-380). Montreal Children's Hosp~tal,2300 rue Tupper, Montrtal, Canada H3H 1P3. Supported.- by the Association Pulmonaire du Qutbec and, in .part, by. the Medical Creases both ventilation and metabolic rate in a sustained Research Council of Canada (Grant MA-10258) and the Heart and Stroke Foundation of fashion (10, 11). The decrease in metabolic rate in response to Canada. hypoxemia is viewed as an adaptation of the newborn to low ' Visiting physician from the University of Cali, Colombia. Research Scholar of the Fonds de la recherche en santt du Qutbec. Oxygen Content (9, 12, 13). Nonetheless, if theophylline abol- METABOLIC EFFECT OF THE(3PHYLLINE IN HYPOXEMIA 927 ishes the decrease in metabolic rate that occurs in response to We measured total body 0, consumption and CO, produc- hypoxemia, the consequences could be deleterious for those tion by continuously recording the drop in Fio, and the in- hypoxemic newborns so often given theophylline. So far, no crease in Fico, in the animals' metabolic chamber. An electro- study has explored the interaction of theophylline and hypox- chemical cell oxygen analyzer (model S-3M1) and sensor emia with metabolic rate and ventilation in newborns. (model N-22 M; AMETEK, Applied Electrochemistry, Pitts- The purpose of the present study was to determine whether, burgh, PA) was used to measure the Fio, (dry). An infrared in newborn piglets, theophylline modifies the metabolic re- CO, analyzer (model CD-3A) and sensor (model P-61B; sponse to hypoxemia. We elected to measure metabolic and AMETEK, Applied Electrochemistry) was used to measure the ventilatory parameters during quiet sleep to avoid any stimu- Fico,. This equipment is sensitive to a +0.01% change in Fio, lation of respiration caused by the general stimulation of or Fico, and has an accuracy of +0.02% and a 90% response activity, secondary to theophylline, during the awake state. We time of 100 ms. We calibrated these instruments with three have shown in the past that, after administration of theophyl- different gas mixtures of 0, and CO,, the range of which line, quiet sleep is present in sufficient amounts to make such corresponded to the range of values used during the studies. To study feasible, but that active sleep is greatly diminished (10). ensure that all measurements of Vo, and Vco, were accurate, we regularly tested our system's ability to measure a small, METHODS precisely known change in 0, and CO, levels, both in room air and with a hypoxic mixture similar to that used for the study. Animal preparation. A total of 16 piglets were studied at a For that purpose, we added 0, (or CO,) to the chamber, mean age of 7 f 1 d (mean f SD) and a mean weight of 2.7 through a precisely calibrated flowmeter, at a rate of 20 and 30 + 0.5 kg. The animals were obtained a few days before the mL1min for a known period of time (usually between 5 and 10 experiments and kept in our animal quarters. We provided min). This flow rate represents a value for oxygen consumption them with sow milk replacer (Wetnurse; JEFF0 Co. St- (or CO, production) of 10-15 mL/kg/min in our piglets. We Hyacinthe, QuCbec, Canada) ad libitum throughout the day and were able to measure the expected changes exactly within night and monitored their weight gains closely. Only those f0.01% of Fio, or Fico,. In the past, we obtained very animals that had a normal weight gain (=I00 glday) were used reproducible Vo, and vco, results using that method (10). for the experiments. The environmental temperature in the Measurements were made over a period of approximately 15 animal quarters was kept between 26 and 30°C; the sleeping min (sufficient time to measure 0, consumption and CO, area was provided with sufficient additional heat to maintain a production), during which time neither Fio, nor Fico, changed thermoneutral environment (32-33°C) (14). by more than 0.3 (gas circulation system closed). The mea- We performed surgery for catheter insertion 2-3 d before the surements were made when the piglets were in quiet sleep, experiments, using halothane/N,O/O, anesthesia. One catheter starting at least 30 s after that state had been clearly identified was placed in the axillary vein for theophylline or placebo by EEG and behavioral criteria. The time delay of 30 s was infusion; a second cathether was placed in the descending aorta chosen because, with our system, it represents three times the from a femoral artery for blood gas sampling and monitoring of delay to see a change in the Fio, or Fico,. We calculated 0, blood pressure and heart rate (polyvinyl, TYGON microbore consumption as follows: tubing; Norton Co., Wayne, NJ). We also instrumented the animals with s.c. electrodes (platinum subdermal type E2; Vo, (mLO, . minp' . kgp') Grass Instruments, Quincy, MA) for EEG recording. Once the anesthesia was stopped, the animals woke rather rapidly (with- A Fio, (%/min) X [vol of chamber (mL) - vol of animal] - in 15-20 min); a few hours after surgery, they seemed to weight of animal (kg) behave, walk, and feed normally. Measurement of total body 0, consumption and CO, pro- We calculated CO, production (Vco,) in much the same way, duction, and alveolar ventilation. We used the closed-system replacing AFio, in the equation by AFico,. We derived alveo- method for the measurement of these parameters. The animals larventilation from the value of CO, production and Paco, as were studied in a totally sealed plexiglass chamber (44.6 X 45 follows: X 60 cm; volume, 120.4 L). We kept the chamber's tempera- ture in the thermoneutral range for piglets; that is, between 32 CO, production Alveolar ventilation = X K and 33°C with the ambient temperature adjusted to fO.l°C PAco, (Telethermometer; Yellow Spring Instruments, Yellow where K is a constant used as a correcting factor to convert Springs, OH).
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