Hypotension Flashcards
(37 cards)
Blood pressure formula
BP = CO x SVR
Cardiac output is the product of
heart rate and stroke volume (CO = HR x SV)
What is stroke volume?
The volume of blood ejected with each heart beat
List the factors that affects stroke volume
Preload, contractility and afterload
Vascular resistance refers to…
the effects of vascular tone and blood viscosity on flow through a blood vessel
At which point during the cardiac cycle does coronary perfusion occur?
Diastole - diastolic pressure is therefore important here whereas MAP is more important when considering general organ perfusion
Poiseuille’s equation:
Vessel resistance (R) is directly proportional to the length (L) of the vessel and the viscosity (n) of the blood, and inversely proportional to the radius to the fourth power (r4)
Describe the autonomic innervation of blood vessels and how it affects vascular resistance
Sympathetic fibres innervating smooth muscle of resistance vessels are noradrenergic (vasoconstricting). Sympathetic fibres innervating vessels of skeletal muscle are cholinergic (vasodilating)
How is tone of resistance vessels maintained and altered by innervating fibres?
Noradrenergic fibres coursing through adventitia tonically release norepinephrine. Vessels constrict when frequency of neurotransmitter release increases and vice versa
Describe the autonomic innervation affects heart rate and force of heart contraction
Sympathetic innervation increases (positive chronotropic / positive inotropic)
Cholinergic (parasympathetic) fibres carried by the vagus nerve oppose the sympathetic effects.
Both discharge tonically with PNS dominating during rest
What structure controls the sympathetic output to the cardiovascular system
The vasomotor centre; group of neurons in the medulla oblongata
Describe the pathway of sympathetic innervation from the brain to blood vessels
Vasomotor centre neurons (medulla oblongata) activate preganglionic sympathetic neurons on the intermediolateral grey matter of the spinal cord –> fibres exit spinal cord –> course to sympathetic ganglia –> post ganglionic nerves leave from ganglia –> course to and terminate in blood vessel adventitia
What are baroreceptors and where are they located?
Stretch receptors located in the left atrium, aortic arch and carotid sinus
Describe the consequences of increased stretch / pressure detected by a baroreceptor
Impulses from baroreceptors to the vasomotor centre increase –> inhibits SNS (tonic discharges from vasomotor centre decrease) –> PNS overrides (there is activation of PNS) –> results in vasodilation / bradycardia / decreased contractility –> MAP is reduced
Describe the consequences of decreased stretch / pressure detected by a baroreceptor
Impulses from baroreceptors to the vasomotor centre decrease –> reduced inhibition of SNS (tonic discharges from vasomotor centre increase) –> concurrent inhibition of PNS –> results in vasoconstriction / increased HR / increased contractility –> MAP is increased
The baroreceptor feedback mechanism controls arterial blood pressure between physiologic limits of …
Approximately 50mmHg and 150mmHg. Outside of these limits, baroreceptor activation/inhibition has been maximised; further changes cannot be made by baroreceptors
Describe the effects of hypoxia / hypercapnia on blood pressure
Stimulated chemoreceptors in the carotid sinus / aortic bodies –> afferent fibres to vasomotor centre –> vasomotor centre stimulation –> vasopressor response / tachycardic response
Is the chemoreceptor feedback system more or less important in maintaining blood pressure than the baroreceptor feedback system?
Less important
Stretch receptors of the venous system are located in the…
Left atrium and pulmonary artery
Describe the response pathway to stimulation of stretch receptors in the atria and pulmonary artery
Respond to distension of the venous system. Afferent fibres inhibit vasomotor centre –> reduced frequency of tonic discharge –> reduced SNS input –> vasodilation –> decreased BP
Describe the response pathway of stretch receptors in the atria and pulmonary artery to reduced venous distension (absence of venous distension and increased atrial filling pressure)
Decreased input of afferent fibres to vasomotor centre –> decreased inhibition of SNS –> increased frequency of tonic discharge of SNS –> vasoconstriction / increased HR / increased contractility –> increased BP
What is the most important initial physiologic response of the body to hypotension?
The baroreceptor-mediated increase in sympathetic output
Which is the more important response in the face of hypotension: increased concentration of circulating catecholamines from the adrenal medulla or local release of NE by post ganglionic nerve fibres at the level of the arterioles?
Local NE release is responsible for the response of generalised vasoconstriction; it is therefore more important
During hypotension, which other substances are produced to assist with improving blood pressure? What are their purpose?
Catecholamines, angiotensin II, aldosterone, vasopressin. They assist by causing vasoconstriction and /or by expanding intravascular volume
