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

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

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.

Thermodilution curves

Higher flow

Reference curve dashed

Lower flow

Reference curve dashed

Larger amount of indicator…

… or smaller distribution volume for indicator

Reference curve dashed

m = amount of indicatorAUC = area under the curve

Example:10 mg of indicator is injectedand concentration over time is determined down stream

Modified for temperature as indicator:

Factor K:corrects for differences in heat conductance and heat capacitance between injectate and blood

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

…distribution frequency histogram of transit times

MTt = mean transit time

Distribution volume for indicator

For intravascular indicator:

For thermal indicator:

The distribution volume for a thermal indicator extends beyond the vessel bed

Dilution curve with exponential decay

Lin-log transformation allows circumvention of the problem with recirculation

Time, linear

Time, linear

Time, linear

Small distribution volume

Large distribution volume

Slope is proportional to distribution volume and flow

Small

Large

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

In transpulmonary thermodilution, the single largest distribution volume is Pulmonary Thermal Volume (PTV)

Therefore, it is PTV and CO that determines the slope of the indicator dilution curve

PTV is pulmonary blood volume and surrounding structures involved in thermal equilibrium

Circulation 1951;4;735-746

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

ITTV = CO ×MTt

PTV = CO×DSt

ITBV = CO ×MTt ICG

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

intrathoracic blood volume

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

RA RC PBV LA LC

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

Can this really be true?

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

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

Extravascular lung water

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 pneumoniain pulmonary edemain alveolar fluidin ARDS

…alternatively PTV-PBV=EVLW

Prognostic value?

In an ICU population the mortality was 65% for EVLW>15mL/kg

and 33% 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

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

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

ITTV=CO*MTt

PTV=CO*DSt

per.moller@vgregion.se