Understanding Ventricular Pressure-Volume Calibration and Experimental Design

Dr. Dimitrios "Jim" GeorgakopoulosChief Scientific Officer, Sunshine Heart, Inc.

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Today’s Lecture…

1. Introduction to PV

2. Starting with the end in mind

3. Stroke Volume Calibration

4. Parallel Volume Correction

5. What does it all mean?

Why Pressure Volume?

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ESPVR: Pes = 6.857 * Ves + 29.165, r² = 0.9909

EDPVR: Ped = 0.1509 * exp(0.1488 * Ved), r² = 0.9512

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• Intrinsic cardiovascular condition can be assessed during changing load conditions

• Load-Independent can be data important when making hemodynamic assessment of conditions that may effect preload or afterload.

• Multi-segments useful assessing dysynchrony

• The only method which provides the gold standard of diastolic function, EDPVR.

Catheter-based Pressure-Volume studies provide global assessment of the heart and vascular system.

Introduction to PV Theory

• Catheters consist of a single pressure sensor and 4 or more electrodes for volume measurement

• Outer electrodes provideexcitation to the ventricle, inner electrodes measure conductance between them.

• During diastole, blood volume increases and conductance increases. Both decrease during systole.

Introduction to PV Theory

Conductance to Volume…

A

LR r=

2

RL

RL

LLAV rr ===

L A

R = Resistance

V = Volume

r = Blood Resistivity

Ohm’s Law:

where…

I = current (constant)

V∞ R

Introduction to PV Theory

Why must we calibrate PV?

Location…Location…Location!

The necessity of calibration is derived from the variability inherent to the measurement, and variability between animals

• Catheter placement is the most important variable

• The composition of the myocardium can vary between every animal, especially in in disease conditions.

• Can directly effect the electrical properties of the myocardium, and by extension the measured conductance

• Calibration is important if done correctly, reduces variability of the data minimizing the number of animals required to show an effect, or lack of it.

Calibration of Volume Signal

Reference methods as MRI, Echo, Angio

Parallel conductance by hypertonic saline (provide EF)

SV calibration flow probe, thermodilution

Start With The End In Mind

Start With The End In Mind

• Care should be taken to understand which parameters are needed for your study.

– What physiological changes are taking place? And what PV parameters reflect these changes.

• Relative changes may be sufficient, depending on your experimental requirements (eg. Testing effects of a drug, acute changes in PV)

• All parameters, except EF, can be derived from SV calibration alone

– EDV, ESV, Max and Min Vol must be used as relative measures without parallel volume correction

?

How do we calibrate PV?

• First, we attempt to quantify how changes in the volume of the blood pool affect the measured conductance

– Converts the measured conductance changes to stroke volume

– Can be derived from blood resistivity only, or can be made more accurate with adjustment based on a known Stroke Volume reference.

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How do we calibrate PV?

Blood Resistivity

• The simplest method to calibrate volume is using a set of reference blood volumes.

– Conductance is measured in a known cylindrical volume, this relationship is applied to the data.

– This is the most common method for calibrating stroke volume.

• May not account for all of the variability inherent to Pressure Volume recording

Blood Resistivity

• Well volumes are recorded in your software and a calibration curve can be calculated

Blood Conductance/Resistivity

• Blood conductance will rarely change… However, it may not always be possible to do a blood conductance measurement on every animal

– Multiple doses of a compound known to alter blood resistivity

– Effective calibration grouping should be considered when appropriate

• You may consider an adjustment based on a second measure of known stroke volume

• Consider using a secondary measurement of SV or CO to provide further validation of your data accuracy

• These methods attempt to adjust the recorded SV, with or without a resistivity measurement, and match it with another known stroke volume

• Can be derived from Transit-Time Ultrasound measurement, thermodilution, or echocardiography

• Linear or non-linear approaches can be used based on your resources

Methods for SV Adjustment

SV Adjustment

• Assumes a linear relationship between conductance and blood volume

• Favored when an alternative SV measure is available only as a discrete or single-sample value (Echo, Thermodilution, Transit-Time Ultrasound)

KnownPV SVSV /=

Methods for SV Adjustment

Linear SV Adjustment

Linear SV Adjustment

Favored when an alternative method for continuous SV measurement is available (Transit-Time Ultrasound)

Non-Linear SV adjustment is foundational to admittance systems, and can be another approach to SV correction.

Linear SV Adjustment

Linear SV Adjustment

Favored when an alternative method for continuous SV measurement is available (Transit-Time Ultrasound)

Non-Linear SV adjustment is foundational to admittance systems, and can be another approach to SV correction.

What is a Known SV?

• Consider that a SV correction attempts to fit your PV loop data to a known SV

• Fitting all of your animals, or all animals in a group to a single SV value will invalidate this approach

• It is imperative to understand the variability in SV among groups

– Disease conditions often present a varying degree of change from control

– SV can only be considered an input that increases accuracy in Pressure-Volume data when the SV for the specific animal is known

“range of SV in control mouse strains is 14-26 μL”

Measurement of cardiac function using pressure–volume conductance catheter technique in mice and rats

Pál Pacher, Takahiro Nagayama, Partha Mukhopadhyay, Sándor Bátkai, and David A Kass

Nat Protoc. 2008; 3(9): 1422–1434.

Considerations for SV Calibration

• This is the most critical part of your calibration

– All outputs parameters of your PV Loops are affected

• If using only resistivity as the calibration, consider if that value may be transiently affected by your interventions

• If SV adjustment is to be used, ensure that the SV applied to the fit is based on reasonable assumptions

Considerations for SV Calibration

How do we calibrate PV?

• Next, we attempt to remove anyoffset in the measurement

– The largest offset in the measurement is typically due to the conductance of the myocardium contributing to the measured signal

– Termed parallel conductance or parallel volume

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How do we calibrate PV

Saline Bolus Injection

Increased Conductivity

x xx xxx

x x x x x

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Saline Bolus Injection

Saline Bolus Data Quality

PV Loop Demo.adicht

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7/28/2011 11:57:50.839 AM

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Vp throughout the cardiac cycle

• Change in constant current parallel conductance has been shown to be minimal throughout the cardiac cycle

– E.B. Lankford, et. al “Does volume catheter parallel conductance vary during the cardiac cycle,” Am. J. Physiol. Heart Circ. Physiol. 258: H1933-H1942, 1990.

• The time-varying method used can be applied to conductance data if desired

• Later work has shown that complex admittance (using AC excitation) does change throughout the cardiac cycle

– Cl Wei, et.al “Evidence of time-varying myocardial contribution by in vivo magnitude and phase measurement in mice,” IEEE Eng Med Biol Soc. 2004;5:3674-7.

Vp throughout the cardiac cycle

• Change in constant current parallel conductance has been shown to be minimal throughout the cardiac cycle

– E.B. Lankford, et. al “Does volume catheter parallel conductance vary during the cardiac cycle,” Am. J. Physiol. Heart Circ. Physiol. 258: H1933-H1942, 1990.

• The time-varying method used can be applied to conductance data if desired

• Later work has shown that complex admittance (using AC excitation) does change throughout the cardiac cycle

– Cl Wei, et.al “Evidence of time-varying myocardial contribution by in vivo magnitude and phase measurement in mice,” IEEE Eng Med Biol Soc. 2004;5:3674-7.

Considerations for Parallel Volume

Considerations for Parallel Volume

• Ejection Fraction (EF) is the most important value for which the parallel volume adjustment is critical

• Degree of remodeling in disease conditions, along with individual variation in heart morphology and muscle fitness makes introduce variability between animals.

• Repeated saline calibrations can be difficult in some disease models.

• Saline can effect the utilization of calcium in the myocardium

• Consider whether transient changes in Vp could be present in your study LV Volume (µL)

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SV

EDV

EF = SV/EDV

Other affected parameters: all absolute Volume Parameters (ESV, EDV, Max and Min Vol, etc. (relative changes still meaningful)

Other Accuracy Considerations

∙ ∙ ∙ ∙

1. Making Analysis and Calibration selections: - Linear Sections vs. Non Linear

2. Double Check Pressure Calibration at the study’s end

Other Accuracy Considerations

Summary

• Ultimately, this is a hugely powerful set of data

• Important to consider the ultimate goal of your study in making decisions about study design

• Understand how the intervention you are making might change the calibration values

• Consider the inputs of your calibration, where do they come from, and how can you minimize variability?

• Choose your calibration procedure accordingly…try to be consistent

• No matter the procedure used, proper calibration should be your default procedure and should be done for each experiment

Proven within the research community

Millar Mikro-tip catheters

and the conductance

method of calibration have

been tested and validated

as an accurate and

trustworthy combination

for research.

Click on publications

to view online…

Thank You!For additional information on ADInstruments solutions for pressure-volume loops, including Millar catheters, Pressure-Volume Hardware and associated data acquisition and analysis software please visit:

www.adinstruments.com/partners/millar