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)