The present study confirmed that transient changes in pleural pressure in nonventilated lung existed during one-lung positive pressure ventilation before opening of the thoracic cavity of 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 lateral position was in the range of 21 mL to 180 mL.
The measurement of the volume of TGM was performed using a pediatric spirometry sensor. The measurement range is 5 to 300 ml with accuracy of ± 6% or 4 mL. It is more accurate to measure small gas movement than adult spirometer. The volume of TGM in the present study was smaller than that in other studies (65–265 mL) . The authors attribute this to the race difference.
Previous studies used different ways 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 . As compare to above methods, the pediatric spirometry sensor used in the present study makes the measurement simpler and more accurate. In addition, the volume of TGM can be measured in a real-time and repeatable way by using the pediatric spirometry sensor.
Since there was no research about analysis of factors associated with the volume of TGM, eleven variables were chosen for the single factor correlation analysis. These variables are the most commonly used clinical indicators, including patients’ general information, lung function variables and basic anesthesia-related factors. Among these factors, ABW-IBW was included to study the effect of obesity on the TGM. According to the multiple regression analysis, gender, side of OLV, FVC or dynamic lung compliance independently correlated with the volumes of the TGM. To the best of our knowledge, it was the first time that a regression model was established to predict the volume of the TGM. Using this model, anesthesiologists can calculate the volume of TGM before surgery and evaluate the impact of TGM on the collapse of the nonventilated lung.
In the present study, male patients had more TGM than female ones. Gender independently correlated with the volume of TGM (P < 0.001) (Table 2, Figure 2). The correlation between male and the volume of TGM was positive, while the correlation between female and the volume of TGM was negative. The B value of gender was 16.452 mL, which is the largest coefficient of the four correlated variables. The underlying reason may be related to sexual anatomical differences. The lung volume of adult female is typically 10–12% smaller than that of male who have the same height and age as female. It results from the differences in thoracic dimensions between male and female. Because of a greater inclination of ribs, female rib cages could accommodate 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 also independently correlated with the volume of TGM. Patients with right side OLV produced less volume of TGM than those with left side OLV (Figure 2, Table 2). It may be caused by the anatomical difference between the left and right lung. The volume of right lung is bigger than that of the left lung, which shares space in chest with 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 same between left OLV and right OLV, the mediastinal movement was more significant during left OLV than that during right OLV. Therefore, the volume of TGM was bigger during left OLV than that during right OLV.
The FVC from the pulmonary function tests before surgery positively correlated with the volume of TGM (Figure 1, Table 2). The B value is 14.017mL. FVC is the volume of air that can be forcibly blown out after full inspiration. It is an important indicator in the pulmonary function test. Larger FVC means larger ventilation potential of the lung, which may be reasonable to produce more TGM of the nonventilated lung by the same magnitude of the mediastinal movement.
Dynamic lung compliance is an important variable in respiratory physiology. It represents pulmonary compliance during periods of gas flow. In the present study, it was measured during dual lung ventilation immediately after DLT intubation using spirometry sensor. The results indicated that the Cdyn positively correlated with the volume TGM (Figure 1, Table 2). The Lung with better Cdyn produces more tidal gas under the same external force. However, the effect of Cdyn was smaller than the other three variables (B = 0.206).
There were several limitations in the present study. Firstly, as a single-center research, sample size was small. Multicenter, large sample observation study need to be performed in the future. Secondly, the 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 volume.
In conclusion, the tidal gas movement of the non-ventilated lung during one-lung ventilation does exist. Gender, FVC, side of OLV and Cdyn independently correlated with the volume of TGM. The regression model in the present study may be used to predict the volume of TGM before surgery and to guide OLV management.