Assessment of cardiac output

Determinants of cardiac output and methods of its measurement were asked about in Question 2a, Question 2b and Question 2c from the second paper of 2000. Subsequent years have not brought any further questions of this sort. A brief survey of primary exam candidates has revealed that questions like this have submerged into the CICM Part I repertoir of vivas.

Determinants of cardiac output

Cardiac output = heart rate × stroke volume.

  • Determinants of heart rate:
    • Sympathetic tone
    • Exogenous sympathomimetics
    • Exogenous rhythm device (pacemakers)
    • Arrhytmias
  • Determinants of stroke volume:
    • Preload
      • Volume status
      • Intrathoracic pressure eg PEEP
      • Pulmonary arterial flow (eg. obstructed by PE)
      • Atrial arrhythmias (loss of atrial kick)
      • Ventricular rate (allowing for diastolic filling)
      • Ventricular compliance
    • Afterload
      • Outflow tract resistance
      • Aortic/pulmonic valve resistance
      • peripheral vascular resistance
      • Haematocrit (as component of cardiac wotrkload)
    • Contractility
      • exogenous sympathomimetics
      • electrolyte balance
      • cardiac conduction system, presence of resynchronisation therapy eg. biventricular pacemaker

Indications for cardiac output monitoring

Indications for cardiac output monitoring

Influence on management

Deranged cardiac function in the context of shock

Titration of fluids, inotropes and vasopressors

Goal-directed resuscitation of complex multifactorial shock states

Continuous  monitoring following cardiac and non-cardiac surgery for high risk patients

Early intervention for depressed cardiac function

Protocol-driven  management of  hemodynamically unstable patients

Standardised management algorithms driven by cardiac output monitoring can support management decisions

Techniques of measuring cardiac output

Technique of cardiac output monitoring



PA catheter

“Gold standard” of CO monitoring
Easy to insert

Risk of vascular access
Unreliable with septal defects or tricuspid regurgitation
Thrombotic complications
Potential for valve damage


Easy to insert

Risk of vascular access
Unreliable with septal defects , tricuspid regurgitation or arrhytmia
Invalidated by rapid changes in vascular tone

Transthoracic Doppler


Poor reproducibility in serial assessments
Depends heavily on image quality

Oesophageal Doppler

Minimally invasive

Positional; risk of gastric or oesophageal perforation

SvO2 measurments

Easy to insert 

No information on  regional oxygen extraction
Assumptions regarding cardiac output

Pulse dye densitometry


Difficult to perform
Specialized equipment required
Exposure  to dye may be undesirable 
Studies of validity give conflicting results

Bioimpedance cardiography


Thus far not validated for clinical use


Mathews, Lailu, and Kalyan RK Singh. "Cardiac output monitoring." Annals of cardiac anaesthesia 11.1 (2008).

de Waal, Eric EC, Frank Wappler, and Wolfgang F. Buhre. "Cardiac output monitoring." Current Opinion in Anesthesiology 22.1 (2009): 71-77.

Pinsky, Michael R. "Hemodynamic evaluation and monitoring in the ICU."CHEST Journal 132.6 (2007): 2020-2029.