1. Transpulmonary indicator dilution method
SSAI Gothenburg November 2019
Transpulmonary indicator dilution method
Per Werner Möller – M.D.
Head of section Operating theatres and Anaesthesia, Department of Anaesthesiology and Intensive Care Medicine, Östra sjukhuset, Sahlgrenska University Hospital, Gothenburg
Department of Anaesthesiology and Intensive Care Medicine, Institute of Clinical Sciences at the Sahlgrenska Academy, University of Gothenburg, Sahlgrenska University Hospital, Gothenburg, Sweden
Visiting Investigator, Department of Intensive Care Medicine, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland

3. Continous pulse contour cardiac output monitoring
AUC of systolic part of pulse curve is proportional to SV
Thermodilution gives instantaneous CO and actual SV is calculated from HR
Arterial impedance (a combination of vessel tree resistance and compliance) is calculated
Under the assumption of unchanged impedance, the system provides beat-to-beat SV from continous pulse contour analysis
Volumetric measures derived from transpulmonary thermodilution
Global End-Diastolic Volym; GEDV
Extravascular Lung Water; EVLW

4. The software takes into account the individual aortic compliance and systemic vascular resistance based on the following considerations.
During systole, more blood is ejected from the left ventricle into the aorta than actually leaves the aorta. During the subsequent diastole, the volume remaining in the aorta flows into the arterial network at a rate
determined by the aortic compliance (C), systemic vascular resistance (R), and the blood pressure (Windkessel effect)
The shape of the arterial pressure curve after the dicrotic notch is representative for this passive emptying of the aorta (exponential decay time = R × C). The systemic vascular resistance, R, is determined by the
quotient of mean arterial pressure (MAP) and cardiac output measured by the reference method (R = MAP ⁄ CO). As the decay time and R are known, the compliance, C, can be computed.

5. Thermodilution curves

7. Higher flow
Reference curve dashed

8. Lower flow
Reference curve dashed

9. Reference curve dashed
Larger amount of indicator…
… or smaller distribution volume for indicator
Reference curve dashed

10. m = amount of indicator
AUC = area under the curve

11. Example:
10 mg of indicator is injected and concentration over time is determined down stream

12. Modified for temperature as indicator:
Factor K:
corrects for differences in heat conductance and heat capacitance between injectate and blood

13. So – if you know the transit time and the flow, you can calculate the distribution volume.

14. Transit time for particle 1, travelling with flow Q1 equal to:

15. …distribution frequency histogram of transit times
MTt = mean transit time

16. Distribution volume for indicator
For intravascular indicator:
For thermal indicator:
The distribution volume for a thermal indicator extends beyond the vessel bed

17. Dilution curve with exponential decay
Time, linear
Time, linear
Lin-log transformation allows circumvention of the problem with recirculation

18. Small distribution volume
Large distribution volume
Time, linear

19. Slope is proportional to distribution volume and flow
Small
Large

20. The flow thru the system and the size of the largest distribution volume will decide the properties of the dilution curve…
…it is irrelevant where in the series the ”bathtub” is located

21. In transpulmonary thermodilution, the single largest distribution volume is Pulmonary Thermal Volume (PTV)
PTV is pulmonary blood volume and surrounding structures involved in thermal equilibrium
Therefore, it is PTV and CO that determines the slope of the indicator dilution curve

22. Circulation 1951;4;

27. Down Slope Time, DSt = the time that multiplied with CO gives PTV
AUC and known amount of indicator gives CO
MTt ×CO = ITTV = total distribution volume
DSt ×CO = PTV = largest included volume

28. ITTV = CO ×MTt
PTV = CO×DSt
ITBV = CO ×MTt ICG

29. Intrathoracic blood volume
ITBV = GEDV+PBV+part of caval vein+part of aorta to tip of catheter
A volumetric measure of central blood volume
Method dependent rather true anatomical definition
Determined by use of intravascular indicator; ITBV = CO×MTt ICG

30. intrathoracic blood volume
RA RC PBV LA LC
Several studies demonstrate how ITBV correlates to SV or CO
Whereas CVP and PAOP does not;
In septic shock
In hemorrhage
Intraoperatively for different types of surgery

31. Comparison Between Intrathoracic Blood Volume and Cardiac Filling Pressures in the Early Phase of Hemodynamic Instability of Patients With Sepsis or Septic Shock
Sakka et al. Journal of Critical Care, Vol 14, No 2 (June), 1999, pp 78-83

33. Can this really be true?

37. There is a physiological relation between GEDV and PBV
…in porcine models of extreme hemorrhage
…for different rates of cathecholamine infusion
…not affected by infusion of dobutamine
…not affected by concomitant pulmonary hypertension

38. After pulmonary resection, the ratio of PBV/GEDV is changed;
Pulmonary blood volume decreases in relation to GEDV
ITBV is overestimated by approx. 10% after pulmectomy
Extravascular lung water (EVLW) is underestimated

39. Extravascular lung water

40. A measure of the amount of water in the lung...
…alternatively PTV-PBV=EVLW
A measure of the amount of water in the lung...
…or the amount of tissue in the thorax, other than ITBV, involved in thermal equilibrium
EVLW normally <7mL/kg (Predicted Body Weight)
EVLW >7mL/kg indicates hydrostatic or inflammatory edema
EVLW increases: in pneumonia
in pulmonary edema
in alveolar fluid
in ARDS

41. Prognostic value?
In an ICU population the mortality was 65% for EVLW>15mL/kg
and % for EVLW<10mL/kg
The prognostic value of EVLW at ICU admission was highar than APACHE II score
Diagnostic value?
To better characterize patients with ARDS
The ratio of EVLW/ITBV (or EVLW/GEDV) is significantly higher in ”permeability edema” than in hydrostatic edema
Therapeutic value?
Hemodynamic management based on fluid restriction guided by ITBV (or GEDV) and EVLW vs. therapy guided by PAOP gave fewer days on ventilator and fewer days in the ICU

42. Potential sources of error
Experimentally - EVLW is underestimated by obstruction of pulmonary vessels with diameter >0.5 mm
thermal indicator does not get access to the ”true” distribution volume
However – temperature is excellently conducted in water and can reach equilibrium in spite of vascular obstruction
Clinically, obstruction of smaller pulmonary vessels is more problematic (compare ARDS or PEEP effect)
High levels of PEEP (in respect to central blood volume) can induce perfusion defects (West zone 1) leading to underestimation of EVLW
Recruitment maneuvers can open pulmonary perfusion and thereby give the thermal indicator access to to higher Vd reporting higher EVLW
But of course, PEEP also give CO or PAOP and decrease EVLW
…in summary PEEP can affect the measurement and the prevalence of EVLW

43. Potential sources of error
Focal pulmonary injury from ALI or acte cardiac edema gives a severely diminished HPV;
therefore pulmonary flow is not so much deviated away from the injured areas and this makes the method robust

44. ITTV=CO*MTt
PTV=CO*DSt