Three groups of patients were studied: group 1—normal lungs; group 2—pulmonary parenchymal abnormality; and group 3—chronic obstructive pulmonary disease (Table 1). The patients were initially ventilated for at least 30 min using the same mode and settings on which each patient had been ventilated during the mechanical ventilatory period. In this baseline condition, gross alveolar ventilation (V’gA in liters per minute) was measured as respiratory rate (f) times tidal volume (Vt) minus series dead space ventilation (f°VT—f’VdS) at baseline. The ventilator was then switched to ALV with a target V’gA identical to the baseline condition.
After 30 min, the target V’gA was reduced by 66% to encourage weaning. If after 2 h a patient required no more than 5 cm of pressure support and no more than four mechanical ventilatory breaths while still meeting the standard weaning criteria, the patient was considered to be weaned. He or she was then placed on a T-piece for another 24 h. The wean was documented as being successful if the patient required no further ventilatory support and continued to meet the standard wean criteria after 24 h. The wean was considered to have failed if the patient continued to require more than 5-cm pressure support or more than four mechanical breaths or no longer met the weaning criteria, eg, PaC02 increased to >6 kPa, respiratory rate increased to >30/ min, etc. The wean was also documented as having failed if the patient required further ventilatory support after an apparently successful wean.
To document the weaning process, recordings of ventilatory parameters (see below), hemodynamic parameters, and arterial blood gases were made at baseline, after 5 min on ALV, at 30 min, and then at 2 h intervals. The patients were also continuously monitored by ECG, pulse oximetry, and capnography. website
Results are reported as median values and were compared using the Mann-Whitney U test with Bonferroni correction where appropriate with a level of significance of 0.05.
Table 1—Patient Grouping, Outcome, and Primary Diagnoses
|Normal||S||20||74||3||Fat embolism syndrome|
|Parenchymal abnormality||S||4||49||11||ARDS / Septicemia|
|Parenchymal abnormality||S||21||49||7||Pulmonary edema/renal failure|
|Parenchymal abnormality||S||11||46||5||Legionnaires’ disease|
|Parenchymal abnormality||Failed||30||35||19||ARDS / septicemia|
|Parenchymal abnormality||t||12||52||2||Septicemia/fibrosing alveolitis|
|COPD||S||3||68||6||Congestive heart failure in COPD|
|COPD||s||9||71||3||Blunt chest trauma in COPD|
|COPD||s||33||58||6||Pneumonia in COPD|
|COPD||s||13||61||4||Pneumonia in COPD|
|COPD||s||32||28||2||Bronchiectasis in COPD|
|COPD||Failed||24||35||7||Pneumonia in COPD|
|COPD||Failed||23||33||4||Pneumonia in COPD|
|COPD||Failed||28||42||30||Pneumonia in COPD|
|COPD||S||29||55||18||Pneumonia in COPD|