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Impact of Sampling Frequency on the Accuracy and Diagnostic Efficacy of Flow Index in Monitoring Inspiratory Effort
Yi-Min Zhou1,2, Ming-Yue Miao1, Xu-Ying Luo2
1Drs. Zhou, Miao, and Prof. Zhou are affiliated with Department of Critical Care Medicine, Beijing Shijitan Hospital, Capital Medical University, Beijing, China.
Background:
The flow index (FI) is a noninvasive method for monitoring inspiratory effort during pressure support ventilation (PSV). However, the impact of different sampling frequencies on FI accuracy remains unclear. This study investigates how sampling frequency affects FI measurements and its diagnostic efficacy in identifying low or high inspiratory effort.
Methods:
A retrospective analysis of prospectively collected data from patients with acute brain injury (ABI) and acute respiratory failure (ARF) undergoing PSV with esophageal pressure monitoring was conducted. Flow, airway pressure, and esophageal pressure data were collected at 100 (ABI) and 200 Hz (ARF), then downsampled to 100, 50, 25, and 20 Hz. FI was calculated using a Python script, and respiratory mechanics were analyzed. Bland-Altman analysis compared FI at lower frequencies to the 100 Hz reference (both ABI and ARF cohorts). Receiver operating characteristic (ROC) curve analysis was conducted to evaluate the diagnostic accuracy of FI in identifying low or high inspiratory effort across sampling frequencies.
Results:
A total of 178 records from 76 subjects were analyzed. FI values were significantly lower at 50, 25, and 20 Hz compared with 100 Hz (P < .001). Bland-Altman analysis revealed biases of -0.08 (95% CI -0.17 to 0.00), -0.20 (95% CI -0.36 to -0.04), and -0.23 (95% CI -0.39 to -0.06) for 50, 25, and 20 Hz, respectively. The corresponding limits of agreement all exceeded the predefined clinically acceptable range. Despite these differences, the diagnostic efficacy of FI in identifying low or high inspiratory effort remained consistent across all frequencies, with area under the ROC ranging from 0.84 to 0.85 and 0.85 to 0.90, respectively.
Conclusions:
Although FI demonstrated robust diagnostic performance for low or high inspiratory effort across sampling frequencies, its reduced accuracy at lower frequencies calls for cautious use and frequency-specific cutoffs for effective FI monitoring.
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