Cardiac Reflexes

Describe the cardiac reflexes

Cardiac reflexes are fast-acting reflex loops between the CVS and CNS which contribute to the maintenance of cardiovascular haemostasis.

They include:

• Baroreceptor reflex
  Aortic arch and carotid sinus reflexes.

• Bainbridge reflex
  Atrial stretch receptor reflexes.

• Chemoreceptor reflex
  Decreased PaO₂ < 50mmHg or decreased pH sensed by peripheral chemoreceptors causes subsequent tachycardia and hypertension.

• Cushing reflex
  Brainstem compression causes ischaemia of the vasomotor centre leading to Cushings' Triad:
  • Hypertension
    May have a wide pulse pressure.
  • Bradycardia
    Due to baroreceptor response from hypertension.
  • Irregular respirations

• Bezold-Jarisch reflex
  Stimulation of C fibres of the vagus nerve in the cardiopulmonary region.
  • This causes:
    • Significant bradycardia
    • Hypotension
    • Apnoea, followed by rapid shallow breathing. These fibres can be stimulated by a number of substances, including:
  • Capsaicin
  • Serotonin
  • Those produced in myocardial ischaemia

• Oculocardiac reflex
  Pressure on the globe or traction on ocular muscles causes a decrease in heart rate. This is mediated by the:
  • Trigeminal nerve (afferent limb)
  • Vagus nerve (efferent limb)
    Increased vagal tone reduces SA nodal activity.

References

1. CICM September/November 2013
2. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
3. Open Anaesthesia - Oculocardiac reflex: afferent path