Cardiac reflexes, How to prevent it.

Dr Imran Sheikh.

Afferent nerve ↓ Symp and para symp N input processed in the CNS ↓ Efferent nerve ↓ Heart /Systemic circulation ↓ Particular Reaction

Cardiac reflexes Fast acting reflex loops between the

heart and CNS.

Contribute to regulation of cardiac function and maintenance of homeostasis.

Cardiac receptors –linked to CNS by myelinated or un myelinated afferent fibres that travel along the vagus N.

Cardiac receptors--- found in atria, venticles, pericardium and coronary arteries.

Extra cardiac receptors--- Great vessels and Carotid.A.

Response varies with age and the duration of underlying condition that elicited the reflex in the first instance.

Baro receptor reflex(Carotid sinus reflex)

• Responsible for maintenance of BP• Capable of regulating arterial BP around a

preset value through a negative feed back loop

• Also capable of establishing a prevailing set point of BP when the preset value has been reset because of c/c HTN.

Changes in BP---monitored by circumferential and longitudinal stretch receptors located in carotid sinus and aortic arch

Carotid sinus At the bifurcation of

the common carotid arteries

the root of internal carotid artery shows a little bulge

has stretch receptors in the adventitia

are sensitive to arterial pressure fluctuations

Carotid sinus (Contd…)

Afferent nerves from these stretch receptors travel in the carotid sinus nerve

which is a branch of the glossopharyngeal nerve. (IXth cranial nerve)

Aortic arch

baroreceptors are also present in the adventitia of the arch of aorta

have functional characteristics similar to the carotid sinus receptors.

their afferent nerve fibers travel in the aortic nerve,

which is a branch of the vagus nerve. (Xth cranial nerve)

Cardiovascular centre in medulla• Nucleus solitarius

• CV centre has

Area for inc BP Area for dec BP Located laterally and Located centrally and rostrally. Caudally.

Concept and mechanism of baroreceptor reflex

Any drop in systemic arterial pressure decreases the discharge in the buffer nerves,

and there is a compensatory rise in blood pressure and cardiac output.

Any rise in blood pressure produce dilation of the

arterioles and decreases cardiac output until the blood pressure returns to its previous normal level.

• vagus nerve

• • ① ↓ Sympathetic activity

• ↓↓• ↓Cardiac contractility• ↓HR• ↓Vascular tone

• + of Para symp system--- ↓HR and ↓myo contr:

Arterial

PressureBaroreceptor

Carotid Sinus

Aortic Arch

Sinus Nerve

Nucleus solitarius

Reverse effects ----- onset of hypotension.

The reflex arch loses its capacity at BP < 50 mm Hg

Hormonal diff and hence sex diff have been implicated in altered baroreceptor responses.

Volatile anesth (esp Halothane) inhibit the HR component of the reflex.

Concomitant use of CCBs and ACEIs OR PDEIs will lessen the CV response of raising BP through baroreceptor reflex

→ Direct effect on peripheral vasculature →Interference in CNS signalling pathway(imp)

Baroreceptor Resetting

Baroreceptor will adapt to the long term change of blood pressure.

That is, if the blood pressure is elevated for a long period of time, several days or years, the set point will transfer to the elevated mean blood pressure.

Obviously, the adaptation of the baroreceptor prevents the baroreceptor reflex from acting as a long term control system.

That makes the baroreceptor system unimportant for long-term regulation of arterial pressure

Chemoreceptor reflex

o Chemo sensitive cells present in carotid and aortic bodies.

o Respond to →changes in Ph →Blood tension

Art < 50 mmHg or Acidosis Chemoreceptors Impulses through Sinus N of hering and Vagus N Chemosensitive area of medulla

Response

Response: Stimulation of chemoreceptors leads to a reflex increase in vasomotor tone,

which causes generalized vasoconstriction and hence a rise in blood pressure.

Importance: Chemoreceptor mechanism is important in regulation of blood pressure when it fall below the range in which baroreceptors act (70 mmHg).

Stimulates resp centre and causes increase in ventilatory drive.

Also stimulates para symp system

Bain bridge ReflexElicited by stretch receptors located in the Rt atrial

wall and cavoatrial junction.

↑Rt sided filling pressure Vagal afferent Cardiovascular centre in medulla inhibition of parasymp system

↑HR

o ↑ HR ---also from direct effect on SA node by stretching the atrium.

o The changes in HR dependent on underlying HR before stimulation.

• The Bainbridge reflex and the baroreceptor act antagonistically to control heart rate.

• The baroreceptor reflex acts to decrease heart rate when blood pressure rises.

• When blood volume is increased, the Bainbridge reflex is dominant; when blood volume is decreased, the baroreceptor reflex is dominant.

Bezold-Jarish Reflex• Responds to noxius ventricular stimuli

• Sensed by chemoreceptors and mechanoreceptors with in LV wall,

by inducing triad of →Hypotension →Bradycardia →Coronary artery dilatation

Afferent-----Unmyelinated vagal afferent type C fibres

Reflexively increase parasymp: tone.As it involves bradycardia, this reflex is thought of as a cardio protective reflex

Implicated in the physiologic response to a range of cardio vascular conditions… →Myocardial ischemia/infarction →thrombolysis →revascularisation →Syncope

Cushing Reflex(CNS ischemic response)

Result of cerebral ischemia due to ↑ICP. Chemoreceptor reflex is useful in regulation of

blood pressure when it falls to a level between 40 and 70 mmHg.

But if the blood pressure below 40 mmHg, the last ray of hope for survival is the central nervous system (CNS) ischemia response.

So it sometimes called the “last ditch stand” pressure control mechanism.

As the name indicates, it is evoked by ischemia (poor blood flow) of the central nervous system.

CNS ischemia reduces blood flow to the vasomotor centre (VMC).

Reduction in blood flow to the VMC leads to reduced Po2 and elevated Pco2 in the medulla region.

Both these factors stimulate the VMC directly, leading to vasoconstriction and consequently rise in blood pressure.

One of the most powerful of all the activators of the sympathetic vasoconstrictor system.

Not one of the usual mechanisms of regulating normal pressure.

It is an emergency arterial pressure control system that acts rapidly & powerfully to prevent further decrease in arterial pressure whenever blood flow to the brain decreases dangerously close to the lethal level.

Blood flow to

vasomotor centre

decreased significantl

y

Effect is due to

failure of slowly

flowing blood to carry C02

away from vasomotor

centre.

Neurons in vasomotor

center respond to ischemia directly

Systemic arterial

pressure rises as high as

the heart can

pump.

Degree of vasoconst

riction can be intense enough

to totally occlude

some peripheral vessels

Eg : The kidneys

may totally cease urine

production because

of arteriolar constricti

on

Oculocardiac reflexPressure applied to the globe of the

eye or traction on the surrounding structures.

Incidence ---- 30%---90%

Stretch receptors--- present in Extraocular Ms

Afferent through short & long ciliary Ns.The ciliary Ns will merge with ophthalmic division of the Trigeminal N at the ciliary ganglion. Gasserian ganglion.

↑Para symp tone & Bradycardia

Valsalva maneuverForced exp against a closed glottis→↑intra thoracic pressure,↑CVP, ↓Venous return. ↓CO & BP

This decrease will stimulate baroreceptors

↑HR,↑Myocardial contractility by sympathetic (+)

When glottis opens Venous return ↑

Causes heart to respond by vigorous contraction & ↑BP.

Sensed by Baroreceptors

Stimulation of para sympathetic system.

Recto cardiac reflexDilatation of the anal canal and instrumentation of the anal rectum also may evoke cardiovascular responses.Bradycardia.Hypotension.Afferents ----- chiefly over the pelvic N.Efferents ----- Vagus.

Pelvic reflexes Mobilisation of uterus Traction on the uterus (more usual)

Hypotension.

Circulatory depression-----when large tumours are lifted from pelvis

(Reflex or Mechanical).

Celiac plexus Reflex

Manifested by marked falls in BP and absence of systolic sounds while taking BP.

Pulse may be slow/imperceptible Bradycardia.

↓BP ----- narrowing of pulse pressureMore likely when stomach is pulled.Traction on gallbladder, hilum of liver,

or retraction of the duodenum.

Diaphragmatic traction reflex(Brewer-Luckhardt reflex)

• Manipulation or traction on the diaphragm often results in drop in BP, esp the systolic, accompanied by bradycardia.

• Downward traction on the liver or traction on the gallbladder, produces similar acute effects.

• Should be distinguished from mechanical hypotension.

Nasocardiac reflex

Stimulation of the nasal cavity by a nasal speculum, a nasal retractor, or an ET tube, when anaesthesia is in adequate or in the absence of topical anaesthesia

Bradycardia (predominant manifestation). Hypotension.

Afferent- Maxillary div of trigeminal.N & ethmoidal N.Efferent- Vagus N.

Response- Bradycardia, drop in BP.

Intrathoracic Reflexes• Stimuli to Vagal N endings with in the thoraxEsophageal reflexPericardial reflexPleural reflexHilar reflex( Prophylaxis and Rx- Atropinisation and infiltration of the hilar area with 1% procaine before manipulation)

Tracheal reflexAfferent and efferent Vagus. VAGO VAGAL REFLEX.

Stimulation – Layngospasm, and/or bronchospasm will occur in light anaesth:Ineffective breathing movt called Bucking.

Bradycardia and arrhythmias and hypotension may result

Causes:

Tracheal intubationInflation of the endotracheal cuffPresence of mucus or other foreign

materialStimulation from a tracheal suction

catheter through the endotracheal tube

Peritoneal and mesenteric reflex• Pulling or stretching the peritoneum or pulling the

mesenteries

Bradycardia and Hypotension

Often accompanied by spasm of the larynx and even apnoea.

Traction on the ovaries--- similar response.

Periosteal reflexAfferent- Somatic N fibres Efferent- Vagus N

Apnea occurs often followed by tachypnea with varying degrees of laryngospasm

Hypotension freq folowed by tachycardia

Prevention and RxAtropine- most widely used and

effective agent in prevention and Rx of parasympathomimetic reflex responses.

Topical anaesthesia- can eliminate the reflex at the afferent component.*

Intravascular Lignocaine is more eff than topical and obtunds the cardiovascular responses to upper resp and thoracic induced reflexes.

CV resp during abdominal Sx.

Prevention and Rx (Cond…)Continous infusion of lignocaine (2mg/min) after an initial loading dose of 100 mg IV is recommended.

During Sx…..Cessation of the applied stimulus

IV Atropine (5 – 10 μg/kg)

Vasopressors- If persistent hypotensive response.(ephedrine in fractional doses of 5mg)

Increase the depth of anaesthesia.