Single-breath CO2 analysis as a predictor of lung volume change in a model of acute lung injury

Typical patterns of expiratory flow and carbon dioxide in mechanically ventilated patients with spontaneous breathing

Incomplete expiration of tidal volume can lead to dynamic hyperinflation and auto-PEEP. Methods are available for assessing these, but are not appropriate for patients with respiratory muscle activity, as occurs in pressure support. Information may exist in expiratory flow and carbon dioxide measurements, which, when taken together, may help characterize dynamic hyperinflation. This paper postulates such patterns and investigates whether these can be seen systematically in data. Two variables are proposed summarizing the number of incomplete expirations quantified as a lack of return to zero flow in expiration (IncExp), and the end tidal CO₂ variability (varETCO₂), over 20 breaths. Using these variables, three patterns of ventilation are postulated: (a) few incomplete expirations (IncExp < 2) and small varETCO₂; (b) a variable number of incomplete expirations (2 ≤ IncExp ≤ 18) and large varETCO₂; and (c) a large number of incomplete expirations (IncExp > 18) and small varETCO₂. IncExp and varETCO₂ were calculated from data describing respiratory flow and CO₂ signals in 11 patients mechanically ventilated at 5 levels of pressure support. Data analysis showed that the three patterns presented systematically in the data, with periods of IncExp < 2 or IncExp > 18 having significantly lower variability in end-tidal CO₂ than periods with 2 ≤ IncExp ≤ 18 (p < 0.05). It was also shown that sudden change in IncExp from either IncExp < 2 or IncExp > 18 to 2 ≤ IncExp ≤ 18 results in significant, rapid, change in the variability of end-tidal CO₂ p < 0.05. This study illustrates that systematic patterns of expiratory flow and end-tidal CO₂ are present in patients in supported mechanical ventilation, and that changes between these patterns can be identified. Further studies are required to see if these patterns characterize dynamic hyperinflation. If so, then their combination may provide a useful addition to understanding the patient at the bedside.

Tracheostomy in the critically ill: the myth of dead space

Benefits and advantages of tracheostomy have been vigorously debated. There is a lack of consensus as to whether perceived clinical improvement is attributable to fundamental changes in respiratory dynamics. We compare the effect of tracheostomy versus endotracheal tube on dead space, airway resistance and other lung parameters in critically ill ventilated patients. Data collected included patients who were admitted to surgical, burn and neurosurgical intensive care units at the University of North Carolina. Twenty-four intubated patients were included in our analysis with various aetiologies of respiratory failure. Tracheostomy was deemed necessary either for severe neurological devastation or failure to wean from the ventilator. The diameter of the endotracheal tubes ranged from 6-8 mm and the tracheostomy tube diameters were from 6.4-8.9 mm. Internal diameters between endotracheal tube and tracheostomy tubes, ventilator settings and sedation were kept consistent throughout the study. Respiratory parameters were measured using the Respironics' non-invasive cardiac output 2 device (Phillips, Andover, MA) immediately prior to tracheostomy and repeated within 24 hours of tracheostomy. Only two (8%) of the patients had slight improvement (>6% decrease in dead space). The average dead space of endotracheal versus tracheostomy tubes was 41±12.6% and 40±14.6%, respectively (P=0.75). The remaining 22 patients (92%) had no significant change in dead space, compliance or other respiratory parameters. This study shows that there is no significant difference in respiratory mechanics and dead space with a tracheostomy versus endotracheal tube.

The role of dead space ventilation in predicting outcome of successful weaning from mechanical ventilation

BACKGROUND

The exact mechanism by which tracheostomy results in clinical improvement in respiratory function and liberation from mechanical ventilation remains unknown. Physiologic dead space, which includes both normal and abnormal components of non-gas exchange tidal volume, is a clinical measure of the efficiency of ventilation. Theoretically, tracheostomy should reduce dead space ventilation and improve pulmonary mechanics, thereby facilitating weaning from mechanical ventilation.

METHODS

This study compares arterial blood gases (ABG), pulmonary mechanics, including minute ventilation (VE) and dead space ventilation (Vd/Vt) within 24 hours before and after tracheostomy in 45 patients admitted to a surgical intensive care unit.

RESULTS

There was no difference noted in patients' ABG or VE. Pre- and posttracheostomy change in Vd/Vt was negligible (50.7 and 10 vs. 51.9 and 11; p = NS). On subgroup analysis, those patients that were weaned from mechanical ventilation with 72 hours of tracheostomy (T3) were compared with those patients weaned from mechanical ventilation 5 days or more after tracheostomy (T+5). Again, no difference was found in pulmonary mechanics or Vd/Vt pre- and posttracheostomy.

CONCLUSION

There is minimal improvement in pulmonary mechanics after tracheostomy. The change in physiologic dead space posttracheostomy does not predict the outcome of weaning from mechanical ventilation. Tracheostomy does allow better pulmonary toilet, and easier initiation and removal of mechanical ventilation and control of the upper airway.

Macrophage cyclooxygenase expression, immunosuppression, and cardiopulmonary dysfunction after blunt chest trauma

BACKGROUND

Two series of experiments were performed in swine who received severe blunt chest trauma. The goals were to determine the time course of constitutive and inducible cyclooxygenase (COX) isozyme expression in pulmonary macrophages (Mphis), and to determine whether COX expression and cardiopulmonary dysfunction were altered when neutrophils (PMNs) were pharmacologically depleted with cyclophosphamide (CYC).

METHODS

In series 1 (n = 17), anesthetized, mechanically ventilated swine were subjected to right chest trauma via captive bolt gun, hemorrhage, and a 60-minute shock period. In series 2 (n = 41), CYC (50 mg/kg intravenously) was administered 4 days before trauma, and the shock period was shortened to 30 minutes. In both series, hemodynamic support and supplemental oxygen were provided for an additional 60 to 90 minutes after shock. Mphis were isolated from serial bilateral bronchoalveolar lavages (BALs) and COX protein expression was measured with Western blots.

RESULTS

In series 1, death occurred in 11 of 17. In survivors, Mphi COX-1 peaked at > 100 times baseline in both right BAL and left BAL by 60 minutes (before resuscitation). Changes in Mphi COX-2 were minimal. In series 2, before trauma, CYC (n = 16) reduced circulating and BAL PMNs by > 90% relative to control (n = 25, both p < 0.05) with no complicating side effects. After trauma, death occurred in 11 of 25 controls versus 9 of 16 with CYC. In survivors, PaO2/FIO2 was < 250 and PaCO2 was 25% higher on constant minute ventilation, indicating mismatched ventilation/perfusion; both changes were reduced with CYC (p < 0.05). In controls, bilateral histologic damage included edema, alveolar hemorrhage, and interstitial infiltrates. These changes were reduced by one third with CYC (p = 0.08). Trauma-induced changes in BAL protein, BAL elastase, or Mphi COX expression were not lessened by CYC.

CONCLUSION

After unilateral chest trauma, Mphi COX-1, not COX-2, is induced bilaterally and before fluid resuscitation; CYC prevented PMN infiltration and attenuated structural and functional changes after resuscitation, which suggests that PMNs have a role in the pathogenic mechanism of secondary lung injury; Mphi COX expression and other injury markers were not altered by CYC; and since Mphis continued to express proinflammatory COX protein even after pretreatment with a powerful nonspecific immunosuppressant, and since there is residual alveolar capillary damage even in the absence of PMNs, it is logical to conclude that no single cell type or mediator is a practical therapeutic target and that novel resuscitation strategies must address multiple elements in the inflammatory cascade.