Assessment of sedation in patients with pressure-controlled ventilation: tidal-volume variance, sedation scores or bispectral index?

Background

Mechanical ventilation is nonphysiological and usually requires sedation. The assessment of sedation is not standardised. Until now, clinical criteria and several scores are used to assess sedation (Ramsay Score [RS], Cook and Palma Score [CPS], Cohen and Kelly Score [CKS], Chamorro Score [CS], linear Sedation Score [LSS] and RASS). Time-consuming apparative methods such as EEG are not routinely used. The bispectral index (BIS) is promising, but not routinely available. Biologically, variability of ventilation parameters is a well-known phenomenon; however, it is not used to assess sedation. It was therefore the aim of our study to investigate the correlation of tidal-volume variance (TVV) and the aforementioned sedation scores.

Materials and methods

In 80 patients of an internal ICU with MOF, pressure-controlled ventilation and continuous sedation, RS, CPS, CKS, CS and LSS were independently documented by physician (P), nurse (N) and investigator (I; not working in the ICU). Subsequently 200 consecutive tidal volumes were documented. TVV was calculated and compared with the aforementioned sedation scores. Additionally TVV was compared with the BIS. Ventilation was according to ARDSnet recommendations, Siemens Servo 900C or Servo300, Trigger -2 cmH₂O. Statistics: SPSS software, Spearman correlation.

Results

Patient characteristics (mean ± SD): age 57.4 ± 15.4 years, 28 female, 52 male, APACHE II score 28.2 ± 6.6; number of organ failures 4.0 ± 1.12; preceding ICU period 8.5 ± 9.3 days; continuous sedation with midazolam 31.2 ± 34.2 mg/hour, fentanyl 0.12 ± 0.08 mg/hour, propofol 45.6 ± 105.2 mg/hour; sedation assessment according to RS 5.65 ± 0.63, CPS 5.15 ± 1.67, CKS 0.65 ± 0.69, CS 9.34 ± 2.13 and LSS 1.78 ± 1.69, RASS -4.50 ± 1.27, FiO₂ 0.52 ± 0.17, PEEP 8.2 ± 2.4 cmH₂O, ventilatory frequency 20.5 ± 4.8/min, pressure control 16.8 ± 4.4 cmH₂O, tidal volume 540 ± 115 ml, TVV 2525.6 ± 11,366 ml (minimum 1.52; maximum 91,586).

Despite the limited number of patients there was a significant correlation of TVV and the sedation scores (coefficient of correlation of TVV compared with the sedation scores of P, N and I, respectively: *P < 0.05; **P ≤ 0.01, NS = not significant): CKS: 0.32**; 0.408**; 0.396**; CS: 0.212^NS; 0.371**; 0.394**; LSS: 0.345**; 0.407**; 0.423**; CPS: 0.413**; 0.363**; 0.388**; RS: -0.2^NS; -0.20^NS; -0.261*; RASS: 0.23*; 0.188; 0.289**. Additionally, TVV significantly correlated with BIS values (41.3 ± 18.8; Spearman coefficient 0.461**).

Conclusion

Calculating TVV is a simple and cheap tool for sedation assessment in patients with pressure-controlled ventilation.