Expiratory Flow-Volume Loop Profile and Patient Outcome in Chronic Obstructive Pulmonary Disease in Acute Respiratory Failure: A Prospective Observational Study in a Single Intensive Care Unit

Abstract

Background: Expiratory flow-volume (EFV) loops are continuously displayed on the screen of intensive care unit (ICU) ventilators. Objectives: It was the aim of this study to investigate the relationships of EFV to chronic obstructive pulmonary disease (COPD) patient outcome. Methods: This is a prospective study on COPD patients who received invasive mechanical ventilation for acute respiratory failure in the ICU. Within the 24-hour post-intubation period, the angle of the EFV slope during the last 50% of expiration was computed and patients were stratified into 4 quartiles. Resistance, compliance of the respiratory system and change in end-expiratory lung volume above relaxation volume were assessed. Patients were followed up to hospital discharge. The main outcome was hospital mortality. Secondary outcomes were ICU mortality, length of ICU stay, duration of invasive ventilation, number of intubations, oxygen and non-invasive ventilation. Results: Thirty-eight patients were analysed. The first quartile comprised 9 patients (median angle 11°, interquartile range 8–12), the second 10 patients (median angle 26°, range 19–30), the third 10 patients (median angle 42°, range 39–46), and the fourth 9 patients (median angle 53°, range 49–64). Hospital and ICU mortality were not different between groups. Lengths of ICU and hospital stay and length of invasive ventilation were significantly different between groups, with the highest values observed in the first quartile. The rate of oxygen use and non-invasive ventilation in the ICU and at hospital discharge was significantly different between groups, with the highest rate observed in the first quartile. There was a significant negative correlation between angle and resistance, compliance of the respiratory system and change in end-expiratory lung volume above the relaxation volume. Conclusion: The slope of the angle during the last 50% of expired volume in the COPD patients was associated with worsened respiratory mechanics and higher morbidity.

Physiology to Assist Bedside Prognostication in Mechanically Ventilated Patients with Chronic Obstructive Pulmonary Disease

“…In this issue of Respiration , Porot et al [19], building on a previous concept [20], reported on the use of the flow-volume curve to predict outcomes such as in-hospital and ICU mortality, length of stay and duration of mechanical ventilation in 38 intubated and mechanically ventilated patients with COPD. More specifically, the angle of the second half of the relaxed expiratory portion of the flow-volume curve was used as a surrogate marker of the severity of airflow obstruction.…”

“…The possibility of having continuous monitoring at no extra-costs is also very appealing. From a physiological standpoint, the validity of the angle of the slope of the expiratory flow-volume curve is supported by its inverse association with traditional respiratory mechanics parameters such as total resistance of the respiratory system and the amount of dynamic hyperinflation [19]. It may be of practical value to note that, according to receiving-operating characteristic analysis, the slope of the second half of the flow-volume curve was a better outcome predictor than more traditional respiratory mechanics measurements such as intrinsic PEEP and total resistance of the respiratory system.…”

“…One difficulty with the study results that complicates its interpretation is that different outcomes were predicted by different angles. Porot et al [19 ]only studied patients early into the mechanical ventilation period and it would have been of interest to evaluate whether the change in the slope of the expiratory-flow volume curve occurring with treatment could be a better outcome predictor. It can be argued that the changes in the slope, indicating improvement in airflow limitation with treatment, could even better predict various clinical outcomes than a single measurement made early on during an episode of respiratory failure.…”