Demand valve ventilation in a swine pneumothorax model

Real-time detection of pneumothorax using electrical impedance tomography

OBJECTIVES

Pneumothorax is a frequent complication during mechanical ventilation. Electrical impedance tomography (EIT) is a noninvasive tool that allows real-time imaging of regional ventilation. The purpose of this study was to 1) identify characteristic changes in the EIT signals associated with pneumothoraces; 2) develop and fine-tune an algorithm for their automatic detection; and 3) prospectively evaluate this algorithm for its sensitivity and specificity in detecting pneumothoraces in real time.

DESIGN

Prospective controlled laboratory animal investigation.

SETTING

Experimental Pulmonology Laboratory of the University of São Paulo.

SUBJECTS

Thirty-nine anesthetized mechanically ventilated supine pigs (31.0 +/- 3.2 kg, mean +/- SD).

INTERVENTIONS

In a first group of 18 animals monitored by EIT, we either injected progressive amounts of air (from 20 to 500 mL) through chest tubes or applied large positive end-expiratory pressure (PEEP) increments to simulate extreme lung overdistension. This first data set was used to calibrate an EIT-based pneumothorax detection algorithm. Subsequently, we evaluated the real-time performance of the detection algorithm in 21 additional animals (with normal or preinjured lungs), submitted to multiple ventilatory interventions or traumatic punctures of the lung.

MEASUREMENTS AND MAIN RESULTS

Primary EIT relative images were acquired online (50 images/sec) and processed according to a few imaging-analysis routines running automatically and in parallel. Pneumothoraces as small as 20 mL could be detected with a sensitivity of 100% and specificity 95% and could be easily distinguished from parenchymal overdistension induced by PEEP or recruiting maneuvers. Their location was correctly identified in all cases, with a total delay of only three respiratory cycles.

CONCLUSIONS

We created an EIT-based algorithm capable of detecting early signs of pneumothoraces in high-risk situations, which also identifies its location. It requires that the pneumothorax occurs or enlarges at least minimally during the monitoring period. Such detection was operator-free and in quasi real-time, opening opportunities for improving patient safety during mechanical ventilation.

A comparison of demand-valve and bag-valve ventilations in a swine pneumothorax model

OBJECTIVE

Two means of delivering artificial ventilation readily available to out-of-hospital personnel are the bag-valve (BV) and the O2-powered demand-valve (OPDV). However, use of the OPDV has been limited because of concerns that it may worsen an underlying pneumothorax. This study compared the changes in size of pneumothorax in swine ventilated with the 2 devices.

METHODS

Three swine were anesthetized, intubated, and instrumented with a femoral arterial line and a pediatric Swan-Ganz catheter. A chest tube was placed, the chest was opened, and the lung parenchyma was visualized. The lung was disrupted by a single stab with a #10 scalpel; the chest was then sealed; and a pneumothorax was created by injecting 30 mL of air through the chest tube. The animals were ventilated by 12 emergency medical technicians using either BV or OPDV. After 10 minutes of ventilation, the pneumothorax volume was measured.

RESULTS

When comparing final pneumothorax volumes after 10 minutes of ventilation with the 2 devices, there was no significant difference (mean +/- SD = 40.8 +/- 28.2 mL vs 52.3 +/- 23.1 mL, p = 0.286).

CONCLUSION

There is no difference in final pneumothorax volumes after OPDV or BV ventilation.

Comparison of automated and manual ventilation in a prehospital pediatric model

OBJECTIVE

Portable transport ventilators (TV) and demand valves (DV) may be effective and easy-to-use alternatives to bag-valve (BV) for prehospital ventilation of adults. The purpose of the study was to determine whether such devices maintain arterial blood gases and airway pressures similar to those for BV in a pediatric swine model.

METHOD

This study was a prospective, randomized, crossover design using immature swine (9.6 +/- 0.9 kg) to model ventilation in small children. Anesthetized, intubated, paralyzed, and cannulated animals were ventilated initially on standard mechanical hospital ventilation (HV). They were then assigned in random order to 10-minute intervals of ventilation using BV, TV, low-frequency jet ventilation (JV), and DV. Data were analyzed using repeated-measures ANOVA and Tukey multiple comparisons (alpha = 0.05).

RESULTS

The PaO2 exceeded 90 mm Hg for all animal/ventilation combinations. Blood PaCO2 was lower for BV and DV than it was for TV, JV, or HV. In contrast, blood pH was higher for BV and DV than it was for TV, JV, or HV. Peak airway pressure was higher for BV than it was for HV, TV, or JV; it was lower for JV than it was for HV, TV, or BV.

CONCLUSION

This animal model suggests that automated TV and JV may provide more effective ventilation of children than do manual BV or DV devices. Although promising, these findings require application in children under prehospital emergent conditions.