Bedside test for estimating stiff index in mechanically ventilated ARDS patient: a pilot study

Abstract

Stiff index is defined as the proportion of lung tissue elasticity in relation to chest wall elasticity to pertain to its cyclic tidal volume. It is meant to reflect the risk of overloading lung pressure. Introducing the Stiff index into bedside management determines the pressure needed to keep the lung open with no risk of barotrauma.Aim of the work The primary endpoint was to highlight the stiff index in ARDS patients as a potential surrogate for detecting the changes in lung mechanics and the assumptions underlying the estimation of relevant respiratory parameters. Secondly, we aimed to assess intrathoracic pressure ITP in view of chest wall elastance Ecw in relation to respiratory system elastance Ers within the same population.Subjects and methods Data were collected from forty ARDS patients, diagnosed according to Berlin definition, who were admitted to ICUS Ain Shams University hospitals during the period between December 2022 and April 2023. These populations were patients’ purposive samples who underwent volume‐controlled ventilation at least 2 days prior to study enrollment. Their mean age was 64.68 ± 15.01 years with males (N = 22) to females (N = 18) ratio 55%. Measurements of peak airway pressure, plateau pressure, and delta airway pressure change ∆Paw measured three times on end-expiratory (ee) and on end-inspiratory (ei) occlusion tests, and the higher of the three readings were reported.Results The calculated mean Stiff index was (0.66 ± 0.159) which was correlated with Peak pressure (r = − 0.377, P = 0.017) and with trans-pulmonary end inspiration TPPei (r = 0.312, P = 0.05). The stiff index was also found to be correlated with intra-thoracic pressure ITP (r = 0.769, P 0.001), but is not statistically significant with inspiratory tidal volume (r = 0.132, P 0.416).Conclusion Lung pressure loading in ARDS could be recognized by estimating an increase in stiff index. The cut point of stiff factor that limited the plateau at or below 30 cmH2o was 0.75 with 71% sensitivity and 0.63% specificity, respectively. The more the Stiff index the more was the ITP, and it had no relation to TV.