Forced vital capacity correlated more strongly with total lung capacity than predicted body weight, and Vt = FVC/8 reduced calculated tidal volume by 22.5% in low compliance patients.
Observational (n=130)
Does forced vital capacity (FVC) better predict total lung capacity and low pulmonary compliance compared to predicted body weight (PBW) for tidal volume selection in thoracic surgery patients?
Using forced vital capacity (FVC) rather than predicted body weight may provide a more accurate method for selecting intraoperative tidal volumes, particularly in patients with low pulmonary compliance.
BACKGROUND: Tidal volume selection during mechanical ventilation utilizes dogmatic formulas that only consider a patient's predicted body weight (PBW). In this study, we investigate whether forced vital capacity (FVC) (1) correlates better to total lung capacity (TLC) than PBW, (2) predicts low pulmonary compliance, and (3) provides an alternative method for tidal volume selection. METHODS: One hundred thirty thoracic surgery patients had their preoperative TLC calculated via 2 methods: (1) pulmonary function test (PFT; TLCPFT) and (2) computed tomography 3D reconstruction (TLCCT). We compared the correlation between TLC and PBW with the correlation between TLC and FVC to determine which was stronger. Dynamic pulmonary compliance was then calculated from intraoperative ventilator data and logistic regression models constructed to determine which clinical measure best predicted low compliance. Ratios of tidal volume/FVC plotted against peak inspiratory pressure were utilized to construct a new model for tidal volume selection. Calculated tidal volumes generated by this model were then compared with those generated by the standard lung-protective formula Vt = 7 cc/kg. RESULTS: The correlation between FVC and TLC (0.82 for TLCPFT and 0.76 for TLCCT) was stronger than the correlation between PBW and TLC (0.65 for TLCPFT and 0.58 for TLCCT). Patients with very low compliance had significantly smaller lung volumes (forced expiratory volume at 1 second, FVC, TLC) and lower diffusion capacity of the lungs for carbon monoxide when compared with patients with normal compliance. An FVC cutoff of 3470 cc was 100% sensitive and 51% specific for predicting low compliance. The proposed equation Vt = FVC/8 significantly reduced calculated tidal volume by a mean of 22.5% in patients with low pulmonary compliance without affecting the mean tidal volume in patients with normal compliance (mean difference 0.9%). CONCLUSIONS: FVC is more strongly correlated to TLC than PBW and a cutoff of about 3.5 L can be utilized to predict low pulmonary compliance. The equation Vt = FVC/8 reduced mean calculated tidal volume in patients with low pulmonary compliance and/or small lungs.
Hoftman et al. (Thu,) conducted a observational in Thoracic surgery (n=130). Forced vital capacity (FVC) for tidal volume selection vs. Predicted body weight (PBW) was evaluated on Correlation between FVC and TLC (TLCPFT) compared to PBW and TLC. Forced vital capacity correlated more strongly with total lung capacity than predicted body weight, and Vt = FVC/8 reduced calculated tidal volume by 22.5% in low compliance patients.