The most exciting thing about PiCCO for me is the ability to determine extravascular lung water and pulmonary vascular permeability. I don’t find GEDI terribly useful or even cardiac output as a number. I can already perform volume responsiveness estimation by manually measuring the sDown on an arterial waveform. Skip to the bottom to see extravascular lung water and pulmonary vascular permeability. But if you are interested in how PiCCO determines these magical numbers, read from the top for a detailed analysis.
Concept and Definitions
Extravascular Lung Water : EVLW
Pulmonary blood volume + EVLW : Pulmonary Thermal Volume (PTV)
RA + RV + LV + LA : Global End-Diastolic Volume (GEDV)
PTV + GEDV : Intra-thoracic Thermal Volume (ITTV)
Global End-Diastolic Volume
Unlike a Swan, the cold saline bolus passes through the lungs before detection. This allows us to determine the total amount of fluid in the thorax also known as intrathoracic thermal volume (ITTV). Using the thermodilution curve, we can estimate the ITTV.
ITTV = CO x MTt
ITTI = ITTV / BSA (normal is 850-1000)
More importantly, we can estimate the PTV because it is the largest mixing chamber and should produce the largest drop in the thermodilution curve (from 85% of max temperature response to 45% of max). The slope of this drop corresponds to the pulmonary thermal volume.
PTV = CO x DSt
ITTV is composed of the blood volumes in the heart (GEDV) as well as the total fluid volume in the lungs (PTV). This means that the GEDV can be calculated by calculated by simple arithmetic.
GEDV = ITTV – PTV
GEDV / BSA = GEDI (normal is 680-800)
Tb : Temperature of blood
Ti : Temperature of infusate
Vi : Volume of infusate
Denominator is AUC
In mechanically ventilated patients with machine triggered and delivered breaths (8mL/kg IBW), one can use stroke volume variation or pulse pressure variation. PiCCO analyzes AUC of pulse pressure wave form to determine SVV.
Extravascular Lung Water
EVLW is my new favorite number. It is the most sensitive and specific measurement of pulmonary edema. It can even detect volumes left in the lung from a BAL within 30mL (Dres, CCM 2014). It is not affected by pleural effusions (Saugel, JCC 2013). There is a fantastic agreement between EVLW and postmortem (within 48h) lung weight (Tagami, CC 2010). There is great sensitivity and specificity with EVLW > 10 and there is 99% specificity with EVLW > 14.
Measuring EVLW is based on the observation that the pulmonary vasculature hold 1.25x of the total blood volume in the heart (GEDV). Since EVLW + Pulmonary Blood Volume = PTV (see diagram) then simple substitution solves for EVLW:
EVLW = PTV – 1.25x GEDV
It’s also possible to use EVLW to calculate the pulmonary vascular permeability index (PVPI), which can help distinguish ARDS and hydrostatic edema.
PVPI = EVLW / (1.25 x GEDV)