List the determinants of cardiac output in the ventilated Intensive Care patient.
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,
drugs (beta-agonistslblockers, calcium agonistslblockers, inodilators, etc.)
electrolytes (especially calcium, magnesium, phosphate)
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
changes in intrathoracic pressure, sympathetic tone (inhibition/paralysis etc), vasodilatation ( .
direct effects of drugs, anaphylaxis etc).
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)
- Determinants of stroke volume:
- 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
- Outflow tract resistance
- Aortic/pulmonic valve resistance
- peripheral vascular resistance
- Haematocrit (as component of cardiac wotrkload)
- 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.
Vincent, Jean-Louis. "Understanding cardiac output." Crit Care 12.4 (2008): 174.