1996 OPEN FORUM Abstracts
Developing Ventilatory Management Strategles - A Pathophysiologic Approach
David J. Durand, MD Wednesday, November 6, 1996
High Frequency Ventilation
High frequency ventilators, regardless of their type, provide a theoretical advantage over conventional ventilators in patients with certain lung diseases. In general, the advantage is most pronounced in disease states where large tidal volumes are needed to provide adequate gas exchange. By not using "real" tidal volumes, HFVs can theoretically avoid the consequences of lung over-distension that result from ventilation with large tidal volumes. Patients who have pulmonary interstitial emphysema (PIE), or bronchopleural fistula, or who are at risk for developing these complications, are the usual candidates for high frequency ventilation. Typically, these are infants with severe hyaline membrane disease, although infants with severe meconium aspiration syndrome, pulmonary hypoplasia, and pulmonary hypertension all may benefit from HFV.
In choosing between HFOV and HFJV, we use the following (unproven) guidelines: * - HFOV for patients with HMD or other dense, uniform disease * - HFJV for patients where air leak is a major problem * - HFJV for patients with clear lungs, or with air-trapping
Patient Triggered Ventilation
Although adult ventilators have incorporated synchronized modes since the late 1970's, attempts to devise neonatal ventilators which could provide SIMV or AC met with little success until the last few years. The development of more sophisticated sensing mechanisms and "smarter" ventilators has provided devices which are capable of responding to the small tidal volumes and rapid respiratory rates of neonates.
None of these triggering mechanisms are without drawbacks. Chest impedance leads must be placed appropriately for maximal signal and may not work if the adhesive becomes loosened or the pads become dry. To optimize triggering, the Graesby capsule's signal must be re-checked or repositioned when the infant is repositioned. Flow-sensing devices, although not usually affected by infant position changes, may lose sensitivity if the leak around the endotracheal tube is too large, or they may become too sensitive if not adjusted properly.
In 1994, Bernstein reported results of a randomized trial of SIMV vs IMV using the Infrasonics machine. A total of 306 infants were randomized to SIMV or IMV. Infants with birth weights < 1000 grams on SIMV, had a reduced need for supplemental oxygen at 35 weeks post conceptual age: SIMV (46%); IMV (77%), p < 0.05. In infants with birthweight > 2000 grams, he reported a significant decrease in the duration of ventilation in SIMV (72 hours) vs IMV (96 hours) p < 0.02.
Donn recently reported a study of 30 preterm infants weighing between 1100 and 1500 grams at birth. Infants were randomized, to AC or IMV. This study demonstrated a significant decrease in the mean time to extubation (IMV 271 ± 218 hours, AC 119 ± 156 hours, p < 0.01). This decrease resulted in lower ventilator-use charges for the AC group (p < 0.02), and a trend towards a reduced amounts of sedation per patient (p < 0.07), but otherwise demonstrated no significant differences between the two groups.
While there is a tremendous amount of variation between nurseries in their approach to patient-triggered ventilation, we feel that SIMV or AC will rapidly become the standard of care for the majority of ventilated patients who do not require HFV.