Manipulation of haemodynamic variables

By Alex Yartsev - 11/07/2015

So, perhaps this exact table has never appeared in the past papers, but there have been numerous questions which have asked the candidate to manage a haemodynamically unstable cardiac surgical patient. And in these questions, inevitably the candidate must discuss the manner in which they would approach the contractility, preload, afterload, and so forth. It is therefore important to consider these variables. An analogous chapter exists in the cardiology section, and applies this framework to the management of heart failure in the ICU.

Haemodynamic variables, their determinants, and their manipulation
Variable	Determinant	Interventions
Preload	Venous return (MSFP -RAP) / SVR	Manipulation of blood volume (eg. fluid resuscitation) Manipulation of venous tone (eg. noradrenaline) Manipulation of muscular pump (abolished by paralysis) Manipulation of posture (eg. supine, or Trendelenburg position)
	Diastolic filling time	A sufficiently low heart rate (i.e. management of the tachycardia) Avoidance of chronotropic agents
	Atrial contribution to diastolic filling	Prevention of atrial arrhythmia If paced, use of atrial pacing and avoidance of ventricular pacing
	Ventricular compliance	Management of pericardial effusion Use of lucitropic drugs to improve diastolic relaxation (eg. levosimendan) Open chest (improves ventricular relaxation)
Afterload	Ventricular size	Optimisation of preload
	Ventricular volume
	Chamber radius
	Wall thickness	Not usually amenable to any sane intervention
	SVR	Manipulated by vasoactive agents Decreased by IABP
	Aortic valve resistance	Aortic valve surgery can make adjustments to this determinant
	Aortic compliance	Aortic aneurysm repair can make adjustments to this determinant
Contractility	Metabolic substrate	Adequate supply of fatty acids, glucose and phosphate Sepsis and SIRS may result in impaired cardiac mitochondrial function
	Coordination of depolarisation	Preservation of native pacemakers and conduction pathways where possible; and atrial pacing or biventricular pacing otherwise.
	Functional muscle mass	Prevention / reversal of muscle loss, by management of protein malnutrition and attention to coronary artery disease
	Autonomic tone	Artifical manipulation of this can be performed with the use of exogenous catecholamines and non-cathecholamine inotropes
	Hormonal contribution	Insulin, glucagon and thyroxine all play a positive inotropic role
	Electrolyte gradients	Digoxin can manipulate the Na+/K+ pump to modify cardiac contractility

References

Norton, James M. "Toward consistent definitions for preload and afterload."Advances in physiology education 25.1 (2001): 53-61.

ROTHE, CARL. "Toward consistent definitions for preload and afterload—revisited." Advances in physiology education 27.1 (2003): 44-45.

Noble, M. I. "Problems in the definition of contractility in terms of myocardial mechanics." European journal of cardiology 1.2 (1973): 209-216.

Solaro, R. John. "Regulation of cardiac contractility." Colloquium Series on Integrated Systems Physiology: From Molecule to Function. Vol. 3. No. 3. Morgan & Claypool Life Sciences, 2011.

Penefsky, Zia J. "The determinants of contractility in the heart." Comparative Biochemistry and Physiology Part A: Physiology 109.1 (1994): 1-22.

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