Flashcards in 5. Haemodynamic Shock Deck (21)
Give 3 ways to calculate mean arterial BP.
1- maBP = CO x TPR
(CO = SV x HR)
2- maBP = DV + 1/3 PP
(PP = SV - DV)
3- maBP = 2/3 DV + 1/3 SV
What is haemodynamic shock? Which 2 mechanisms can cause this?
Acute condition of inadequate blood flow throughout the body due to a catastrophic fall in arterial BP.
- fall in CO
- fall in TPR (ie widespread vasodilation) beyond capacity of heart to cope
Name the 4 different types of haemodynamic shock and whether they affect CO or TPR.
Fall in CO:
- mechanical shock (obstructive, ventricle cannot fill properly)
- cardiogenic shock (pump failure, ventricle cannot empty properly)
- hypovolaemic shock (reduced blood volume leads to poor venous return)
Fall in TPR:
- distributive shock (toxic or anaphylactic)
What is cardiogenic shock? How is it different to heart failure?
- Cardiogenic shock = acute failure of the heart to maintain CO (and consequently BP) - pump failure.
- Heart failure is a chronic condition with adequate BP.
What are the causes of cardiogenic shock?
1- following myocardial infarction - damage/myocyte death in left ventricle
2- serious arrythmias - profound tachycardia (beats so quickly it doesn't have time to pump) or bradychardia
3- acute worsening of heart failure (where BP isn't maintained)
What are the consequences of cardiogenic shock?
Heart fills but fails to pump effectively:
- central venous pressure may be normal or raised (accumuation of blood/pressure in heart which can back up into venous system)
- dramatic drop in arterial BP... tissues poorly perfused, eg. coronary arteries (exacerbates problem) and kidneys (oliguria - reduced urine production)
What are the 2 main causes of mechanical shock?
Ventricle cannot fill properly:
1. cardiac tamponade
2. massive pulmonary embolism (PE)
What are the consequences of mechanical shock - cardiac tamponade?
Blood or fluid builds up in pericardial space (eg. from stab wound, MI)... restricts filling of both sides of the heart - limits end diastolic volume:
- high central venous pressure
- low arterial BP
- heart attempts to beat (continued electrical activity) but SV is too low
What are the consequences of mechanical shock - massive pulmonary embolism (PE)?
Embolus occludes a large pulmonary artery...
- pulmonary artery pressure is high and right ventricle cannot empty... high central venous pressure
- reduced return of blood to left ventricle... reduced filling of left heart... low left atrial pressure and low arterial BP (as low SV).
Causes chest pain and dyspnoea.
Why might an embolus reach the lung?
Deep vein thrombosis - portion of thrombus breaks off and travels in venous system to right side of heart. Pumped out via pulmonary artery to lungs.
Effects of this depends on size of embolus.
What is hypovolaemic shock and what does the severity of this depend on?
- Reduced blood volume, most commonly due to haemorrhage (but also severe burns or severe diarrhoea/vomiting and loss of Na+).
- Severity of shock related to amount and speed of blood loss:
<20% blood loss unlikely to cause shock
20-30% some signs of shock response
30-40% substantial decrease in maBP and serious shock response
What are the consequences of hypovolaemic shock?
1) Haemorrhage: venous pressure falls (so decreased SV)... CO falls (Starling's Law)... maBP falls.
2) Detected by baroreceptors causing compensatory response: increased sympathetic stimulation...
- increased contraction force
- peripheral vasoconstriction
Also get some 'internal transfusion': increased peripheral resistance reduces the capillary hydrostatic pressure... net mov of fluid into capillaries.
3) Compensatory response allows body to deal well <20% volume loss but then get dangerous Decompensation: peripheral vasoconstriction impairs tissue perfusion... tissue damage due to hypoxia... release of vasodilatory chemical mediators... TPR decreases... BP falls dramatically... vital organs can no longer be perfused... multi-system failure.
What are the symptoms of hypovolaemic shock?
2. weak pulse
3. pale skin (peripheral vasoconstriction)
4. cold, clammy extremities (sweat from sympathetic response)
What is hypovolaemia and how does the body deal with this?
Decreased blood volume but <20%.
Would involve longer term responses to restore blood volume - about 3 days if salt and water intake are adequate, via:
- anti-diuretic hormone
What is distributive shock?
Low resistance shock (normovolaemic) due to profound peripheral vasodilation decreasing TPR (ie blood volume is constant but area of circulation has increased).
1. toxic shock
2. anaphylactic shock
What are the consequences of toxic/septic shock?
Sepsis (serious life-threatening response to infection) can lead to septic shock - exotoxins released by circulating bacteria cause profound/excessive inflammatory response:
i) profound vasodilation... dramatic decrease in TPR... decrease in maBP... impaired perfusion of vital organs.
ii) capillaries become leaky... reduced blood volume.
iii) increased coagulation and localised hypo-perfusion (distal to coagulation).
Why does a septic shock patient become tachycardic?
Decreased maBP due to vasodilation (causes warm red extremities initially) in inflammatory response is detected by baroreceptors... increased sympathetic input... vasoconstriction and increase in HR and SV to try to compensate.
What are the consequences of anaphylactic shock?
1. Severe allergic reaction (anaphylaxis)... release of histamine from mast cells - stimulates release of vasodilatory mediators from endothelial cells... powerful vasodilation and decreased TPR... dramatic drop in arterial pressure.
2. Inreased sympathetic response... increased CO but can't overcome vasodilation.
3. Impaired perfusion of vital organs (confused, loss of consciousness). Mediators also cause bronchoconstriction and laryngeal oedema - difficulty breathing.
What are the symptoms of anaphylactic shock and how is this treated?
Acutely life threatening
3. rapid heart rate
4. red, warm extremities
Adrenaline (EpiPen) - causes vasoconstriction at alpha1 adrenoRs
What is cardiac arrest and how is it caused?
- Unresponsiveness associated with lack of pulse - heart has stopped or has ceased to pump effectively - no CO.
1. asystole - loss of electrical and mechanical activity
2. pulseless electrical activity (PEA) - electrical activity with no mechanical activity (contractile machinery not working, eg due to hypoxia)
3. ventricular fibrillation (most common form) - uncoordinated electrical activity, often following MI, electrolyte imbalance or some arrhythmias (eg long QT and Torsades de Pointes). Can cause asystole.