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Differential lung volume measurement in the anesthetized patient

Brimingham Heartlands Hospital, Birmingham, United Kingdom, E-mail: alan.seymour{at}tesco.net

To the Editor:

The Carlens¹ double lumen tube was originally developed in order to carry out differential spirometry in an awake patient. The method gave acceptable results but was cumbersome, finally being abandoned in the late 1960s² with the advent of scanning techniques. However, the merits of conducting differential volume measurements in an anesthetized subject have never been explored. We obtained approval from our Ethical Committee and conducted a feasibility study in 21 adults undergoing thoracotomy for carcinoma of the bronchus. The induction sequence was alfentanil 1 to 2 mg iv, papaveretum iv 0.07 mg·kg^–1 iv, atropine 0.4 to 0.6 mg iv, thiopentone 3.5 mg·kg^–1 iv and cisatracurium 0.2 mg·kg^–1 iv. Following rigid (surgical) bronchoscopy the non-operated lung was intubated with a double-lumen tube. Throughout the measuring sequence great care was taken to ensure freedom from secretions, full paralysis, accurate tube positioning and the absence of leaks. Full monitoring was employed. Before the patient was turned lateral we manually inflated each lung in turn to a plateau pressure of 20 cm H₂O for about five seconds. Then the circuit was opened to air by pulling the connector smoothly off the inspiratory port of the circle absorber. The resulting expired tidal volumes to resting volume, V_TO and V_TN (operated and non-operated sides respectively) were measured using a calibrated electronic Wright respirometer positioned next to the catheter mount. A feature of the technique is that patients form their own controls. As a check we took a third measurement, V_TC (= combined) with both lungs together. The chosen pressure of 20 cm H₂O ensured that the lower inflection point on the pressure/volume loop had been passed. The pressure and flow traces were recorded to allow confirmation that the conditions had been fulfilled. All three readings were usually completed in about five minutes.

The data were entered onto a spreadsheet and analyzed using Excel 97 (Microsoft Corp), 18 sets of results being suitable; 12 males and six females. The individual splits (S_N%) for the non-operated lung, 100 V_TN/(V_TN + V_TC)% were compared with those from a quantitative ventilation scan (V/[Q]) using a Bland-Altman plot.³ This gave limits from –9.8 to +10.2 (bias 0.3)%, even though the two methods were not applied simultaneously. In 12 instances the discrepancy was 4% or less. Differential measurement can have advantages: in our first exclusion the operated lung was shown to have dropped from a V/Q result of 36% ventilation to non-functional. Severe gas trapping caused by partial obstruction of a bronchus may also be more apparent than with a scan. However, it cannot be used in every case; bleeding into the bronchial tree and an unstable tube position caused our other two exclusions.

Applying the calculated split (S_N%) to our third measurement (might be expected to produce a volume close to that for the non-operated lung working alone (V_TN), but in fact the latter was often greater (in one instance by 250 mL). This serendipitous finding is probably explained by mediastinal shift. The difference for each patient, V_TN – (S_N V_TC/100) mL, was plotted on a column chart (Figure). It is well known that, following pneumonectomy, FEV₁ is often better than expected.⁴ Despite one anomalous result, the compensatory expansion demonstrated by the chart could well explain and define this.

View larger version (14K): [in this window] [in a new window]   FIGURE Tidal volume difference generated by the non-operated lung working alone and in tandem (mL): figures from static differential measurements made in 12 male and six female adults undergoing thoracotomy.

Footnotes

Accepted for publication October 31, 2005.

References

1 Carlens E. A new flexible double-lumen catheter for bronchospirometry. J Thorac Surg 1949; 18: 742–6.[Medline]

2 Kristersson S, Lindell SE, Svanberg L. Prediction of pulmonary function loss due to pneumonectomy using ¹³³Xeradiospirometry. Chest 1972; 62: 694–8.[Abstract/Free Full Text]

3 Altman DG, Bland JM. Measurement in medicine: the analysis of method comparison studies. Statistician 1983; 32: 307–17.

4 Zeiher BG, Gross TJ, Kern JA, Lanza LA, Peterson MW. Predicting postoperative pulmonary function in patients undergoing lung resection. Chest 1995; 108: 68–72.[Abstract/Free Full Text]

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