Diagnosing and quantifying incomplete expiration in patients with lung disease.

Editor—Incomplete expiration is common in smokers and patients with chronic obstructive pulmonary disease (COPD). This can increase FRC and auto-PEEP. The increased intrathoracic pressure may compromise venous return or, if great, could cause a pneumothorax. We report a simple new method for detecting this, which appears quick and reliable, and can quantify the increase in lung volume. During mechanical ventilation of an anaesthetized subject, we set the APL valve to zero and then discontinued ventilation for approximately 10 s, allowing complete expiration to occur. Complete expiration was verified by confirming that the expiratory flow rate shown on the flow–volume loop was zero. The ventilator was restarted using the previously set rate and volume and the flow– volume loop was observed (Fig. 1). With the GE s/5 spirometer (D-lite), which displays successive individual inspired and expired volumes, the differences between inspired and expired tidal volumes were added for the next three respiratory cycles. We have called this the volume of incomplete expiration, since it represents the volume retained by the respiratory system when the previous pattern of mechanical ventilation is resumed. A female aged 71 yr with a history of 40 pack years of smoking and symptoms and signs of severe COPD underwent a total abdominal hysterectomy. The volume of incomplete expiration was 380 ml. This feature was reproducible, with other measures of incomplete expiration, resulting in volumes of 310, 400, and 290 ml (mean 345 ml) during the anaesthetic (Fig. 2). A further patient with a long history of smoking, although no formal diagnosis of COPD showed incomplete expiration with a mean total volume of incomplete expiration of 215 ml (200–250 ml) over four trials. As a control patient, a man of 62 yr with no respiratory disease and a non-smoker undergoing a hemi-colectomy had a mean volume of incomplete respiration of 10 ml (250 to 50 ml) over four trials. Similarly a man of 65 yr also a non-smoker with no respiratory disease undergoing an open right radical nephrectomy had a mean volume difference of 40 ml (10–100 ml) over four trials. This technique was used in various patients and seems to be useful in both confirming and quantifying incomplete expiration. There appears to be a difference between observed values of our four patients, which correlates with the severity of clinically observed airways disease. We cannot find previous reports of this manoeuvre, of discontinuing ventilation and measuring subsequent volume changes when mechanical ventilation is recommenced. This could be particularly useful as most other methods of detecting incomplete expiration rely on pressure measurement, as opposed to volume, and do not quantify the degree of incomplete expiration. Detecting incomplete expiration is clinically useful. Various strategies can reduce incomplete expiration, such as reducing the I:E ratio or the tidal volume. Perhaps, the Fig 1 Flow–time plot from GE s/5 monitor, with volume measurements from d-lite spirometer module. There are three breaths on this screen. Breath 1 is the equilibrium condition as both inspiratory and expiratory tidal volumes are 450 ml. Ventilation was discontinued after breath 2. The flow rate is zero for a sustained period ensuring complete expiration. When ventilation was recommenced, with breath 3, the expiratory tidal volume decreased to 390 ml, despite an inspiratory volume of 450 ml. Thus, this patient had incomplete expiration of 60 ml. Plotting breaths 1 and 3 together, one can see the horizontal distance, which indicates expired volume, is noticeably smaller. Although the increase in expired volume on prolonged exhalation (breath 2) was evidence of incomplete expiration, this appeared inconsistent and unreliable.