Manipulation of Haemodynamic Variables

Created on Sat, 07/11/2015 - 19:17
Last updated on Wed, 11/18/2015 - 04:46

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.