Association of circulating angiotensin converting enzyme activity with respiratory muscle function in infants

Flow-resistive loading and diaphragmatic muscle function in term and preterm infants

PURPOSE

We aimed to assess diaphragmatic function in term and preterm infants with and without history of bronchopulmonary dysplasia (BPD), before and after the application of inspiratory flow resistive loading.

METHODS

Forty infants of a median (range) gestational age of 34 (25-40) weeks were studied. BPD was defined as supplemental oxygen requirement for >28 days of life. Seventeen infants were term, 17 preterm without history of BPD, and six preterm with a history of BPD. The diaphragmatic pressure-time index (PTIdi) was calculated as the mean to maximum trans-diaphragmatic pressure ratio times the inspiratory duty cycle. The PTIdi was calculated before and after the application of an inspiratory-flow resistance for 120 s. Airflow was measured by a pneumotachograph and the transdiaphragmatic pressure by a dual pressure catheter.

RESULTS

The median (interquartile range [IQR]) pre-resistance PTIdi was higher in preterm infants without BPD (0.064 [0.050-0.077]) compared with term infants (0.052 [0.044-0.062], p = .029) and was higher in preterm infants with BPD (0.119 [0.086-0.132]) compared with a subgroup of preterm infants without BPD (0.062 [0.056-0.072], p = .004). The median (IQR) postresistance PTIdi was higher in preterm infants without BPD (0.101 [0.084-0.132]) compared with term infants (0.067 [0.055-0.083], p < .001) and was higher in preterm infants with BPD [0.201(0.172-0.272)] compared with the preterm subgroup without BPD (0.091 [0.081-0.108],p = .004). The median (IQR) percentage change of the PTIdi after the application of the resistance was higher in preterm infants without BPD (65 [51-92] %) compared with term infants (34 [20-39] %, p < .001).

CONCLUSIONS

Preterm infants, especially those recovering from BPD, are at increased risk of diaphragmatic muscle fatigue under conditions of increased inspiratory loading.

Validation of a non-invasive pressure-time index of the inspiratory muscles in spontaneously breathing newborn infants

To validate the pressure-time index of the inspiratory muscles as a non-invasive index of inspiratory muscle function in spontaneously breathing infants by comparing it against the gold-standard pressure-time index of the diaphragm. Prospective observational cohort study of consecutive infants breathing unsupported in room air in a tertiary neonatal intensive care unit, studied prior to discharge from neonatal care. The invasive pressure-time index of the diaphragm was calculated using a transdiaphragmatic dual-pressure catheter that measured transdiaphragmatic pressure by subtraction of the oesophageal from the gastric pressure. The non-invasive pressure-time index of the inspiratory muscles was calculated using pressure measurements at the level of the mouth via a differential pressure transducer connected to a face mask. Both indices were calculated as the product of the ratio of the mean inspiratory pressure divided by the maximum inspiratory pressure and the ratio of the inspiratory time divided by the total time of a respiratory cycle. One hundred and thirty infants (79 male) were included with a mean (SD) gestational age of 35.2 (3.2) weeks, studied at a median (IQR) postnatal age of 9 (6-20) days. The mean (SD) pressure-time index of the diaphragm was 0.063 (0.019) and the mean (SD) pressure-time index of the inspiratory muscles was 0.065 (0.023). The correlation coefficient for the two indices was 0.509 (p < 0.001). The mean (SD) absolute difference between the pressure-time index of the inspiratory muscles and pressure-time index of the diaphragm was 0.002 (0.021). In convalescent infants, the non-invasive pressure-time index of the inspiratory muscles had a moderate degree of correlation with the invasively derived pressure time index of the diaphragm measured with a transdiaphragmatic catheter.