Biomedical Engineering - Bioengineering of Physiological Control Systems

Baroflex

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BAROREFLEX Physiological reaction (or reflex) to arterial pressure changes It is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels . The control output is generated by balancing the sympathetic action that tends to increase pressure and the parasympathetic one that lows it. At normal conditions it provides 2 closed -loop mechanism s to regulate blood pressure (the 2 branches in the schema) : x The HEART RATE ADAPTATION It provides a rapid negative feedback loop: in normal condition the action is performed in less than the duration of a cardiac cycle � this allows to deal with postural hypotension , then to restore a normal blood pressure level after a decrease on standing due to gravity The mechanism reacts when blood pressure increases by decreasing the heart rate and vice versa . x The VASOCONSTRICTION MECHANISM A slower -acting system that regulates the vessels ’ tone to increa se the blood pressure by increasing the peripheral resistance . BARORECEPTORS They are specialized neurons located in vessels ’ walls , mainly in the aortic arch and carotid sinuses. They have the capability of monitoring changes in blood pressure : � If the b lood pressure increases th e tension on vessel s’ wall increases � Pressure induced stretching of baroreceptors � Receptors catch the input and relay to the medulla oblongata CONTROLLER CNS PLANT CONTROL VARIABLE S CONTROL LED VARIABLE SAP FEEDBACK SENSOR Baroreceptive fibers FEEDBACK SIGNAL Afferent neural signal PR EMBEDDED REFERENCE Intrinsic set point RR Arteries Heart + - + - + - RR = Cardiac Period, i.e. the interval between two R peaks PR = Peripheral Resistance CNS = Central Nervous System SAP = Systolic Arterial Pressure Blood pressure changes with respect to normal values ( = intrinsic WP ) ↑ BLOOD PRESURE INCREASE ↑ Baroreceptors neural activity (afferent path) increases ↓ Heart rate is slowed = contractility is lowered = parasympathetic (or vagal) activity is increa sed = sympathetic activity decreases ↑ Vaso dilatation increases = vasoconstriction firing rate decreases ↓ BLOOD PRESURE DECREASES ↓ Baroreceptors neural activity (afferent path) decreases ↑ Heart rate increases = contractility is raised = sympathetic activity increases = parasympathetic activity is decreased ↑ Vaso constriction increases = vasoconstriction firing rate increases ARTIFICIA LLY INCREASED BLOOD PRESSURE LEVELS When vasopressure drugs - like phenylephrine - are administered to a subject � the blood pressure is rais ed above the equilibr ium point ( + δSAP ) d ue to the induced vasoconstriction ( + δPR ) � Baroceptors ’ firing rate increases, but ramains syncronize d with the HR � The ANS, consequently , lowers the HR ( + δRR period ) (= bradycardia) to lower the the blood pressure ( - δSAP ) � The SAP gets back to the equilibrium FEEDBACK SENSOR Barorecep tors BARO -REFL EX SENSITIVITY ( BRS slope ) By applying line ar regression to data relating RR (hear t rate expressed by the RR period) SAP (blood pressure) at each time inst ant, during a phenylephrine inje ction a slope, depicting the relatio n between SAP and HR changes can be obtained BRS slope α in ms/mmHg >✗SLOPEcanbeseemastheratiobetweentheShapesofthetwoCurvesinthatraisingInterval(approxienatcdcostantbyMearnsofREGRESSIONI "¥!theexplicitviewoftimeSequenceisLost+NB: KRRandKSAP areneuercomputedin practice unstheyhaveonly annotabile Meaning intheTIMEDOMAINVA L S AVAMANOEVRE fortheBAROREFLEXGAINASSESSMENT1.TheSubjectblowsintoamanometerS→os→OI1qpLTHATCORRESPONDtoREALITY:✗•anIncreaseinSAP]issensedbyBARORECEPTORSAss. .ie??...~TheBAROREFl--lXtuendecreasesRR uptotheCardiacperiodthatallonsSAPtoLowbacktVENTRICULARPREMATUREBEAT ( VPB,orextrasistole) itisanabnormaluemtricnlarcontractorinaduancewrttotheCurrentRRperiodthattakesplaceduetoamanomalouselectricalactivatiom*andthatCausesaTURBOLENCEinRRtrend 2 .RRCHANGESprovateSAPCHANGESextrabeat@SmollerRR RR decreases