Metabolic response of preterm infants to variable degrees of respiratory illness.

In older children and adults, physiologic instability associated with severe illness causes increased cellular oxygen consumption (VO2), increased serum lactate and cortisol levels, and more negative nitrogen balance. To determine the metabolic response of preterm infants to severity of respiratory illness, we analyzed VO2, nitrogen balance, urinary 3-methyl-histidine and norepinephrine concentrations, and serum levels of lactate and cortisol as a function of ventilatory index (VI). Twelve 2-day-old premature infants who were appropriate in size for gestational age (mean +/- SEM birth weight: 1460 +/- 251 gm) and who required mechanical ventilation for respiratory distress syndrome had VO2 and carbon dioxide production measured by indirect calorimetry and blood and urine samples obtained concurrently. All infants received amino acids, 1.0 gm/kg per day, and a mean energy intake of 27 +/- 3 kcal/kg per day, provided as a parenteral dextrose solution. The resting energy expenditure exceeded energy intake in all infants. The VO2 value ranged from 5.5 to 9.2 ml/kg per minute and was directly correlated with VI (r = 0.79; p = 0.002). Nitrogen balance ranged from -160 to 53 mg/kg per day (mean: -33 +/- 21 mg/kg per day) but was not dependent on VI (r = 0.04) or VO2 (r = 0.01). The serum lactate level correlated directly with VI (r = 0.82; p = 0.002) and VO2 (r = 0.60; p = 0.05), but cortisol and urinary norepinephrine levels did not. We conclude that preterm infants with respiratory distress syndrome have increased VO2 rates and serum lactate concentrations directly related to the degree of respiratory illness. They are generally in a state of mildly negative nitrogen balance, the degree of which is not related to severity of illness. Although these infants may require increased energy delivery during illness, they do not appear to require excessive amounts of amino acids.

Necrotizing tracheobronchitis: a complication of high-frequency ventilation.

The tracheobronchial histopathologic findings in eight neonates who died after treatment with high-frequency jet ventilation (HFJV) were compared with those in eight similar infants who died after treatment with conventional mechanical ventilation. The HFJV and conventionally treated groups were matched as closely as possible for birth weight, gestational age, and duration of mechanical ventilation. A 4-point, nine-variable histologic scoring system was used to grade tissue changes in the trachea, carina, and mainstem bronchi. The patients who received HFJV had significantly more histologic damage in their tracheas, carinas, and right and left mainstem bronchi. At all levels of the airway examined, HFJV was associated with more inflammation, greater losses of ciliated epithelium, and more mucus within the lumen of the airway than was conventional mechanical ventilation.

Comparison of high-frequency jet ventilation and conventional mechanical ventilation in a meconium aspiration model.

Ten adult cats received alternately high-frequency jet ventilation and conventional mechanical ventilation after aspirating 2 ml/kg 25% human meconium in saline. Equivalent mean airway pressures were maintained during the hourly ventilator changes. Aortic pressures, pulmonary artery pressures, and central venous pressures were continuously monitored. Cardiac outputs were measured, and pulmonary and systemic vascular resistances, intrapulmonary shunts, and alveolar arterial oxygen gradients were determined at regular intervals. During the first hour after aspiration, AaDO2 and Qs/Qt were lower during HFJV (P less than 0.05); PVR and Pa were always higher during HFJV (P less than 0.05). Overall, PVR, Pa, AaDO2, and Qs/Qt rose during HFJV; these changes occurred at equivalent Paw within 15 minutes of each ventilator change (P less than 0.05). In this meconium aspiration model, conventional mechanical ventilation was the superior form of ventilatory therapy.

A comparison of the effects of high frequency--low tidal volume and low frequency--high tidal volume mechanical ventilation.

Ten neonates with severe lung disease were studied while mechanically ventilated with standard volume preset infant ventilators, using two different ventilatory patterns. Slow ventilatory rates and high tidal volumes were alternated with rapid rates and low tidal volumes; minute ventilation, FIo2, PEEP, and I:E ratios were held constant. Peak inspiratory pressure, mean airway pressure, expiratory time, Pao2, Paco2, pH, and arterial blood pressure were measured and compared for each frequency-tidal volume combination. The best arterial oxygenation occurred at the combination of settings that produced the highest mean airway pressure and always during low frequency-high tidal volume ventilation (P less than 0.001). Changes in oxygenation appeared to be directly related to changes in MAP. A second experiment examined two different ventilator systems' responses to changes in ventilatory rate. When the rate of one ventilator (Bourns LS104 volume preset) was increased, MAP increased. When the rate of the other ventilator (Bennett PR2 pressure preset) increased, MAP decreased. These observations suggest that there is a direct relationship between MAP and orterial oxygenation, and that the supposed advantages of one ventilatory pattern over the other may be secondary to inadvertent changes in subtle pressure-time relationships within the respiratory cycle and incidental changes in MAP. These changes may vary from one ventilator to another.

Variations in inspiratory:expiratory ratio and airway pressure wave form during mechanical ventilation: the significance of mean airway pressure.

Twelve neonates with severe lung disease were studied while mechanically ventilated with volume pre-set infant ventilators, using different I:E ratios and different airway pressure waves. While FIO2 tidal volume, respiratory rate, and PEEP remained constant, I:E ratios were increased, first by reducing inspiratory flow rate, which produced a triangular pressure wave, and then by using an inspiratory time hold mechanism, which produced an inspiratory plateau or squared pressure wave. Peak inspiratory pressure, mean airway pressure, PaO2, PaCO2, pH, and blood pressure were measured and compared for each I:E ratio and pressure wave combination. In all patients, increases in oxygenation appeared to be directly related to increases in MAP. Optimum oxygenation and ventilation occurred with the I:E ratio and pressure wave combination that produced the highest MAP. Because MAP changes with any alteration in PEEP, I:E ratio, or airway pressure wave, it is a clinically useful composite measure of all pressures transmitted to the airways by a mechanical ventilator.

The effect of independent variations in inspiratory-expiratory ratio and end expiratory pressure during mechanical ventilation in hyaline membrane disease: the significance of mean airway pressure.

The specific aspects of the respiratory cycle during mechanical ventilation that allow for optimum gas exchange are still controversial. To further clarify the relationship of inspiratory:expiratory ratio and positive end expiratory pressure to optimum ventilation and oxygenation, five premature lambs with severe hyaline membrane disease were ventilated with volume-present infant ventilators at I:E ratios of 1:4 and 1:1 and PEEP levels of 0, 5, and 10 cm H2O. For each I:E ratio/PEEP combination, pH, Pao2, PaCO2, PAO2, PACO2, peak inspiratory pressure, mean inspiratory pressure, and mean airway pressure were measured and compared. Optimum ventilation and oxygenation were related to MAP, but not to I:E ratio, PIP, or MIP. As MAP increased from 6 to 14 cm H2O, progressive improvement in Pao2, PaCO2 (A-a) DO2 and (a-A) DCO2 was evident. Above 14 cm H2O, there was progressive deterioration in these measurements. There was also a direct relationship between MAP and mean pleural pressure. These results indicate that during mechanical ventilation there is an optimum MAP at which gas exchange is best. Since MAP changes with any change in PIP, PEEP, or I:E ratio, it provides a useful composite measure of all pressures transmitted to the airways by the ventilator.

Leg growth following umbilical artery catheter-associated thrombus formation: a 4-year follow-up.

Fourteen children were re-examined 4 years following the radiographic demonstration of umbilical artery catheter associated thrombus formation. Positive correlations could not be established between radiographically determined femoral and/or tibial lengths, length differences, pulse pressure differences, and the location of thrombi noted on the 1971 aortograms.