Tilo Gerhardt

Serial determination of pulmonary function in infants with chronic lung disease

Pulmonary function was measured in 39 infants with chronic lung disease who had required mechanical ventilation starting during the first week of life for a median of 9 days (range 1 to 46 days) and supplemental oxygen for a median of 48 days (range 28-162 days). Their mean birth weight was 1140 g (range 550 to 2325 g), and mean gestational age 29.8 weeks (range 26 to 37 weeks). Ventilation was measured by pneumotachography, esophageal pressure through a water-filled feeding tube, and functional residual capacity (FRC) by a modified nitrogen washout technique. Lung compliance, pulmonary conductance, and FRC were determined at 1, 3, 6, 12, 18, 24, and 36 months after birth. Pulmonary function was also determined in 40 normal children, ranging in age from neonates to 5 years, who served as controls. In infants with chronic lung disease, growth in weight and length followed the 10th to 25th percentiles of the normal curve. Minute ventilation and respiratory effort remained elevated throughout the follow-up. FRC per kilogram of body weight was decreased at 1, 3, and 6 months after birth, but thereafter was in the normal range. FRC increased in proportion to weight at the same rate as in the controls. Lung compliance was only half of normal at 1 month, increased with growth in close correlation with weight, and was approximately 80% of normal at the end of follow-up. Pulmonary conductance was 50% of normal at 1 month, increased little during the first 6 months, but reached 85% of normal at 3 years of age. There was no evidence of gas trapping. These results indicate that in infants with chronic lung disease after mechanical ventilation, lung volume increases normally, probably by formation of new alveoli, which also leads to improvement in lung compliance. Airway growth is slow during the first 6 months after birth, but the subsequent faster growth leads to conductance values close to normal at 3 years of age.

Chestwall compliance in full-term and premature infants.

Abstract. Gerhardt, T. and Bancalari, E. (Department of Pediatrics, University of Miami, Florida, U.S.A.) Chestwall compliance in full‐term and premature infants. Acta Pediatr Scand, 69: 359, 1980.—Chestwall compliance was determined in 26 premature infants (BW 1 320±410 g, gest. age 32 weeks) and in 10 full‐term infants (BW 3 155±810 g) who were ventilated mechanically. Chestwall compliance in premature infants was 6.4 ml/ (cmH2O×kg), decreasing with advancing gestational age to 4.2 ml/(cmH2O×kg) in full‐term infants. There was a linear correlation (r= 0.95 and 0.79 respectively) between tidal volume and the pressure transmitted to the esophagus throughout the tidal volume range. The portion of airway pressure transmitted to the esophagus depended on the infants lung compliance. Only 5% was transmitted in infants with hyaline membrane disease, 12% in newborns with a patent ductus arteriosus, 17 % in normal prematures and 25% in normal full‐term infants. The findings suggest that during mechanical ventilation the high chestwall compliance and low lung compliance of premature infants prevent a significant rise in intrapleural pressure which could interfere with central venous return and cardiac output. However, using high inspiratory pressures and continuous distending airway pressure in the absence of lung pathology may result in a decreased cardiac output. The highly compliant chestwall of the premature infant may exert insufficient outward recoil and might be one of the causes of a low functional residual capacity and chronic pulmonary failure in the premature infant.

Proportional assist ventilation in low birth weight infants with acute respiratory disease: A comparison to assist/control and conventional mechanical ventilation ☆ ☆☆ ★

OBJECTIVES To compare the physiologic efficacy and safety aspects of proportional assist (PA), assist/control (A/C), and intermittent mandatory ventilation (IMV) in very low birth weight infants with acute respiratory illness and to test the hypothesis that ventilatory pressure requirements are lower and arterial oxygenation is improved during PA when compared with IMV or A/C at an equivalent inspired oxygen fraction. STUDY DESIGN Randomized, 3-period, crossover design. METHODS Thirty-six infants were stratified by birth weight (600 to 750, 751 to 900, and 901 to 1200 g) and exposed to consecutive 45-minute epochs of the 3 modalities in a sequence chosen at random. Tidal volumes of 4 to 6 mL/kg were targeted during A/C and IMV. The IMV rate was matched to the rate during an A/C test period. PA was adjusted to unload the resistance of the endotracheal tube and the disease-related increase in lung elastic recoil. RESULTS Compared with A/C and IMV, PA maintained similar arterial oxygenation with lower airway and transpulmonary pressures (15% to 44% reduction depending on the index variable). The oxygenation index decreased by 28% during PA. No adverse events were observed. The number and severity of apneic episodes and periods of arterial oxygen desaturations were similar with the 3 modes. Similar results were obtained within each birth weight subgroup. CONCLUSIONS PA safely maintains gas exchange with smaller transpulmonary pressure changes compared with A/C and IMV. It may therefore offer a way of reducing the incidence of chronic lung disease in low birth weight infants.

Mechanisms for episodes of hypoxemia in preterm infants undergoing mechanical ventilation

OBJECTIVE To ascertain possible mechanisms implicated in the development of transient episodes of hypoxemia (oxygen saturation < 85%) frequently observed in preterm infants undergoing mechanical ventilation, even after the acute phase of respiratory failure has passed. STUDY DESIGN Tidal flow, airway and esophageal pressure, and oxygen saturation were continuously recorded in 10 infants (mean +/- SD, birth weight 733 +/- 149 gm, gestational age 25.5 +/- 2.2 weeks, age 26.3 +/- 11.9 days) who had repeated episodes of hypoxemia without any evident cause. Measurements of minute ventilation (VE) inspiratory compliance (Ci), and inspiratory resistance (Ri) were compared before and during episodes of hypoxemia. RESULTS All episodes of hypoxemia were preceded by an active exhalation that produced a mean decrease in end-expiratory lung volume of 6.4 +/- 2.8 ml/kg. The reduction in lung volume was immediately followed by a sudden decrease in tidal flow and volume, despite continuation of mechanical ventilation at the same rate and peak pressure. The resulting hypoventilation was associated with a drop in Ci to approximately one half and an increase in Ri to more than double the baseline values. Approximately 30 seconds after the beginning of hypoventilation, the arterial oxygen saturation reached a hypoxemic level (oxygen saturation < 85%)> CONCLUSION Most hypoxemic episodes were triggered by an expiratory effort that produced a large decrease in lung volume. This reduction in lung volume probably leads to closure of small airways and the development of intrapulmonary shunts, which would explain the rapid development of hypoxemia.

Lung Compliance in Newborns with Patent Ductus Arteriosus before and after Surgical Ligation

Pressure volume curves of the lungs were determined in 10 premature infants (mean gestational age 31.4 weeks, mean birth weight 1,260 g) before and after surgical ligation of a patent ductus arteriosus (PDA). 7 infants who had low compliance initially showed a significant improvement in lung compliance after surgery, while 3 infants whose compliance was close to normal before surgery had a decrease after ligation. In conclusion, a PDA with left to right shunt is frequently associated with a decrease in lung compliance that improves after ligation. Measurement of lung compliance in infants with PDA can be helpful in predicting the degree of improvement in lung function that may result from the closure of the ductus.

Influence of different methods of synchronized mechanical ventilation on ventilation, gas exchange, patient effort, and blood pressure fluctuations in premature neonates

We studied the effects of two methods of synchronized mechanical ventilation [synchronized intermittent mandatory ventilation (SIMV) and assist/control (A/C)] on ventilation, gas exchange, patient effort, and arterial blood pressure (ABP) fluctuations. SIMV and A/C were applied in random order in 12 preterm neonates (gestational age, 29.7 ± 2.3 weeks; birth weight, 1,217 ± 402 g). We measured total (Vetot) and mechanical (Vemech) minute ventilation, spontaneous (Vtspont) and ventilator supported (Vtmech) tidal volume, transcutaneous oxygen saturation (SpO2), transculaneous PO2 (TcPO2), and PCO2, (TcPCO2), mean airway pressure (Paw), phasic esophageal pressure deflections (Pe) as an estimate of inspiratory effort, mean arterial blood pressure (ABP), and beat‐to‐beat ABP fluctuations. The measurements obtained during conventional intermittent mandatory ventilation (IMV) were compared with the recordings during SIMV and A/C. To make the measurement conditions comparable and to prevent hyperventilation, peak inspiratory pressure was reduced during the A/C mode so that Vetot remained in the same range as during the IMV mode. Whereas Vetot was similar in all three conditions by study design, Vemech was larger during SIMV and A/C than during IMV. Vtmech increased during SIMV and by study design was smaller during A/C than during IMV. Pe decreased during SIMV and A/C compared with IMV, and Paw was higher during A/C than during IMV or SIMV. Beat‐to‐beat ABP fluctuations were reduced during SIMV and A/C compared with IMV and showed a close positive correlation with Pe changes. We conclude that SIMV increases Vemech and reduces Pe compared with IMV, resulting in smaller intrathoracic and ABP fluctuations. During A/C, a substantial portion of the spontaneous respiratory effort is shifted to the ventilator, resulting in a further decrease in Pe and ABP fluctuations Pediatr Pulmonol. 1996; 22:305–313.

Functional residual capacity in normal neonates and children up to 5 years of age determined by a N2 washout method

ABSTRACT. Functional residual capacity (FRC) was determined in 50 infants by a simplified N2 washout method. Fourteen infants were preterm, four full-term newborns and the rest were 1 month to 5 yr of age. Weight ranged from 1.19 to 25.8 kg. The method gave well reproducible values with a mean coefficient of variation of 3.9%. The FRC values are equally well correlated to weight and length (r = 0.98). The correlation with weight is linear, intercepting the x axis (FRC = 0) at a weight of 480 g, the one with length is best described by a power curve. The course of the regression lines reflects the observation that FRC per kg weight or per cm length is lower in neonates than in larger infants. The FRC measurements are in the same range as values obtained by other investigators using the N2 washout or He-dilution techniques. The values are significantly smaller than thoracic gas volume measurements obtained by plethysmography. This difference may be due to air trapping or to possible methodological problems with the plethysmographic technique. The data demonstrate that FRC can be measured easily and accurately in preterm and older infants using a N2 washout technique.

A simple method for measuring functional residual capacity by N2 washout in small animals and newborn infants

ABSTRACT: An open circuit N2 washout technique is described for the determination of functional residual capacity in infants. Either 100% O2 or any oxygen/helium mixture can be used as the washing gas. The subject breathes the washing gas through a T-tube and the washed out nitrogen is mixed with this gas in a mixing chamber, placed into the exhalation part of the circuit. The N2 concentration of the mixed gas is analyzed continuously, and the concentration signal is electronically integrated over time. Calibration of the system is accomplished by injecting known amounts of nitrogen or room air into the circuit. The gas flow through the system must remain constant and is adjusted to approximate peak inspiratory flow of the infant. In vitro testing of the system showed that the technique gives reproducible values (coefficient of variance <1.0%) and that the integrated signal output has a close linear correlation with the amount of N2 washed out (r = 0.99). In vivo measurements in 10 cats confirmed the accuracy and reproducibility of the method when compared with N2 collection. The technical advantages of the system are simplicity of components, absence of valves, easy calibration, low dead space, and no need to collect or measure expired gases. For the infant this means no added resistance during washout and no risk of hypoxia, hyperoxia, or hypercapnea. In the presence of pulmonary disease and poor gas mixing the washout period can be prolonged as needed. There is no lower limit of weight or size for functional residual capacity measurements in small infants or animals.

Randomized crossover comparison of proportional assist ventilation and patient-triggered ventilation in extremely low birth weight infants with evolving chronic lung disease

Background: Refinement of ventilatory techniques remains a challenge given the persistence of chronic lung disease of preterm infants. Objective: To test the hypothesis that proportional assist ventilation (PAV) will allow to lower the ventilator pressure at equivalent fractions of inspiratory oxygen (FiO2) and arterial hemoglobin oxygen saturation in ventilator-dependent extremely low birth weight infants in comparison with standard patient-triggered ventilation (PTV). Methods:Design: Randomized crossover design. Setting:Two level-3 university perinatal centers. Patients: 22 infants (mean (SD): birth weight, 705 g (215); gestational age, 25.6 weeks (2.0); age at study, 22.9 days (15.6)). Interventions: One 4-hour period of PAV was applied on each of 2 consecutive days and compared with epochs of standard PTV. Results: Mean airway pressure was 5.64 (SD, 0.81) cm H2O during PAV and 6.59 (SD, 1.26) cm H2O during PTV (p < 0.0001), the mean peak inspiratory pressure was 10.3 (SD, 2.48) cm H2O and 15.1 (SD, 3.64) cm H2O (p < 0.001), respectively. The FiO2 (0.34 (0.13) vs. 0.34 (0.14)) and pulse oximetry readings were not significantly different. The incidence of arterial oxygen desaturations was not different (3.48 (3.2) vs. 3.34 (3.0) episodes/h) but desaturations lasted longer during PAV (2.60 (2.8) vs. 1.85 (2.2) min of desaturation/h, p = 0.049). PaCO2 measured transcutaneously in a subgroup of 12 infants was similar. One infant met prespecified PAV failure criteria. No adverse events occurred during the 164 cumulative hours of PAV application. Conclusions: PAV safely maintains gas exchange at lower mean airway pressures compared with PTV without adverse effects in this population. Backup conventional ventilation breaths must be provided to prevent apnea-related desaturations.

Gas trapping with high-frequency ventilation: Jet versus oscillatory ventilation

Gas trapping was evaluated during high-frequency jet ventilation (HFJV) and high-frequency oscillatory ventilation (HFOV) in nine adult rabbits under basal conditions and after instillation of a mixture of 20% human meconium (2 mL/kg). The anesthetized animals underwent tracheostomy and were placed inside a body plethysmograph. Respiratory compliance and resistance were calculated from airway pressure and simultaneous flow, and volume was measured with a pneumotachograph. Gas trapping was measured as the change in volume observed in the plethysmograph after clamping the jet or the oscillatory line at respiratory rates of 10 and 15 Hz and tidal volumes of 1.0 and 2.0 mL/kg. Mean airway pressure was similar with both ventilators. Inspiratory/expiratory ratios were 1:4 at 10 Hz and 1:2 at 15 Hz with HFJV, and 1:1 during HFOV. Under all conditions, gas trapping was significantly greater with HFJV than with HFOV. More gas trapping was observed with higher tidal volume (2 mL/kg) and respiratory rate (15 Hz) during HFJV, before and after meconium instillation. After meconium instillation, gas trapping during HFJV at 15 Hz and tidal volume 2 mL/kg decreased significantly (32.7 +/- 10.4 to 24.9 +/- 10.3; P less than 0.05), compared with basal conditions. This finding may be explained by the shorter time constant of the respiratory system after meconium instillation (0.118 vs 0.083 seconds, P less than 0.01). Thus gas trapping was significantly greater with HFJV than with HFOV, a difference most likely related to the active expiratory phase of HFOV.