Session 11 Flashcards
How do you calculate mean arterial blood pressure?
CO x TPR
SV x HR x TPR
diastolic pressure + 1/3 pulse pressure
How do you calculate cardiac output?
HR x SV
What is haemodynamic shock?
An acute condition of inadequate blood flow throughout the body - fall in arterial blood pressure leading to circulatory shock
What can cause haemodynamic shock?
- Fall in cardiac output
- Fall in TPR beyond capacity of the heart to cope
What can cause shock due to fall in cardiac output?
- Cardiogenic shock
- Mechanical shock
- Hypovolaemic shock
What is cardiogenic shock?
Pump failure where the ventricle cannot empty properly
What is mechanical shock?
An obstruction where the ventricle cannot fill properly - cardiac output drops (eg. cardiac tamponade)
What is hypovolaemic shock?
Loss of blood volume leading to poor venous return
What can cause cardiogenic shock?
- Damage to LV during MI
- Serious arrhythmias (heart block) - REDUCED HR
- Profound tachycardia (not enough time to fill)
- Acute deterioration of the heart
- DECREASE IN STROKE VOLUME
What are the features of cardiogenic shock?
- Normal or raised central venous pressure
- Dramatic drop in arterial BP!!! (hallmark of haemodynamic shock)
- Poor tissue perfusion
- Poor kidney perfusion causing oliguria
What is the pulse pressure?
The difference between the diastolic and systolic pressure
How do you manage someone in cardiac arrest?
- Basic life support (ventilation/compressions)
- Advanced support
- Adrenaline
What does defibrillation do?
Puts all cells into refractory period and allows coordinated activity to restart
What does adrenaline do in cardiac arrest?
- Enhancing myocardial function
- Increasing peripheral resistance to manage cardiac output and therefore MABP
What is cardiac tamponade?
Blood or fluid buildup in pericardial space that will constrict the filling of the heart and limit EDV
What are the features of a cardiac tamponade?
- High central venous pressure
- Low arterial blood pressure
- HR will increase to try and bring up BP
How can a pulmonary embolism cause mechanical shock?
- Occludes a large pulmonary artery
- High pulmonary pressure
- Right ventricle unable to empty giving reduced blood return and filling to left heart (occlusion)
- Left atrial pressure is low (less blood returned to the left side of the heart)
- Arterial BP low
= Shock due to low CO and SV
How does an embolus reach the lungs?
- DVT (most common)
- Portion of thrombus breaks off and travels in venous system to the right side of the heart
- Embolus is pumped out via the pulmonary artery to the lungs
- Effect corresponds to the size of the embolus
What is hypovolaemic shock?
Shock caused by reduced blood volume (20-30% of blood loss will cause some shocks signs, 30-40% serious shock), most likely due to HAEMORRHAGE
How does haemorrhage cause hypovolaemic shock?*
- Venous pressure falls
- Cardiac output falls (Starling’s law)
- Arterial BP falls as CO falls
- Fall in arterial blood pressure detected by baroreceptors
What is the compensatory response to hypovolaemic shock?*
- Increased sympathetic stimulation to try and raise MABP
- Therefore tachycardia
- Increased force of contraction (steeper Starling’s curve)
- Peripheral vasoconstriction (BP = CO x TPR)
- Venoconstriction
What else can cause hypovolaemic shock?
- Severe diarrhoea/vomiting, loss of Na+
- Severe burns
What are the signs and symptoms of hypovolaemic shock?
- Tachycardia
- Weak pulse (low SV)
- Pale skin (peripheral vasoconstriction)
- Cold and clammy extremities (peripheral vasoconstriction + sympathetic system)
- Low central venous pressure (less blood in system)
What are the complications of shock?*
Multi-organ failure due to severely reduced perfusion
How do capillary pressures change in hypovolaemic shock?*
Normal: small net movement of fluid into tissues
- Re-enters via venous system by lymphatic drainage
REVERSED in SHOCK
- Very low hydrostatic pressure due to the hypovolaemia
- Arterioles constrict to maintain ABP = pressure in capillaries is therefore lower
= Net movement of fluid into capillary (reabsorption)
What reduces hydrostatic pressure?
- Hypovolaemia
- Arteriole constriction to maintain ABP causing a drop in capillaries as blood goes through the resistance
What is an issue with peripheral vasoconstriction in hypovolaemic shock?
Impaired tissue perfusion = danger of DECOMPENSATION
- Peripheral tissue damage due to hypoxia
- Release of vasodilator chemical mediators that will overcome sympathetic nervous system - TPR falls
- Blood pressure falls dramatically when TPR falls
- Vital organs not perfused and so multi-system failure
What are the longer-term responses to hypovolaemia?
- Renin-angiotensin-aldosterone system
- Anti-diuretic hormone that promotes water retention and rebuilding of the blood volume
- Salt and water retention by kidneys promoted
What is distributive shock?
Normovolaemic shock caused by low total peripheral resistance (blood volume stays constant but circulation volume has increased)
What are examples of distributive shock?
Toxic shock and anaphylactic shock
What is cardiac arrest?
Unresponsiveness associated with the lack of pulse due to the heart stopping or not pumping effectively
What are the features of a cardiac arrest?
- Asystole (no electrical and mechanical activity)
- Pulseless electrical activity - no pulse, so no cardiac output (dissociation between electrical and mechanical activity)
- Ventricular fibrillation (uncoordinated electrical activity, most common)
What is toxic shock?
Sepsis: a life threatening response to infection that can lead to toxic shock
How does toxic shock occur?
- Endotoxins get released by circulating bacteria
- Causes massive inflammatory response
- Profound vasodilation so massive fall in TPR
- Fall in arterial BP
- Impaired organ perfusion
- Leaky capillaries
- Increased coagulation
What is the definition of septic shock?
Persisting hypotension requiring treatment to maintain blood pressure despite fluid resuscitation
What are the features of septic shock?
- Decreased arterial blood pressure
- Detected by baroreceptors and causing increased sympathetic output
- Vasoconstriction effect > vasodilators
- Higher HR and SV to increase CO
What are the patient signs of septic shock?
- Tachycardia
- Warm, red extremities at first)
- Then vasoconstriction so localised hypo-perfusion
What is anaphylactic shock?
A severe allergic relation that causes the release of histamine from mast cells + other vasodilators
How does anaphylaxis cause shock?
- Inflammatory mediators are powerful vasodilators
- Vasodilation = dramatic drop in TPR
- Increased sympathetic response that cannot override vasodilators, so low arterial BP
- Impaired vital organ perfusion
Why do people in anaphylactic shock have difficulty breathing?
Local mediators also cause bronchoconstriction and laryngeal oedema
What are the signs of anaphylactic shock?
- Difficulty breathing
- Collapse and syncope
- Rapid heart rate (trying to get CO up)
- Red and warm extremities due to vasodilation
How is anaphylactic shock immediately treated?
Adrenaline (epipen)
- Vasoconstriction via action of alpha 1 adrenoceptors
- Builds the total peripheral resistance and therefore mean arterial blood pressure up
