21. Haemodynamic Shock Flashcards
What is the equation for mean arterial BP?
CO x TPR
OR
Diastolic pressure + 1/3 pulse pressure
What is the equation for cardiac output?
SV x HR
What is haemodynamic shock?
Acute condition of inadequate blood flow throughout the body
A catastrophic fall in arterial blood pressure leads to circulatory shock
What can haemodynamic shock be due to?
Fall in CO
Fall in TPR beyond capacity of the heart to cope
Mean arterial BP = CO x TPR
How can a fall in cardiac output occur?
Mechanical - pump cannot fill
Pump failure
Loss of blood volume
How can a fall in peripheral resistance occur?
Excessive vasodilation
Leads to redistribution of blood and an increased volume of the circulatory system
What are the different shocks due to a fall in cardiac output?
Cardiogenic shock - ventricles cannot empty properly
Mechanical shock - ventricle cannot fill properly
Hypovolaemic shock - reduced blood volume leads to poor venous return
What is cardiogenic shock?
Acute failure of the heart to maintain cardiac output
What are the potential causes of cardiogenic shock?
Following myocardial infarction - damage to LV
Due to serious arrhythmias
Acute worsening of heart failure
Which arrhythmias can lead to cardiogenic shock?
Heart block as causes bradycardia
Tachycardia as not enough time for ventricle filling
Ventricle fibrillation
How can cardiogenic shock cause an increased central venous pressure?
Blood not being pumped out of the heart effectively, end systolic volume high, systolic pressure high, harder to fill the heart, therefore increased venous pressure
Which tissues may be poorly perfused in cardiogenic shock?
Coronary arteries - exacerbates problem
Kidneys - reduced urine production (oliguria)
When may a ventricular fibrillation cardiac arrest occur?
Often following MI or electrolyte imbalance or some arrhythmias (long QT and torsades de pointes)
What advanced life support is given in a cardiac arrest?
Defibrillation
- electric current delivered to the heart
- depolarised all the cells and puts them into refractory period
- allows coordinated electrical activity to restart
Why is adrenaline given in cardiac arrest?
Enhances myocardial function
Increases peripheral resistance
Give 2 examples of mechanical shock?
Cardiac tamponade
Massive pulmonary embolism
What is cardiac tamponade?
Blood or fluid build up in pericardial space
Restricts filling of the heart - limits end diastolic volume
Affects left and right sides of the heart
How does cardiac tamponade change blood pressure?
High central venous pressure as heart cannot fill properly so systolic pressure increases, harder to fill the heart so venous pressure increases
Low arterial pressure as stroke volume is decreased so cardiac output is deceased, so blood pressure is decreased
What effect does a massive pulmonary embolism have on the blood pressure?
Embolus occludes a large pulmonary artery So pulmonary artery pressure is high So right ventricle cannot empty Leads to high central venous pressure Leads to reduced return of blood to left heart from lungs Limits filling of left heart Left arterial pressure is low Arterial blood pressure is low Leads to mechanical shock
How might an embolus reach the lungs?
Typically due to deep vein thrombosis
Portion of thrombus breaks off
Travels in venous system to right side of heart
Pumped out via pulmonary artery to lungs
Effect of this will depend on the size of embolus
What is hypovalaemic shock?
Reduced blood volume
Most commonly due to haemorrhage
How much blood is lost to give hypovalaemic shock?
20-30% some signs of shock response
30-40% substantial decrease in mean aBP and senior shock response
Severity of shock is related to amount and speed of blood loss
What happens during haemorrhage?
Venous pressure falls therefore cardiac output falls (starlings law)
Arterial pressure falls
Drop in BP detected by baroreceptors
What is the compensatory response to haemorrhage and hypovalaemic shock?
Increased sympathetic stimulation Tachycardia Increased force of contraction Peripheral vasoconstriction Venoconstriction - increase TPR to increase BP
What happens in the capillaries in hypovalaemic shock?
Increased peripheral resistance reduces the capillary hydrostatic pressure
Net movement of fluid into capillaries to try to increase the blood pressure
What does a patient with hypovalaemic shock present with?
Tachycardia Weak pulse Pale skin Cold, clammy extremities Low central venous pressure
What else can cause hypovalaemic shock?
Severe burns
Severe diarrhoea or vomiting and loss of Na+
What is the danger in hypovalaemic shock and how does this happen?
Danger of decompensationn
Peripheral vasoconstriction impairs tissue perfusion
Tissue damage due to hypoxia
Release of chemical mediators - build up of vasodilators which overcome sympathetic NS ability for vasoconstriction
Leads to peripheral vasodilation, BP falls, vital organs no longer perfused, multi system failure
What are the long term responses to restore blood volume?
Renin-angiotensin-aldosterone system - promote water and Na+ reabsorption in kidneys
Anti-diuretic hormone - promote retention of fluid at kidney
Takes about 3 days if salt and water intake are adequate
What types of cardiac arrest can you get?
Asystole - loss of mechanical and electrical activity
Pulseless electrical activity
Ventricular fibrillation
What is distributive shock?
Low resistance shock (normovolaemic)
Profound peripheral vasodilation - decreased TPR
Give 2 examples of distributive shock
Toxic shock
Anaphylactic shock
What are the processes in toxic septic shock in relation to the circulatory system?
Endotoxins released by circulating bacteria
Profound inflammatory response
Causes profound vasodilation
Dramatic fall in TPR
Fall in arterial pressure
Impaired perfusion of vital organs
Capillaries become leaky, reduced blood volume
Increased coagulation and localised hypo-perfusion
What is the body’s response to decreased arterial pressure in septic shock?
Detected by baroreceptors - increased sympathetic output
Vasoconstrictor effect overridden by mediators of vasodilation
Heart rate and stroke volume increased
What can a patient present with if they have septic shock?
Tachycardia
Warm, red extremities initially but later takes of sepsis, vasoconstriction, localised hypo-perfusion
What happens in anaphylactic shock?
Severe allergic reaction
Release of histamine from mast cells
Powerful vasodilator effect - fall in TPR
Dramatic drop in arterial pressure - increased sympathetic response, increased cardiac output
Impaired perfusion of vital organs
Mediators also cause bronchoconstriction and laryngeal oedema
What signs will a patient with anayphylactic shock?
Difficulty breathing
Collapsed
Rapid heart rate
Red, warm extremities
Why is adrenaline given in anaphylactic shock?
Vasoconstriction via action at alpha 1 adrenoceptors
