29 Oct

Cardiac function and monitoring

In surgery, we occasionally need to deal with patients who are very sick and in the intensive care unit. Although most centres rely on anaesthetist in the management of the cardiac and respiration, it is good to have a basic understanding.

Important points and concept

Cardiac output: volume of blood pumped by the ventricle per minute

CO = Heart rate (HR) X Stroke volume (SV)

SV = volume of blood ejected by the ventricle per heartbeat ~ 70ml of blood with each beat

Factors influence the SV to include preload, afterload and muscle contraction.

Let’s have a look at the factors more closely.

Heart rate (HR) and rhythm

Any changes in the rate and rhythm will cause a disturbance within the cardiac output. Bradycardia, for instance, will cause increase myocardial oxygen demand and may lead to ischaemia and infarction. In arrhythmias such as atrial fibrillation (AF), the loss of atrial filling will compromise the ventricular pumping.

Preload

Definition: end-diastolic volume of the left ventricle

The determinants include:

  • Circulating blood volume – more volume increases preload
  • Venous tone – venoconstriction increases preload
  • Afterload – any increment will cause reduction of ejection which leads a higher residual volume
  • Atrial contraction
  • Intrathoracic pressure – important especially in mechanical ventilation

Frank-Starling law: The force of ventricular contraction is related to myocardial fibre length. Hence higher preload increases the cardiac output. However, the heart may decompensate if excessive preload is overwhelming the ventricle’s pumping action.

Afterload

Definition: tension or stress the ventricle must develop for ejection

Law of Laplace: tension is proportional to the pressure and radius divided by wall thickness

Tension ~ (pressure x radius) / (wall thickness x2)

  • increases with larger pressure or becomes dilated
  • interdependent with preload
  •  does not affect the SV in normal heart but may worsen a failing ventricle

Compliance

The relationship between volume and pressure.

Compliance = volume/ pressure

This concept is applied to the heart and diastolic function.

  • less compliance ventricle has a higher ventricular diastolic pressure despite a same end-diastolic volume.

Contractility/ inotropic

Definition: The intrinsic ability of cardiac muscle cells to shorten to a stimulus.

This shortens muscle leads to ejection of blood and increase contractility leads to a higher SV. Any changes in inotropy reduce ejection fraction.

The autonomic nervous system that is responsible for the inotropic control.

  • cathecolamines – increases contractility and increase heart rate. Mediated by adrenergic beta receptor (contractility and rate) and alpha receptors (increase vascular resistance)
  • inotropic medications (dopamine, dobutamine, adrenaline)

More info here

 

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