Cardiovascular Reflexes
Dr Katherine Howell
Learning Outcomes
•To understand how MAP is regulated•To know the location and function of arterial baroreceptors•To understand the baroreceptor reflex and effects on MAP•To learn about hormonal control of MAP and other cardiovascular regulatory processes•To know how the ANS can be assessed
Mean ARTERIAL PRESSURE
MAP = CO x TPR
HR x SV
Mean arterialpressure
Cardiacoutput
Total peripheralresistance
Heartrate
Strokevolume
Intrinsic Properties of the Heart
•Frank Starling’s Mechanism–Extrinsic control (ANS + hormones)–Intrinsic–‘When venous return changes, the heart automatically adjusts its output to match inflow’
–↑EDV →↑force of contraction →↑SV →↑CO
FRANK-Starling’s MECHANISM
•↑EDV →↑ length of muscle fibres →↑ force of contraction–Cardiac muscle has optimum length > resting–Stretching muscles fibres •→↑affinity of troponin for Calcium •→↑number of activated cross bridges
•Starling’s Law regulates •the size of heart–Prevents heart failure
Factors affecting Stroke Volume
•Sympathetic activity controlling ventricular contractility
•EDV–Affected by preload (end diastolic pressure)–↑CVP →↑preload →↑EDV →↑SV
•Afterload–Arterial pressure
Factors affecting SV
↑ Sympathetic activity or Adrenaline
↑ Venous return
↑ End-diastolic Volume
↑ Contractility
↑ Stroke Volume
↓ Arterial Pressure (afterloa
d)
Ventricle
Regulation of Mean Arterial Pressure
•MAP = HR * SV * TPR•Maintain adequate blood flow to all organs
•Short term regulation –Extrinsic/reflexes
•Long term regulation
Sensory Receptors
•Arterial baroreceptors
•Low pressure baroreceptors
•Chemoreceptors
•Proprioreceptors
•Cerebral cortex and hypothalamus
Firing pattern of baroreceptor in response to arterial pressures of increasing magnitude
Arterial Baroreceptors
Sensory receptor neuron in blood vesselsResponds to changes in pressureAortic archCarotid sinuses
Cardiovascular Control Centre
•Location: Medulla Oblongata•Sensory Input: from baroreceptors–Parasympathetic (vagus) to SAN and AVN–Sympathetic and parasympathetic to SAN–Sympathetic to ventricular myocardium–Sympathetic to Arterioles / resistance vessels–Sympathetic to veins
Baroreceptor Reflex
•Increase in pressure detected by arterial baroreceptors•Increase in firing of baroreceptors•Decreased Sympathetic•Increased Parasympathetic•Decrease • HR• SV• Vascular resistance
BARORECEPTOR REFLEX
Arterial pressure decreases
Baroreceptor firing decreases activity to Medullary CV Centre
1.Increased HR ( symp para)2.Increased contractility ( symp)3.Arteriolar constriction ( symp)4.Increased venous constriction ( symp)
Increased COIncreased TPR
BP returned to normal
MAP = CO x TPRCO = HR x SV
LONG TERM REGULATION OF MAP
•‘Quick fix’
•Variation of BP from normal for few days?
•Hormonal control –Adrenaline•Sympathetic stimulation increases HR, SV and TPR–Vasopressin•Vasoconstriction – increases TPR and MAP•Reduces urine output–Angiotensin II•Vasoconstriction – increases TPR and MAP•Reduces urine output, causes vasoconstriction, thirst
Low Pressure Baroreceptors
Walls of large systemic veins
Wall of Right Atrium
Sensitive to changes in stretch
Detect changes in blood volume in veins
Increase sympathetic nerve activity
Stimulate vasopressin release
Respiratory Sinus Arrhythmia
•Rhythmic variation in HR associated with breathing•Inspiration decreases parasympathetic and HR increases•Expiration increases parasympathetic and decreases HR InspirationExpiration
Autonomic testing
•Respiratory sinus arrhythmia•Valsalva manoeuvre•Tilt table•Cold pressor test•Lower body negative pressure•Static Handgrip exercise•Power spectral analysis of heart rate•Muscle sympathetic nerve activity
Valsalva Manoeuvre
Valsalva Manoeuvre
1. Initial pressure rise Increased pressure in chest forces pulmonary blood into left
atrium. Blood pressure increases which is detected by baroreceptors
2. Reduced venous return and compensation: Venous return impeded by elevated thoracic pressure CO reduced therefore BP falls (5 to 14 secs)
3. Pressure release: The pressure on the chest is released Allows the pulmonary vessels and the aorta to re-expand
causing a further initial slight fall in pressure (20 to 23 seconds)
Venous return increases and cardiac output begins to increase.
4. Return of cardiac output: Significant venous return causes rapid increase in CO and BP
Upright tilt test
The normal response
Orthostatic Hypotension
Defined by the consensus group of the American Autonomic Society as a sustained decrease in blood pressure exceeding 20 mmHg systolic or 10 mmHg diastolic occurring within 3 minutes of upright tilt.
Orthostatic hypotension and lightheadedness which lasts for more than a minute is probably abnormal.
Syncope
Symptom or finding Diagnostic consideration
After sudden unexpected pain, unpleasant sight, sound, or smell
Vasovagal syncope
During or immediately after micturition, cough, swallow, or defecation
Situational syncope
With neuralgia (glossopharyngeal or trigeminal) Bradycardia or vasodepressor reaction
Upon standing Orthostatic hypotension
Prolonged standing at attention Vasovagal
Well trained athlete after exertion Vasovagal
Changing position (from sitting to lying, bending) Atrial myxoma, thrombus
Syncope with exertion Aortic stenosis, pulmonary hypertension, mitral stenosis, IHSS, coronary artery disease
With head rotation, pressure on carotid sinus (as in tumors, shaving, tight collars)
Carotid sinus syncope
Associated with vertigo, dysarthria, diplopia, and other motor and sensory symptoms of brainstem ischemia
TIA, subclavian steal
With arm exercise Subclavian steal
IHSS, Idiopathic hypertrophic subaortic stenosis;TIA, transient ischemic attack.
Summary
•Starling’s Law–Increased venous return causes increased CO
• MAP = HR * SV * TPR•MAP – short and long term regulation•Arterial baroreceptors –Aortic arch–Carotid sinus–Respond to stretch–Increased pressure causes increased APs to medulla
Summary
Baroreceptor reflex–↓MAP –↓ APs to CNS–↓ parasympathetic and ↑sympathetic activity–↑SAN APs (↑HR)–↑ventricular contractility (↑SV)–↑venomotor control (↑EDV)–↑vasoconstriction (↑TPR)–↑MAP
Summary
Low pressure receptorsRespiratory Sinus Arrhythmia–Inspiration increases sympathetic (↑HR)–Expiration increases parasympathetic (↓HR)–Chemoreceptor reflexesDetect changes in CO2
Increased CO2 ↓ HR ↑TPR
ANS can be assessed using several methods incluing Valsalva Manoeuvre and change in posture (tilt table)