Cardiovascular Responses — Shock

Shock

Shock is defined as a state of cellular and tissue hypoxia due to reduced oxygen delivery and/or increased oxygen consumption or inadequate oxygen utilization. This most commonly occurs when there is circulatory failure manifested as hypotension (ie, reduced tissue perfusion).

Distributive:

Distributive shock is characterized by severe peripheral vasodilatation (vasodilatory shock)

               e.g: Sepsis with shock

Cardiogenic:

Cardiogenic shock is due to intracardiac causes of cardiac pump failure that result in reduced cardiac output

               e.g: Cardiomyopathy from MI

Hypovolemic:

Hypovolemic shock is due to reduced intravascular volume (ie, reduced preload), which, in turn, reduces CO.

               e.g: Haemorrhage

Obstructive:

Obstructive shock is mostly due to extracardiac causes of cardiac pump failure and often associated with poor right ventricle output.

               e.g: Pulmonary Embolism

Mixed / Unknown:

Unknown or more than one cause of shock contributing.

               e.g: Pancreatitis with Septic (Distributive) and Hypovolemic shock

Cardiovascular Responses

• Arterial Baroreceptor Reflex:
  • Short term regulation of arterial blood pressure
  • Sensor: Stretch sensitive baroreceptor nerve endings in walls of arteries, nonarterial baroceptor units
  • Senses: pressure (stretch)
  • Integrator: Medullary cardiovascular centres
  • Efferent pathways: Cardiovascular sympathetic and cardiac parasympathetic
  • Effector organs: Heart, peripheral blood vessels
  • Effect: Operates such that an increase in arterial pressure leads to an essentially immediate decrease in sympathetic nerve activity and a simultaneous increase in parasympathetic nerve activity (and vice versa).
• Reflexes from Receptors in Heart and Lungs:
  • Mechanoreceptors and chemo receptors in atria, ventricles, coronaries, lungs
  • Neurohumoral control of CVS
  • Regulate blood volume and body fluid balance
• Chemoreceptor reflexes:
  • Hypoxia (low Po2), hypercapnia (high Pco2), and/or acidosis (low pH) levels in the arterial blood cause reflex increases in respiratory rate and mean arterial pressure
  • Arterial Chemoreceptors in carotid bodies at bifurcation of CCAs, and aortic bodies in arch of aorta
  • Central Chemoreceptors in CNS respond to hypercapnia and acidification of CSF to regulate resp patterns and influence autonomic output
• Exercise responses:
  • Reflexes from Receptors in Exercising Skeletal Muscle
    • Chemoreceptors respond to muscle ischaemia
    • Increased blood pressure
  • Central command
    • Input from cerebral cortex to lower brain centres during voluntary exercise simultaneous to cortical drives initiating exercise
    • Cause reflex tachycardia and increased arterial pressure
• The Dive Reflex
  • Bradycardia produced by enhanced cardiac parasympathetic activity and peripheral vasoconstriction caused by sympathetic activity
• Cardiovascular Responses associated with Emotion and Stress
  • Originate in cerebral cortex – corticohypothalamic pathways – medullary cardiovascular centers
  • Response is complex and depends on multiple factors
  • E.g blushing, excitement, vasovagal syncope
• Reflex responses to Pain
  • Superficial pain: rise in BP
  • Deep pain: response similar to vasovagal syncope with dec sympathetic tone, inc parasympathetic tone, dec BP/shock
• Temperature Regulation Reflexes
  • Controlled by hypothalamus, operate through cardiovascular centers, discretely control sympathetic activity

JC 2019

Answer by JC