Transient decrease in PaCO2 and asymmetric chest wall dynamics in early progressing pneumothorax

Purpose

Diagnosis of pneumothorax (PTX) in newborn infants has been reported as late. To explore diagnostic indices for early detection of progressing PTX, and offer explanations for delayed diagnoses.

Methods

Progressing PTX was created in rabbits (2.3?±?0.5?kg, n?=?7) by injecting 1?ml/min of air into the pleural space. Hemodynamic parameters, tidal volume, EtCO₂, SpO₂, blood gas analyses and chest wall tidal displacements (TDi) on both sides of the chest were recorded.

Results

(Mean?±?SD): A decrease in SpO₂ below 90?% was detected only after 46.6?±?11.3?min in six experiments. In contrary to the expected gradual increase of CO₂, there was a prolonged transient decrease of 14.2?±?4.5?% in EtCO₂ (p?<?0.01), and a similar decrease in PaCO₂ (p?<?0.025). EtCO₂ returned back to baseline only after 55.2?±?24.7?min, and continued to rise thereafter. The decrease in CO₂ was a mirror image of the 14.6?±?5.3?% increase in tidal volume. The analysis of endotracheal flow and pressure dynamics revealed a paradoxical transient increase in the apparent compliance. Significant decrease in mean arterial blood pressure was observed after 46.2?±?40.1?min. TDi provided the most sensitive and earliest sign of PTX, decreasing on the PTX side after 16.1?±?7.2?min. The TDi progressively decreased faster and lower on the PTX side, thus enabling detection of asymmetric ventilation.

Conclusions

The counterintuitive transient prolonged decrease in CO₂ without changes in SpO₂ may explain the delay in diagnosis of PTX encountered in the clinical environment. An earlier indication of asymmetrically decreased ventilation on the affected side was achieved by monitoring the TDi.