Pediatric Simulation of Intrinsic PEEP and Patient-Ventilator Trigger Asynchrony During Mechanical Ventilation

BACKGROUND

Intrinsic PEEP during mechanical ventilation occurs when there is insufficient time for expiration to functional residual capacity before the next inspiration, resulting in air trapping. Increased expiratory resistance (R_E), too rapid of a patient or ventilator breathing rate, or a longer inspiratory to expiratory time ratio (T_I/T_E) can all be causes of intrinsic PEEP. Intrinsic PEEP can result in increased work of breathing and patient-ventilator asynchrony (PVA) during patient-triggered breaths. We hypothesized that the difference between intrinsic PEEP and ventilator PEEP acts as an inspiratory load resulting in trigger asynchrony that needs to be overcome by increased respiratory muscle pressure (P_mus).

METHODS

Using a Servo lung model (ASL 5000) and LTV 1200 ventilator in pressure control mode, we developed a passive model demonstrating how elevated R_E increases intrinsic PEEP above ventilator PEEP. We also developed an active model investigating the effects of R_E and intrinsic PEEP on trigger asynchrony (expressed as percentage of patient-initiated breaths that failed to trigger). We then studied if trigger asynchrony could be reduced by increased P_mus.

RESULTS

Intrinsic PEEP increased significantly with increasing R_E (r = 0.97, P = .006). Multivariate logistic regression analysis showed that both R_E and negative P_mus levels affect trigger asynchrony (P < .001).

CONCLUSIONS

A passive lung model describes the development of increasing intrinsic PEEP with increasing R_E at a given ventilator breathing rate. An active lung model shows how this can lead to trigger asynchrony since the P_mus needed to trigger a breath is greater with increased R_E, as the inspiratory muscles must overcome intrinsic PEEP. This model will lend itself to the study of intrinsic PEEP engendered by a higher ventilator breathing rate, as well as higher T_I/T_E, and will be useful in ventilator simulation scenarios of PVA. The model also suggests that increasing ventilator PEEP to match intrinsic PEEP can improve trigger asynchrony through a reduction in R_E.

Predicting neonatal outcomes in infants with giant omphalocele using prenatal magnetic resonance imaging calculated observed-to-expected fetal lung volumes

OBJECTIVE

To examine the association between prenatal magnetic resonance imaging (MRI) based observed/expected total lung volume (O/E TLV) and outcome in neonates with giant omphalocele (GO).

METHODS

Between 06/2004 and 12/2019, 67 cases with isolated GO underwent prenatal and postnatal care at our institution. MRI-based O/E TLVs were calculated based on normative data from Meyers and from Rypens and correlated with postnatal survival and morbidities. O/E TLV scores were grouped based on severity into <25% (severe), between 25% and 50% (moderate), and >50% (mild) for risk stratification.

RESULTS

O/E TLV was calculated for all patients according to Meyers nomograms and for 49 patients according to Rypens nomograms. Survival for GO neonates with severe, moderate, and mild pulmonary hypoplasia based on Meyers O/E TLV categories was 60%, 92%, and 96%, respectively (p = 0.04). There was a significant inverse association between Meyers O/E TLV and risk of neonatal morbidities (p < 0.05). A similar trend was observed with Rypens O/E TLV, but associations were less often significant likely related to the smaller sample size.

CONCLUSION

Neonatal outcomes are related to fetal lung size in isolated GO. Assessment of Meyers O/E TLV allows identification of GO fetuses at greatest risk for complications secondary to pulmonary hypoplasia.

Therapy at 30 days of life predicts lung function at 6 to 12 months in infants with congenital diaphragmatic hernia

INTRODUCTION

Congenital diaphragmatic hernia (CDH) is associated with variable degrees of lung hypoplasia. Pulmonary support at 30 days postnatal age was found to be the strongest predictor of inpatient mortality and morbidity among CDH infants and was also associated with higher pulmonary morbidity at 1 and 5 years. It is not known, however, if there is a relationship between the need for medical therapy at 30 days of life and subsequent abnormalities in lung function as reflected in infant pulmonary function test (iPFT) measurements.

OBJECTIVE

We hypothesized that CDH infants who require more intensive therapy at 30 days would have more abnormal iPFT values at the time of their first infant pulmonary function study, reflecting the more severe spectrum of lung hypoplasia.

METHODS

A single-institution chart review of all CDH survivors who were enrolled in a Pulmonary Hypoplasia Program (PHP) through July 2019, and treated from 2002 to 2019 was performed. All infants were divided into groups based on their need for noninvasive (supplemental oxygen, high flow therapy, noninvasive mechanical ventilation) or invasive (mechanical ventilation, extracorporeal membrane oxygenation) respiratory assistance, bronchodilators, diuretic use, and pulmonary hypertension (PH) therapy (inhaled and/or systemic drugs) at 30 days. Descriptive and statistical analyses were performed between groups comparing subsequent lung function measurements.

RESULTS

A total of 382 infants (median gestational age [GA] 38.4 [interquartile range (IQR) = 37.1-39] weeks, 41.8% female, 70.9% Caucasian) with CDH were enrolled in the PHP through July 2019, and 118 infants underwent iPFT. The median age of the first iPFT was 6.6 (IQR = 5.3-11.7) months. Those requiring any pulmonary support at 30 days had a higher functional residual capacity (FRC) (z) (P = .03), residual volume (RV) (z) (P = .008), ratio of RV to total lung capacity (RV/TLC) (z) (P = .0001), and ratio of FRC to TLC (FRC/TLC) (z) (P = .001); a lower forced expiratory volume at 0.5 seconds (FEV0.5) (z) (P = .03) and a lower respiratory system compliance (Crs) (P = .01) than those who did not require any support. Similarly, those requiring diuretics and/or PH therapy at 30 days had higher fractional lung volumes, lower forced expiratory flows and Crs than infants who did not require such support (P < .05).

CONCLUSIONS

Infants requiring any pulmonary support, diuretics and/or PH therapy at 30 postnatal days have lower forced expiratory flows and higher fractional lung volumes, suggesting a greater degree of lung hypoplasia. Our study suggests that the continued need for PH, diuretic or pulmonary support therapy at 30 days can be used as additional risk-stratification measurements for evaluation of infants with CDH.