Shock And Hypotension Flashcards
What is shock
Shock (or more specifically circulatory shock) is a generalised loss of blood flow to organs, whereby organs are inadequately perfused, leading to hypoxia, necrosis and ultimately tissue death
What is hypovolaemic shock
Hypovolaemic shock arises from a loss of blood or blood fluid (such as plasma)
Hypovolaemic shock affects blood pressure by decreasing stroke volume
If we take a haemorrhage for example, the subsequent loss of blood volume would lead to a decrease in venous return (because there is less blood to return to the heart), therefore a decrease in stroke volume (because there is less blood to eject from the heart), a decrease in cardiac output and subsequently a decrease in blood pressure
Causes
Trauma
Burns
Vomiting
Diarrhoea
How does body respond to hypovolaemic shock
Increases heart rate initially to compensate for cardiac output by sympathetic nervous system which also increases vasoconstriction also by catecholamines (adrenaline)
In the long term, activation of the renin-angiotensin-aldosterone system will increase blood pressure through the direct actions of angiotensin II on vessel diameter, as well as its effects on blood volume through increased sodium and water retention from the kidneys and increased thirst
What is cardiogenic shock
Cardiogenic shock refers to a reduction in perfusion caused by a problem with the heart.
Any condition which affects the pumping ability of the heart can eventually lead to cardiogenic shock
For example congestive heart failure, which results in cardiac hypertrophy reduces both the pumping ability and the ventricular filling space of the heart, thereby reducing stroke volume
How does myocardial failure lead to cardiogenic shock
Myocardial failure and subsequent reduced ejection fraction leads to an increase in end systolic volume
This increases the preload experienced by the heart, increasing left ventricular diastolic filling pressure, resulting in elevated pressure in the left atrium
This elevated LA pressure leads to congestion (hence congestive heart failure) of blood in the pulmonary circulation, which in turn increases the hydrostatic pressure of blood in the lungs
This elevated hydrostatic pressure pushes fluid out of the pulmonary circulation into the lungs, leading to pulmonary oedema
This reduces the ability for the lungs to absorb oxygen and deliver it to the circulation, further exacerbating myocardial failure
The increased ESV leads to LV overfilling and may be heard as an additional S3 sound (see cardiac cycle lecture)
How is the stroke volumes calculated
subtracting end diastolic volume from end systolic volume (that is the blood in the heart at the end of diastolic filling and the volume of blood left in the heart at the end of systole)
What is septic shock
Septic shock occurs when bacteria or fungi enter the blood stream and release toxic chemicals. Gram negative bacteria release endotoxins (such as lipopolysaccharide), gram positive bacteria release exotoxins.
These toxins travel in the blood stream where they directly damage cells, as well as activating mast cells to release pro-inflammatory mediators such as histamine and prostaglandin
These pro-inflammatory chemicals travel through the blood stream where they cause widespread vasodilation (leading to a decrease in total peripheral resistance) and increase the leakiness of blood vessels, leading to fluid leak into tissues, leading to a decrease in cardiac output
What is anaphylactic shock
Anaphylactic shock occurs in response to an allergic reaction
When an allergen is ingested or inhaled, it activates basophils and mast cells to release predominately histamine, which as before travels throughout the body causing widespread vasodilation and an increase in the leakiness of capillaries
This ultimately leads to a decrease in total peripheral resistance and cardiac output
Both septic shock and anaphylactic shock initially appear with an increased temperature, both because of the significant increase in systemic vasodilation, with septic shock the bacterial toxins also induce a pyretic response (compare with cardiogenic and haemorrhagic shock)
What is neurogenic shock
Neurogenic shock occurs in some patients with spinal cord injuries, as well as patients who have undergone concussion and can also occur under deep anaesthesia
Neurogenic shock occurs in the absence of any loss of blood volume, but is the result of a massive increase in vascular capacity, caused by loss of sympathetic tone to the vascular beds
This loss of vascular resistance leads to a reduction in blood pressure and subsequent loss of venous return
What are the 3 main stages of circulatory shock
A non-progressive (or compensated stage), where the normal physiological mechanisms that reflexively mount are sufficient to overcome the loss of flow/pressure
A progressive stage, where without treatment, shock would become progressively worse
And an irreversible stage, whereby the amount of tissue and organ damage is so severe that no current medical treatment is sufficient to save the person and death is inevitable at this stage
Someone is truly in clinical circulatory shock when they reach the progressive stage. At this point, hypotension is a key clinical feature and does not respond to intravenous fluid administration
How do baroreceptors detect and signal to manage shock
In response, baroreceptors signal to vasomotor regions of the medulla, which through communication with other central regions results in peripheral vasoconstriction through activation of the sympathetic nervous system targeting the vasculature, increased renal sympathetic nerve activity, resulting in elevated RAAS activity and release of catecholamines from the adrenal medulla. All of which act to restore blood pressure and volume.
These compensatory mechanisms work better in certain cases of shock than in others. In cardiogenic shock, the increased blood volume leads to increased preload which may increase the burden on an already weakened heart
Where are the baroreceptors that detect blood pressure
baroreceptors in the aortic arch, carotid sinus and right atrium detect decreases in blood pressure and blood volume respectively. In all cases of shock, a decrease in venous return leads to a reduced cardiac output. This in turn results in decreased blood pressure, which is detected by these baroreceptors
