CVS - Haemodynamic Shock Flashcards
Cardiac output (CO) x total peripheral resistance (TPR) = ?
Mean arterial blood pressure
How is cardiac output calculated?
Stroke volume x heart rate
How is mean arterial blood pressure calculated?
Diastolic pressure + 1/3 pulse pressure
What is haemodynamic shock?
An acute condition of inadequate blood flow throughout the body, where a catastrophic fall in arterial blood pressure leads to circulatory shock.
What are the four basic mechanisms of shock?
- fall in cardiac output due to mechanical issues with heart not filling (‘mechanical shock’)
- fall in cardiac output due to pump failure (‘cardiogenic shock’)
- fall in cardiac output due to loss of blood volume (‘hypovolaemic shock’)
- fall in peripheral resistance due to excessive vasodilation (‘distributive shock’)
What is cardiogenic shock?
Acute failure of the heart to maintain cardiac output - ‘pump failure’. The heart fills, but is unable to pump effectively
Give some causes of cardiogenic shock
- myocardial infarction causing damage to left ventricle
- serious arrhythmia means heart cannot fill
- acute worsening of heart failure
Give some characteristics of cardiogenic shock
- central venous pressure may be raised (or normal)
- dramatic drop in arterial blood pressure
- tissues (especially coronary arteries and kidneys) are poorly perfused
- reduced urine production due to kidney failure
Give some different types of cardiac arrest, eg. ventricular fibrillation
- unresponsiveness associated with lack of pulse
- heart stopped or ceased to pump effectively
- asystole
- pulseless electrical activity
- ventricular fibrillation (this is the most common form of cardiac arrest)
How is cardiac arrest treated?
- Basic life support (chest compression and external ventilation)
- advanced life support (defibrillation depolarises all heart cells, which allows coordinated electrical activity to start)
- adrenaline (enhances myocardial function, increases peripheral resistance)
What are the two types of mechanical shock?
Cardiac tamponade and pulmonary embolism
How does cardiac tamponade lead to shock?
Blood/fluid builds up in the pericardial space and restricts filling of the heart. This limits the end diastolic volume, affecting the left and right sides of the heart. There is a high central venous pressure and low arterial blood pressure, meaning the blood pressure is too low and the body goes into shock.
How does a pulmonary embolism lead to mechanical shock?
- An embolus occludes a large pulmonary artery, so pulmonary artery pressure is high.
- The right ventricle cannot empty and central venous pressure rises.
- reduced return of blood to left heart limits filling of left heart.
- left atrial pressure decreases, so arterial blood pressure decreases leading to shock
How does an embolus usually reach the lungs?
Portion of a thrombus breaks off a deep vein thrombosis and travels in the venous system to the right side of the heart. It is pumped out via the pulmonary artery to the lungs.
What is hypovolaemic shock?
Reduced blood volume, usually due to haemorrhage.
How much blood must be lost before someone goes into hypovolaemic shock?
- less than 20% of blood is unlikely to cause shock
- 20-30% some signs of shock response
- 30-40% substantial decrease in mean aBP and serious shock response
What is the compensatory response to hypovolaemic shock?
Baroreceptors detect fall in venous pressure and cause increased sympathetic stimulation leading to tachycardia, increased force of contraction and peripheral vasoconstriction
How does ‘internal transfusion’ work against hypovolaemic shock?
Increased peripheral resistance reduces the capillary hydrostatic pressure leading to net movement of fluid into the capillaries
What are the symptoms of hypovolaemic shock?
- tachycardia
- weak pulse
- pale skin
- cold, clammy extremities
Why is the compensatory effect sometimes dangerous in hypovolaemic shock?
‘Decompensation’ may occur, where peripheral vasoconstriction impairs tissue perfusion. Hypoxia causes tissue damage, and there is a release of chemical mediators (vasodilators). Total peripheral resistance falls, BP falls, vital organs not perfused, multi system failure
What is distributive shock?
Occurs when there is profound peripheral vasodilation, so the volume of the circulation is hugely increased.
What is septic shock?
Persisting hypotension requiring treatment to maintain blood pressure despite fluid resuscitation.
How does sepsis cause shock?
Endotoxins released by circulating bacteria cause excessive inflammatory response and profound vasodilation. TPR drops and arterial pressure drops. Impaired perfusion of vital organs and capillaries become leaky leading to a further reduced blood volume.
How does anaphylaxis lead to shock?
Severe allergic reaction leads to release of histamine from mast cells. This causes a fall in TPR and a dramatic drop in arterial pressure and impaired perfusion of vital organs
