Canonical Question

CO Measuremrent

V5 D6.iv Historical V4 G6.iii 3 appearances

Master answer

Cardiac Output

Cardiac output = Volume of blood ejected by the heart per unit time (minute)

HR determined by autonomic input

Resting HR approx 60
- Sympathetic blockade causes HR to slow to 50
  - Sympathetic output to heart governed by the medullary vasomotor area
- Parasympathetic blockade causes HR to hasten to 110
- Both sympathetic and parasympathetic blockade causes HR to hasten to 100
- Electrolytes
- Drugs

SV = Amount of blood ejected by the heart per contraction

Governed by:

Preload
- Tension applied to myocyte immediately prior to contraction
  - Starling relationship
    - Optimal sarcomere length ~2.2um
    - Over-stretch inhibited by fibrous pericardium
  - Preload governed by
    - Blood volume and MSFP (increases preload)
    - RAP (decreases preload)
    - Resistance to venous return (decreases preload)
    - Negative intrathoracic pressure (aids in venous return)
    - Musculovenous pumps (aids in venous return)
    - One-way venous valves (aids in venous return)
Afterload
- Load against which the ventricle must exert its contractile force
- According to Laplace law
  - Afterload ∝ (Aortic pressure x ventricular radius) / thickness of ventricular wall
- Other factors affecting afterload
  - Resistance
    - Viscosity inversely proportional to flow
      - As viscosity decreases (e.g. Hb decrease), output increases
    - Flow is proportional to vessel radius to the 4^th power
Contractility
- Intrinsic ability of myocardium to contract at given preload and afterload
- Governed
  - Sympathetic increases contractility
  - Parasympathetic decreases contractility
  - Other hormones
  - Electrolytes

Measurement

Cardiac output measurement can be performed:

Invasively
- Pulmonary Artery Catheter
  - Thermodilution
  - Fick Principle
- Indicator Dilution Technique
- TOE
- Arterial waveform analysis
  - PiCCO
  - FloTrak/Vigileo
  - Acumen/Hemosphere
Non-invasively
- TTE
- MRI
- Thoracic impedance

Thermodilution

Indicator Dilution (Stewart-Hamilton)

Introduction

Invasive

Thermodilution remains the gold standard of cardiac output measurement.

Invasive

Equipment

Pulmonary Artery Catheter

A proximal port at the RA/SVC
A temperature probe at the tip: Typically a silicon oxide thermistor.
A balloon at the tip: To float it into position.
A distal (PA) port is required for measuring PAP and the PCWP, but is not required for CO calculation

Can also be done using other devices like PiCCO

dyes: indocyanine green, isobestic or radioactive isotope.
Catheter for injection
Arterial Sampling catheter
Measurement device

Method / Theoretical Basis

Catheter passed into right atrium
- Known quantity of fluid of known temperature injected into right atrium

Second catheter in pulmonary artery
- Measured change in temperature
- Change in temperature over time graphed
- Return of temperature to baseline extrapolated to zero to account for
  recirculation
- Average change in temperature for duration of curve calculated
- CO = temperature of indicator / (average change in temp x duration of curve)

Method:

rapidly inject 10mL of dye into venous circullation in RA.
indicator mixes quickly with cardiac contents.
during next few beats, blood-indicator mixture is measured by continual sampling from proximal artery in arm for one complete circulation (30 seconds).
mean concentration of mixture for one circulation determined.

Q = amount of indicator injected / indicator concentration over time.

Assumptions:

retention of indicator
complete mixing
constant flow rate

Advantages / Disadvantages / Errors

Advantages:

Simple
No toxicity
Repeated measurements possible

Disadvantages / Errors:

Natural variability (reduced by using mean of 3-5 measurements)
Issues with incorrect volume and temperature of injectate
Improper positioning of PAC and complications associated with insertion and use
Tricuspid regurgitation
Results in retrograde ejection of injectate back past the valve.
Arrhythmia

Advantages:

rapid Q determination

Disadvantages:

need to continuously withdraw arterial blood to plot the concentration curve.
dye can build up.
intracardiac shunt will effect recirculation.

Sakurai / JC 2019

Answer by JC, Sakurai

Exam appearances

Exam	Exact wording	Relationship	Success
2011A Q12	Describe the principles, and limitations, of the measurement of cardiac output using an indicator dilution technique	historical_member	—
2017B Q10	Compare and contrast two methods of measuring cardiac output	historical_member	—
2014A Q19	Define cardiac output. (10% of marks) Outline the factors that affect cardiac output. (60% of marks) Briefly describe the thermo dilution method of measuring cardiac output. (30% of marks)	historical_member	—