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.
References and Links
Introduction to ICU Series
Introduction to ICU Series Landing Page
DAY TO DAY ICU: FASTHUG, ICU Ward Round, Clinical Examination, Communication in a Crisis, Documenting the ward round in ICU, Human Factors
AIRWAY: Bag Valve Mask Ventilation, Oropharyngeal Airway, Nasopharyngeal Airway, Endotracheal Tube (ETT), Tracheostomy Tubes
BREATHING: Positive End Expiratory Pressure (PEEP), High Flow Nasal Prongs (HFNP), Intubation and Mechanical Ventilation, Mechanical Ventilation Overview, Non-invasive Ventilation (NIV)
CIRCULATION: Arrhythmias, Atrial Fibrillation, ICU after Cardiac Surgery, Pacing Modes, ECMO, Shock
CNS: Brain Death, Delirium in the ICU, Examination of the Unconscious Patient, External-ventricular Drain (EVD), Sedation in the ICU
GASTROINTESTINAL: Enteral Nutrition vs Parenteral Nutrition, Intolerance to EN, Prokinetics, Stress Ulcer Prophylaxis (SUP), Ileus
GENITOURINARY: Acute Kidney Injury (AKI), CRRT Indications
HAEMATOLOGICAL: Anaemia, Blood Products, Massive Transfusion Protocol (MTP)
INFECTIOUS DISEASE: Antimicrobial Stewardship, Antimicrobial Quick Reference, Central Line Associated Bacterial Infection (CLABSI), Handwashing in ICU, Neutropenic Sepsis, Nosocomial Infections, Sepsis Overview
SPECIAL GROUPS IN ICU: Early Management of the Critically Ill Child, Paediatric Formulas, Paediatric Vital Signs, Pregnancy and ICU, Obesity, Elderly
FLUIDS AND ELECTROLYTES: Albumin vs 0.9% Saline, Assessing Fluid Status, Electrolyte Abnormalities, Hypertonic Saline
PHARMACOLOGY: Drug Infusion Doses, Summary of Vasopressors, Prokinetics, Steroid Conversion, GI Drug Absorption in Critical Illness
PROCEDURES: Arterial line, CVC, Intercostal Catheter (ICC), Intraosseous Needle, Underwater seal drain, Naso- and Orogastric Tubes (NGT/OGT), Rapid Infusion Catheter (RIC)
INVESTIGATIONS: ABG Interpretation, Echo in ICU, CXR in ICU, Routine daily CXR, FBC, TEG/ROTEM, US in Critical Care
ICU MONITORING: NIBP vs Arterial line, Arterial Line Pressure Transduction, Cardiac Output, Central Venous Pressure (CVP), CO2 / Capnography, Pulmonary Artery Catheter (PAC / Swan-Ganz), Pulse Oximeter
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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