Measurement of extravascular lung water using the single indicator method in patients: research and potential clinical value

Abstract

Extravascular lung water includes all of the fluid within the lung but outside of the vasculature. Lung water increases as a result of increased hydrostatic vascular pressure or from an increase in lung endothelial and epithelial permeability or both. Experimentally, extravascular lung water has been measured gravimetrically. Clinically, the chest radiograph is used to determine whether extravascular lung water is present but is an insensitive instrument for determining the quantity of lung water. Bedside measurement of extravascular lung water in patients is now possible using a single indicator thermodilution method. This review critically evaluates the experimental and clinical evidence supporting the potential value of measuring extravascular lung water in patients using the single indicator method.

Fig. 1.

Relationship between pulmonary hydrostatic pressure and pulmonary edema formation under normal conditions and in the presence of increased vascular permeability. Under normal conditions, increased hydrostatic pressure leads to pulmonary edema formation. However, in the presence of increased pulmonary vascular permeability, an increase in hydrostatic pressure amplifies the formation of pulmonary edema. Adapted from manuscript with permission from the American College of Chest Physicians (5).

Fig. 2.

Intrathoracic thermal volume (ITTV) = cardiac output (CO) × mean transit time (Mtt)-cold saline. RAEDV, right atrial end-diastolic volume; RVEDV, right ventricular end-diastolic volume; PTV, pulmonary thermal volume; LAEDV, left atrial end-diastolic volume; LVEDV, left ventricular end-diastolic volume.

Fig. 3.

Intrathoracic blood volume (ITBV) = CO × Mtt-indocyanine green (ICG). PBV, pulmonary blood volume; RAEDV, right atrial end-diastolic volume.

Fig. 4.

Extravascular lung water (EVLW) = ITTV − ITBV.

Fig. 5.

PTV = CO × downslope decay time (Dst)-cold saline.

Fig. 6.

Global end-diastolic volume (GEDV) = ITTV − PTV (ml).

Fig. 7.

ITBV = (1.25 × GEDV) − 28.4 (ml).

Fig. 8.

EVLW (ml/kg) = ITTV − ITBV (ml).

Fig. 9.

EVLW index (EVLWI) measured using the gravimetric method and the single indicator method (PiCCO) (means ± SD). Adapted from manuscript with permission from the Society of Critical Care Medicine (15).

Fig. 10.

Bland-Altman plot for the EVLWI measured using the single indicator method (EVLWI_ST) and the gravimetric method (EVLWI_G) in sheep. The x-axis shows the mean of EVLWI measurements by transpulmonary thermodilution and gravimetry. The y-axis shows the difference between the 2 methods. The bold line indicates the value for the mean difference between EVLWI_ST and EVLWI_G, and each dashed line indicates 2 SDs. Mean difference = EVLWI_ST − EVLWI_G = 4.91 ml/kg (SD 2.54 ml/kg). Adapted from manuscript with permission from Critical Care (17).

Fig. 11.

A study by Mitchell and coworkers (28) randomized 101 patients, 89 of whom had an initial EVLW > 7 ml/kg. This figure depicts the cumulative I−O (intake minus output) (y-axis) after each time interval (x-axis) for the 89 patients in both management groups [EVLW (○) and pulmonary capillary wedge pressure (●)] with an initial EVLW > 7 ml/kg. Numbers in parentheses indicate number of patients at the time interval still in the study. Adapted from manuscript with permission from the American Thoracic Society (28).

Fig. 12.

Mean EVLW (y-axis) after each time interval (x-axis) for the same 89 patients described in Fig. 11. *P < 0.05 compared with baseline measurement of EVLW. Open circles = EVLW management group; closed circles = pulmonary capillary wedge pressure management group. Adapted from manuscript with permission from the American Thoracic Society (28).

Fig. 13.

Time course of EVLWI and arterial Po₂ (Pa_O2)/fraction of inspired oxygen (Fi_O2). Measurements were made preoperatively (Preop) before (Pre) and 30 min after (Post) initiation of thoracic epidural anesthesia (TEA) and on postoperative day (POD) 0 before and 50 min after drug administration. In this crossover design, subjects underwent inhalation of salbutamol followed by ipratropium bromide (●) or ipratropium bromide followed by salbutamol (□). *P < 0.05 vs. preintervention value. Adapted from manuscript with permission from the American College of Chest Physicians (24).