A. Charles Bryan

Effect of surgical repair on respiratory mechanics in congenital diaphragmatic hernia

To determine whether surgical repair of congenital diaphragmatic hernia (CHD) results in improvement in respiratory mechanics, we measured respiratory system compliance in nine patients (five survivors and four nonsurvivors) before and after operation. In all nine infants, CHD was diagnosed within 6 hours of life, and surgical repair was through an abdominal approach after a period of stabilization. Measurements were made noninvasively, using the passive expiratory flow-volume technique. In only one of the nine infants did compliance immediately improve after surgical repair, and in another it showed no change. Both of these infants survived, with an uneventful postoperative course. In the remaining seven infants, however, postoperative compliance immediately decreased to 10% to 77% from the preoperative value. The four infants with more than 50% decrease in compliance died with increasing hypoxemia and acidosis. These results suggest that respiratory mechanics in CHD, far from improving, frequently deteriorate as a result of repair of the hernia. The role of urgent surgery in this malformation should be reevaluated.

Total resistance of the respiratory system in preterm infants with and without an endotracheal tube

The passive compliance and resistance of the respiratory system were measured in 12 spontaneously breathing newborn infants before and after endotracheal extubation. End-inspiratory airway occlusions were used to relax the respiratory muscles, allowing occlusion pressure to be measured and respiratory system compliance and resistance to be calculated from the flow volume relationship of the subsequent passive expiration. Airway pressure was measured from an endotracheal tube or a face mask, expiratory flow from a pneumotachograph, and expiratory volume from the integrated flow signal. In six of the infants, diaphragmatic electromyography was also performed before and after extubation. Resistance and EMG findings were both decreased by extubation (mean decrease 43.9%, P less than 0.001 and 27.3%, P less than 0.05, respectively), but compliance was unchanged. Thus, by substantially increasing resistance, an endotracheal tube causes the diaphragm to increase its activity to maintain ventilation.

Chest Wall Mechanics and Respiratory Muscles in Infants

Aspects in which infants differ from adults in regard to the characteristics of chest wall mechanics and respiratory muscles are discussed. Despite the lack of data on the function of respiratory muscles in infants, it is clear that respiratory muscle failure is a major factor in the response of infants to lung disease.

The consequences of diaphragmatic muscle fatigue in the newborn infant.

We have previously demonstrated that diaphragmatic muscle fatigue can be diagnosed in infants from spectral frequency analysis of the surface diaphragmatic electromyogram. This requires a digital computer, but the analysis takes several days. Spectral frequency changes, however, can be accurately reflected by band pass filtering and expressing the ratio of high-frequency power to low-frequency power. A fall in this ratio of greater than 20% indicates muscle fatigue. Using a simple analog device to obtain this ratio permits the results to be immediately available; we have used this method to study weaning from mechanical ventilators in ten infants. With a successful weaning step there is no significant change in the ratio, whereas an unsuccessful weaning step invariably leads to a decrease in the ratio of greater than 20%, which precedes CO2 retention and clinical deterioration. These data indicate that diaphragmatic muscle fatigue plays an important role in the infants response to lung disease. Monitoring of the high/low frequency ratio may be helpful in weaning infants from assisted ventilation.

Reduction of platelet counts induced by mechanical ventilation in newborn infants.

The association between platelet counts and mechanical ventilation was assessed in 61 newborn infants with respiratory distress syndrome, 10 infants with congenital diaphragmatic hernia, and 10 infants with tracheoesophageal fistula. A significant decrease in platelet counts was observed during mechanical ventilation: (mean +/- SD) reduction of 39% +/- 5%, 42% +/- 5.6%, and 11.9% +/- 5.4% in the three groups, respectively, independent of other causes. In the group with congenital diaphragmatic hernia, there was a significant correlation between mean airway pressure and the reduction in platelet counts. In a subsequent series of experiments, platelet counts were recorded before and during ventilation in rabbits. A significant mean decrease of 37.3% in platelet counts was associated with ventilation with either air or pure oxygen. Results of these studies indicate that mechanical ventilation itself may cause a major decrease in platelet count in newborn infants.

RELATIVE DISTRIBUTION OF GAS AND PERFLUOROCARBON (PFC) DURING PARTIAL LIQUID VENTILATION (PLV). ▴ 258

RELATIVE DISTRIBUTION OF GAS AND PERFLUOROCARBON (PFC) DURING PARTIAL LIQUID VENTILATION (PLV). ▴ 258

Asthma: Current Concepts

Concepts of the pathophysiology and therapeutic management of asthma must take into account more than the appearance and disappearance of the wheeze. The outstanding pathologic finding in the asthmatic lung is the presence of viscid plugs in the bronchi and bronchioles; other features include edema of the bronchial mucosa and submucosa, shedding of superficial columnar epithelial cells into the bronchial lumen, dilatation of submucosal capillaries, mast cell degranulation, infiltration by eosinophils, and marked thickening of the basement membrane.

HIGH FREQUENCY OSCILLATION (HFO) VS. CONVENTIONAL MECHANICAL VENTILATION BAROTRAUMA, SURFACTANT AND SURFACE TENSLOE IN PREMATURE LAMBS

Twelve premature twin lambs delivered by C-section at 133-136 days gestational age were supported for 3 hrs. on an F1 O2= 1.0 on either HFO at 900 breaths/min. or CMV at 20 - 30 breaths/min. Average ± SE blood gas values were similar at sacrifice, but the HFO group required a higher MAP (16 ± 0.78 vs. 13.3 ± 1.43 on CMV). Both groups had similar degrees of epithelial necrosis, hyaline membrane formation and equal disruption of alveolar permeability as evidenced by bidirectional leak of labelled albumin and alveolar wash proteins. The HFO animals had a smaller alveolar pool of endogenous phosphatydyleholine (PC) and larger tissue associated exogenous H3 natural surfactant tracer (H3 -NS) which may represent a decrease in initial secretion of surfactant and/or a stimulation of reuptake in this group. This study failed to prove the superiority of HFO over CMV suggested by our previous studies in a different animal model both in terms of decreasing barotrauma and improving gas exchange and raises important questions with regards to its effect on surfactant turnover.

1360 PREVALENCE OF CENTRAL APNEA IN PREMATURE NEWBORNS

It has recently been suggested that most apneas observed in preterm infants are either mixed or obstructive. However, our results indicate that the majority of the spells are central. We studied 10 preterm infants, BW = 1226 ± 196 gr, gestational age 29 ± 1.6 weeks, and postnatal age 19 ± 10 days (mean ± S.D.). Seven out of the ten infants had intracranial hemorrhage (ICH) diagnosed by ultrasound. We recorded diaphragmatic EMG activity with surface electrodes, motion of the rib cage and abdomen with magnetometers and air flow using a nasal thermistor. Apnea was defined as cessation of air flow for more than 20 sec, or more than 10 sec, if followed by bradycardia (H.R. < 100 bpm). Spells of absent airflow with diaphragmatic EMG activity were called obstructive (O), without EMG activity (C), and spells with both patterns mixed apnea (M).In only two infants (with ICH) was the predominant type mixed apnea. Our results are not in agreement with previous data suggesting that all apneas in infants with ICH are mixed or obstructive. We conclude that apneas in preterm infants are predominantly central.

Distribution of Gas and Flurocarbon Measured by CT Densitometry. 42

Hypothesis: Partial liquid ventilation uses flurocarbons (PFC) which have a great capacity for O2 and CO2 but also present a major diffusion barrier. Therefore, effective gas exchange requires a maximum surface area at the gas/liquid interface.