Assessment of cardiac output

By Alex Yartsev - 09/08/2015

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	Titration of fluids, inotropes and vasopressors
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	Advantages	Disadvantages
PA catheter	“Gold standard” of CO monitoring Easy to insert Interpreter-independent	Risk of vascular access Unreliable with septal defects or tricuspid regurgitation Thrombotic complications Potential for valve damage
PiCCO	Easy to insert Interpreter-independent	Risk of vascular access Unreliable with septal defects , tricuspid regurgitation or arrhytmia Invalidated by rapid changes in vascular tone
Transthoracic Doppler	Non-invasive	Interpreter-dependent 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 Interpreter-independent	No information on regional oxygen extraction Assumptions regarding cardiac output
Pulse dye densitometry	Interpreter-independent	Difficult to perform Specialized equipment required Exposure to dye may be undesirable Studies of validity give conflicting results
Bioimpedance cardiography	Non-invasive	Thus far not validated for clinical use

References

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.

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