Non-invasive measurement of pulse pressure variation using a finger-cuff method (CNAP system): a validation study in patients having neurosurgery

The finger-cuff system CNAP (CNSystems Medizintechnik, Graz, Austria) allows non-invasive automated measurement of pulse pressure variation (PPV(CNAP)). We sought to validate the PPV(CNAP)-algorithm and investigate the agreement between PPV(CNAP) and arterial catheter-derived manually calculated pulse pressure variation (PPV(INV)). This was a prospective method comparison study in patients having neurosurgery. PPV(INV) was the reference method. We applied the PPV(CNAP)-algorithm to arterial catheter-derived blood pressure waveforms (PPV(INV−CNAP)) and to CNAP finger-cuff-derived blood pressure waveforms (PPV(CNAP)). To validate the PPV(CNAP)-algorithm, we compared PPV(INV−CNAP) to PPV(INV). To investigate the clinical performance of PPV(CNAP), we compared PPV(CNAP) to PPV(INV). We used Bland–Altman analysis (absolute agreement), Deming regression, concordance, and Cohen's kappa (predictive agreement for three pulse pressure variation categories). We analyzed 360 measurements from 36 patients. The mean of the differences between PPV(INV−CNAP) and PPV(INV) was −0.1% (95% limits of agreement (95%-LoA) −2.5 to 2.3%). Deming regression showed a slope of 0.99 (95% confidence interval (95%-CI) 0.91 to 1.06) and intercept of −0.02 (95%-CI −0.52 to 0.47). The predictive agreement between PPV(INV−CNAP) and PPV(INV) was 92% and Cohen’s kappa was 0.79. The mean of the differences between PPV(CNAP) and PPV(INV) was −1.0% (95%-LoA−6.3 to 4.3%). Deming regression showed a slope of 0.85 (95%-CI 0.78 to 0.91) and intercept of 0.10 (95%-CI −0.34 to 0.55). The predictive agreement between PPV(CNAP) and PPV(INV) was 82% and Cohen’s kappa was 0.48. The PPV(CNAP)-algorithm reliably calculates pulse pressure variation compared to manual offline pulse pressure variation calculation when applied on the same arterial blood pressure waveform. The absolute and predictive agreement between PPV(CNAP) and PPV(INV) are moderate.