Extravascular lung water assessed by transpulmonary single thermodilution and postmortem gravimetry in sheep

Abstract

Introduction: Acute lung injury is associated with accumulation of extravascular lung water (EVLW). The aim of the present study was to compare two methods for quantification of EVLW: transpulmonary single thermodilution (EVLWST) and postmortem gravimetric (EVLWG).

Methods: Eighteen instrumented and awake sheep were randomly assigned to one of three groups. All groups received Ringer's lactate (5 ml/kg per hour intravenously). To induce lung injury of different severities, sheep received Escherichia coli lipopolysaccharide 15 ng/kg per min intravenously for 6 hours (n = 7) or oleic acid 0.06 ml/kg intravenously over 30 min (n = 7). A third group (n = 4) was subjected to sham operation. Haemodynamic variables, including EVLWST, were measured using a PiCCOplus monitor (Pulsion Medical Systems, Munich, Germany), and the last measurement of EVLWST was compared with EVLWG.

Results: At the end of experiment, values for EVLWST (mean +/- standard error) were 8.9 +/- 0.6, 11.8 +/- 1.0 and 18.2 +/- 0.9 ml/kg in the sham-operated, lipopolysaccharide and oleic acid groups, respectively (P < 0.05). The corresponding values for EVLWIG were 6.2 +/- 0.3, 7.1 +/- 0.6 and 11.8 +/- 0.7 ml/kg (P < 0.05). Ranges of EVLWIST and EVLWIG values were 7.5-21.0 and 4.9-14.5 ml/kg. Regression analysis between in vivo EVLWST and postmortem EVLWG yielded the following relation: EVLWST = 1.30 x EVLWG + 2.32 (n = 18, r = 0.85, P < 0.0001). The mean bias +/- 2 standard deviations between EVLWST and EVLWG was 4.9 +/- 5.1 ml/kg (P < 0.001).

Conclusion: In sheep, EVLW determined using transpulmonary single thermodilution correlates closely with gravimetric measurements over a wide range of changes. However, transpulmonary single thermodilution overestimates EVLW as compared with postmortem gravimetry.

Figure 1

Changes in pulmonary haemodynamics and extravascular lung water in sheep. Data are expressed as mean ± standard error of the mean. *P < 0.05, LPS versus sham-operated group; ^†P < 0.05, OA versus sham-operated group; ^‡P < 0.05, LPS versus OA group; ^§P < 0.05, versus t = 0 hours in LPS group; ^llP < 0.05 versus t = 0 hours in OA group. EVLWI_ST = extravascular lung water index measured by single thermodilution; LPS = lipopolysaccharide; OA = oleic acid; PAP = pulmonary arterial pressure; PVPI = pulmonary vascular permeability index; Sham = sham-operated group.

Figure 2

Changes in oxygenation variables in sheep. Data are expressed as mean ± standard error of the mean. *P < 0.05, LPS versus sham-operated group; ^†P < 0.05, OA versus sham-operated group; ^‡P < 0.05, LPS versus OA group; ^§P < 0.05, versus t = 0 hours in LPS group; ^llP < 0.05, versus t = 0 hours in OA group. DO₂I = oxygen delivery index; LPS = lipopolysaccharide; OA = oleic acid; Qs/Qt = venous admixture; SaO₂ = arterial oxygen saturation; Sham = sham-operated; SvO₂ = venous oxygen saturation.

Figure 3

Linear regression analysis between extravascular lung water index (EVLWI) as determined by transpulmonary single thermodilution (EVLWI_ST) and postmortem gravimetry (EVLWI_G) in sheep. EVLWI_ST = 1.30 × EVLW_G + 2.32 (n = 18, r = 0.85, P < 0.0001). Line of identity is dashed; 95% confidence intervals are indicated by solid lines. LPS, lipopolysaccharide; OA, oleic acid; Sham, sham-operated.

Figure 4

Bland-Altman plot for the extravascular lung water index (EVLWI) measured using transpulmonary single thermodilution (EVLWI_ST) and postmortem gravimetry (EVLWI_G) in sheep. The x-axis shows the mean of EVLWI measurements by single thermodilution and gravimetry. The y-axis shows the difference between the methods. The bold line indicates the value for the mean difference between EVLWI_ST and EVLWI_G (bias), and each dashed line indicates two standard deviations (SDs). Mean difference EVLWI_ST - EVLWI_G = 4.91 ml/kg (SD 2.54 ml/kg).

Figure 5

Gravimetric extravascular lung water index (EVLWI_G) in sheep. Data are expressed as mean ± standard error of the mean. ^†P < 0.05, OA versus sham-operated group; ^‡P < 0.05, LPS versus OA group. LPS = lipopolysaccharide; OA = oleic acid; Sham = sham-operated group.