The present study confirmed that transient changes in the pleural pressure in the nonventilated lung exist during one-lung positive pressure ventilation before opening the thoracic cavity of the nonventilated lung. As a result, it caused tidal movement of gas out of and back into the nonventilated lung. The volume of TGM in the nonventilated lung in the lateral position ranged from 19 to 160 mL.
The measurement of the volume of TGM was performed using a paediatric spirometry sensor. The measurement range was 5 to 300 mL with an accuracy of ± 6% or 4 mL. It is more accurate to measure small gas movements with a paediatric spirometry senor than with an adult spirometer. Previous studies used different tools to measure the volume of TGM, such as a potentiometer attached to the counter-balance wheel of the spirometer , a water-filled spirometer , or an ambient pressure oxygen reservoir bag apparatus . Compared to the above methods, the paediatric spirometry sensor used in the present study makes the measurement simpler and more accurate. In addition, the volume of TGM can be measured in real-time with a repeatable way by using the paediatric spirometry sensor. The volume of TGM in the present study was smaller than those in other studies (65-265 mL) . This result may be related to race differences. Therefore, we calculated the TGM index, which allows for comparing TGM between individuals of different sizes.
The oxygen concentration in the nonventilated lung at 5 min after OLV was negatively correlated with the TGM index in the present study. This could be the result of the nitrogen going into the nonventilated lung by passive ventilation and diluting the oxygen concentration. Since nitrogen is absorbed slower than oxygen, the results suggested that a higher TGM volume may delay lung collapse.
Since there is no research on the analysis of factors associated with the TGM index, ten variables were chosen for the single factor correlation analysis. These variables are the most commonly used clinical indicators and include the patients’ general clinical information, lung function variables and basic anaesthesia-related factors. According to the multiple linear regression analysis, sex, side of OLV, BMI and PIP were independently correlated with the TGM index. To the best of our knowledge, this is the first time that a regression model was established to predict the TGM index. Using this model, anaesthesiologists can calculate the TGM index before surgery and evaluate the impact of TGM on the collapse of the nonventilated lung.
In the present study, male patients had a larger TGM index than female patients. Sex was independently correlated with the TGM index. The B value of sex was 12.770 mL/m2. The underlying reason may be related to anatomical differences. The lung volume of adult females is typically 10-12% smaller than that of males who have the same height and age as females. It results from the differences in the thoracic dimensions between males and females. Because of a greater inclination of ribs, female rib cages could accommodate for a greater volume expansion . During OLV, there is more space for the ventilated lung to expand, less mediastinal movement and less TGM of the nonventilated lung in female patients.
The side of OLV was also independently correlated with the TGM index. Patients with right side OLV produced a lower volume of TGM than those with left side OLV. This may be caused by the anatomical differences between the left and right lungs. The volume of the right lung is larger than that of the left lung, which shares space in the chest with the heart. The right lung has three lobes with 10 segments, while the left lung has only two lobes with 8 segments. Since the tidal volume was the same between left OLV and right OLV, the mediastinal movement was more significant during left OLV than during right OLV. Therefore, the volume of TGM was larger during left OLV than during right OLV.
In the present study, BMI was negatively correlated with the TGM index. The B value is -1.237. BMI is an important determinant of respiratory function, especially in obese patients . The patients with a higher BMI have low pulmonary/chest wall compliance and increased airway resistance. In addition, there is more adipose tissue in the mediastinum in obese patients. These will consequently make the mediastinal movement more difficult.
PIP depends on airway resistance and pulmonary/chest wall compliance. In the present study, PIP was measured during dual lung ventilation immediately after DLT intubation. The results indicated that the PIP was negatively correlated with the TGM index. The B value was -2.664. The lung with less PIP was associated with more TGM in the nonventilated lung under the same external force.
There were several limitations in the present study. First, as a single-centre observational study, the sample size was relatively small. Multicentre, large sample observation studies should be performed in the future. Second, TGM was measured only when the tidal volume was set as 6 mL/kg of ideal body weight during OLV. Although 6 mL/kg of ideal body weight is the commonly used tide volume during OLV, the presented regression model may be unsuitable for other tidal volumes. Third, TGM may indirectly affect oxygenation during surgery by delaying lung collapse. Therefore, the relationship between TGM and oxygenation during surgery needs to be further investigated.
In conclusion, TGM occurs in the nonventilated lung during one-lung ventilation. The TGM index is negatively correlated with the oxygen concentration in the nonventilated lung at 5 min after OLV. A greater volume of TGM might delay lung collapse during thoracic surgery. Sex, FVC, BMI and PIP are independently correlated with the TGM index. The regression model in the present study may be used to predict the TGM index before surgery and to guide OLV management.