List the determinants of cardiac output in the ventilated Intensive Care patient.

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College Answer

The determinants of cardiac output are many  and  varied. A good understanding  is required. The standard four factors usually considered to control cardiac output are: heart rate (and rhythm), myocardial contractility, preload  and afterload. Many  interactions  between  these factors  may be present at one time. (Also could think of in terms of Cardiac Output= Heart Rate • Stroke Volume). Heart rate: loss of atrial kick (AF, nodal rhythms etc.), tachycardias, bradycardias, ectopic beats all may significantly decrease stroke volume and cardiac output.

Myocardial contractility  :generally consider: 
neurally mediated (sympathetic activity [increases], parasympathetic activity [decreases]), hormonally  mediated (adrenal medulla [adrenaline, noradrenaline], adrenal cortex [corticosteroids increase],  thyroid   hormone   (increase),  insulin  increase],  other                                                 [growth   hormone,  glucagon, endothelins  increase;  circulation  myocardial  depressants  including  some  cytokines,  nitric  oxide etc.]) 
oxygen (hypoxia: moderate stimulates, severe depresses),  carbon dioxide (direct: lower increases, 
higher decreases) 
drugs (beta-agonistslblockers, calcium agonistslblockers, inodilators, etc.) 
electrolytes (especially calcium, magnesium, phosphate)

Preload:                                                               . 
decreased   preload   with   normal  left  ventricular   compliance:  absolute   hypovolaemia,   relative 
hypovolaemia  (venodilatation; increased  resistance  to  venous  return:  including  increased intrathoracic  pressure  with IPPV/PEEP; right  heart dysfunction  including  AMI, pulmonary hypertension eg. pulmonary emboli) 
decreased preload with decreased LV compliance: LV hypertrophy, ischaemia, beta-agonists

Afterload: 
changes in  intrathoracic  pressure, sympathetic  tone (inhibition/paralysis etc),  vasodilatation  ( . 
direct effects of drugs, anaphylaxis etc).

Discussion

This is not as complicated as the college answer would have you think.

Cardiac output = heart rate x 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

Jean-Louis Vincent has published a delightful commentary on this topic. It has a cute cartoon of a bicycle rider, in whom the gravelly pavement is the afterload, and the wind to his back the preload. One boggles how this representation might incorporate levosimendan.

References

References

Vincent, Jean-Louis. "Understanding cardiac output." Crit Care 12.4 (2008): 174.