Cardiac Output Measurement

DEFINITIONS

Q = SV x HR (L/min)

CI = SV x HR/BSA (L/min/m2)

Normal CI = 2.5-4.2

SV = end-diastolic volume – end systolic volume

EF = (SV/EDV) x 100%

Shock = failure of tissue perfusion -> end organ injury

THE FICK PRINCIPLE

  • Adolf Eugen Fick (1829-1901) in 1870, was the first to measure cardiac output
  • assumes oxygen consumption is a function of rate of blood flow and rate of oxygen pick pick up by RBC’s.
  • involves measurement of oxygen concentration of arterial and venous blood and subsequent calculation of O2 consumption.
  • Q can then be derived

Measurements

  • VO2 = oxygen consumption/min (from spirometer with subject rebreathing air through a CO2 absorber)
  • Cv = oxygen content of blood taken from pulmonary artery (deoxygenated)
  • Ca = oxygen content of blood taken from a peripheral artery (oxygenated)

Calculations

VO2 = (Q x Ca) – (Q x Cv)

Therefore,

Q = VO2/(Ca-Cv)

Issues:

  • impractical
  • assumes no shunt (pulmonary blood flow = systemic blood flow)
  • assumes arterial blood is equal to pulmonary venous blood

DILUTION TECHNIQUES

  • known quantity of tracer substance introduced into a space to be measured
  • concentration measured after complete mixing

C1 x V1 = C2 x (V1 + V2)

C1 = initial concentration of indicator
C2 = final concentration of indicator
V1 = volume of indicator
V2 = volume to be measured

  • marker injected proximally to right ventricle and concentration measure distally (pulmonary artery or a peripheral artery)
  • concentration vs time plotted -> integration allow calculation of area under curve (SV x HR = Q)
  • suitable substances: radioiosotope, dye, cold water, temperature of blood.

TECHNIQUES

Clinically Used

  • Non-invasive BP monitoring
  • Central venous monitoring
  • Arterial monitoring
  • Pulmonary arterial monitoring
  • ECHO: TOE and TTE
  • Pulse contour analysis (PiCCO)
  • Oesophageal Doppler
  • Cardiac catheterisation and angiography

Experimental

  • Aortovelography – dopper U/S probe in suprasternal notch to measure blood velocity and acceleration in ascending aorta.
  • Ballistocardiography – detection of body motion due to movement of blood within body with each heart beat.
  • Electromagnetic flow meters
  • Oxygen consumption estimation (Fick)
  • Impedance plethymography

TIPS WHEN USING CARDIAC OUTPUT MONITORS

  • there is no ‘normal’ CVP or wedge -> follow trend and look at the response to treatment
  • abnormal hearts (ischaemic, fibrotic, contused) are less compliant so require higher filling pressures to reach ‘normal’ SV.
  • use SV rather than Q as a response to treatment as Q is calculated from HR which may be fast and mask a poorly performing ventricle.
  • low SvO2 usually indicates under-resuscitation.
  • the first treatment for all shock (including cardiogenic) = volume, volume and more volume.
  • a little extravascular lung water is less harmful than vasoactive drugs.
  • there is no formula to calculate the effect of PEEP on PCWP and CVP -> if kept constant, trend should be consistent.
  • during resuscitation if becomes apparent what CVP the patient ‘likes’ -> aim for this.
  • be cautious of all derived variables, particularly SVR.

CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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