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Cardiovascular Medicine > Hypotension > Flashcards

Hypotension Flashcards

1
Q

Blood pressure formula

A

BP = CO x SVR

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2
Q

Cardiac output is the product of

A

heart rate and stroke volume (CO = HR x SV)

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3
Q

What is stroke volume?

A

The volume of blood ejected with each heart beat

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4
Q

List the factors that affects stroke volume

A

Preload, contractility and afterload

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5
Q

Vascular resistance refers to…

A

the effects of vascular tone and blood viscosity on flow through a blood vessel

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6
Q

At which point during the cardiac cycle does coronary perfusion occur?

A

Diastole - diastolic pressure is therefore important here whereas MAP is more important when considering general organ perfusion

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7
Q

Poiseuille’s equation:

A

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)

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8
Q

Describe the autonomic innervation of blood vessels and how it affects vascular resistance

A

Sympathetic fibres innervating smooth muscle of resistance vessels are noradrenergic (vasoconstricting). Sympathetic fibres innervating vessels of skeletal muscle are cholinergic (vasodilating)

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9
Q

How is tone of resistance vessels maintained and altered by innervating fibres?

A

Noradrenergic fibres coursing through adventitia tonically release norepinephrine. Vessels constrict when frequency of neurotransmitter release increases and vice versa

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10
Q

Describe the autonomic innervation affects heart rate and force of heart contraction

A

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

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11
Q

What structure controls the sympathetic output to the cardiovascular system

A

The vasomotor centre; group of neurons in the medulla oblongata

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12
Q

Describe the pathway of sympathetic innervation from the brain to blood vessels

A

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

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13
Q

What are baroreceptors and where are they located?

A

Stretch receptors located in the left atrium, aortic arch and carotid sinus

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14
Q

Describe the consequences of increased stretch / pressure detected by a baroreceptor

A

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

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15
Q

Describe the consequences of decreased stretch / pressure detected by a baroreceptor

A

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

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16
Q

The baroreceptor feedback mechanism controls arterial blood pressure between physiologic limits of …

A

Approximately 50mmHg and 150mmHg. Outside of these limits, baroreceptor activation/inhibition has been maximised; further changes cannot be made by baroreceptors

17
Q

Describe the effects of hypoxia / hypercapnia on blood pressure

A

Stimulated chemoreceptors in the carotid sinus / aortic bodies –> afferent fibres to vasomotor centre –> vasomotor centre stimulation –> vasopressor response / tachycardic response

18
Q

Is the chemoreceptor feedback system more or less important in maintaining blood pressure than the baroreceptor feedback system?

A

Less important

19
Q

Stretch receptors of the venous system are located in the…

A

Left atrium and pulmonary artery

20
Q

Describe the response pathway to stimulation of stretch receptors in the atria and pulmonary artery

A

Respond to distension of the venous system. Afferent fibres inhibit vasomotor centre –> reduced frequency of tonic discharge –> reduced SNS input –> vasodilation –> decreased BP

21
Q

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)

A

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

22
Q

What is the most important initial physiologic response of the body to hypotension?

A

The baroreceptor-mediated increase in sympathetic output

23
Q

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?

A

Local NE release is responsible for the response of generalised vasoconstriction; it is therefore more important

24
Q

During hypotension, which other substances are produced to assist with improving blood pressure? What are their purpose?

A

Catecholamines, angiotensin II, aldosterone, vasopressin. They assist by causing vasoconstriction and /or by expanding intravascular volume

25
Q

What are potential cardiovascular consequences of persistent hypoxia (during shock)?

A
  • Extensive vasodilation of precapillary arterioles
  • Dampened vasomotor function as a result of prolonged cerebral ischemia
  • Increased capillary permeability and fluid loss
  • Myocardial dysfunction
26
Q

Define: Vasoplegia

A

Lack of response of blood vessels to physiologic regulatory mechanisms

27
Q

What are the suspected factors that contribute to vasoplegia and refractory shock?

A
  • Excessive NO production
  • Depletion of endogenous vasopressin
  • Downregulated catecholamine receptors
  • Altered calcium metabolism in vascular smooth muscle
28
Q

Describe the cardiogenic causes of hypotension

A

Primary
Disease originates directly from the heart (e.g. dilated cardiomyopathy, 3rd degree AV block)

Secondary
Disease originates outside the heart. Heart function is supressed or has secondary sites of pathology (e.g. SIRS)

29
Q

Hypotension is commonly defined as…

A

A MAP of below 60mmHg

30
Q

Why should SAP and DAP be interpreted with caution?

A

There is no specific correlation with MAP

31
Q

Why is a MAP of below 60mmHg clinically significant

A

It is below these values that certain tissues become unable to autoregulate perfusion (kidneys, brain, myocardium)

32
Q

What are normal arterial blood pressure values in the dog?

A

SAP 90 - 140 mmHg
DAP 50 - 80 mmHg
MAP 60 - 100 mmHg

33
Q

What are normal arterial blood pressure values in the cat?

A

SAP 80 - 140 mmHg
DAP 55 - 75 mmHg
MAP 60 - 100 mmHg

34
Q

Describe the steps in obtaining a direct blood pressure measurement

A
  • Place catheter into an artery (commonly dorsopedal or femoral)
  • Connect to a pressure transducer; this enables continuous measurement of SAP, DAP and MAP
    (Arterial catheter placement also enables arterial blood sampling)
35
Q

List the disadvantages of pursuing direct blood pressure monitoring

A
  • Technically difficult
  • Invasive
  • Equipment cost
  • Risk of infection
  • Risk of thrombosis
  • Risk of bleeding
  • Risk of inflammation
36
Q

List the advantages of indirect blood pressure monitoring

A

Less expensive
Less time consuming
Less invasive

37
Q

List the disadvantages of indirect blood pressure monitoring

A

Less accurate in very small patients, especially those with sever hypotension, patients with peripheral oedema, vasoconstriction and arrhythmias

Cardiovascular Medicine (1 decks)

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