Lab #10: Cardiovascular Physiology

Transcription

1 Backgrund The heart serves as a pump t drive the flw f bld thrugh the bdy. It des s by underging a cycle f cntractin and relaxatin called the cardiac cycle. During the initial prtin f the cardiac cycle, an electrical signal is generated in s-called pacemaker cells that is distributed thrugh the heart thrugh an electrical cnductin system. In respnse t electrical stimulatin, the mycardium f first the atria and then the ventricles underges cntractin (systle), fllwed by sequential relaxatin (diastle) f the tw sets f chambers a fractin f a secnd later. This cycle f cmpressing n the bld in the ventricles during systle fllwed by the filling f the ventricles during diastle induces pressure changes in the ventricles that cause ne-way valves in the heart t clse audibly at different intervals f the cardiac cycle. The result f the injectin f bld int the arteries by the ventricles underging systle is the generatin f bld pressure, the primary driving frce fr the flw f bld thrugh the bdy. In this exercise we will examine bth electrical and mechanical events that take place during the cardiac cycle as well as measure the resultant bld pressure generated thrugh this cntractile activity. Electrical stimulatin f the heart and electrcardigrams. The heart is aut-excitatry. Actin ptentials are frmed spntaneusly at regular intervals in specialized cells called pacemaker cells. These cells are arranged in a netwrk that enables signals t be cnducted thrughut the mycardium frm the pint f rigin. Fur majr structures are fund within the cnductin netwrk (Fig 10.1). The sinatrial nde (SA nde), which is lcated in the right atrial wall near the junctin fr the superir vena cava, cntains pacemaker cells that underg spntaneus deplarizatins at a higher rate than any f the ther pacemaker cells in the heart. As a result, the SA nde sets the basic temp fr Fig The cnductin system f the heart and the path f cardiac excitatin. Typically, actin ptentials riginate in the sinatrial nde (1) and are cnducted rapidly thrugh the atria (2) t the atriventricular nde (3). Once having passed thrugh the AV nde, the actin ptential prpagates thrugh the Bundle f His (4) and the lateral branches that arise frm it (5). Once the signal has reached the apex f the heart, Purkinje fibers distribute the deplarizatin t the ventricular mycardium. Image frm /mmhe/ heart cntractin (the sinus rhythm), and thus is ften referred t as the pacemaker f the heart. Actin ptentials riginating in the SA nde are cnducted rapidly thrugh bth atria thrugh tracts f pacemaker cells. Lcated in the medial wall f the right atrium, near its junctin with the right ventricle, is the atriventricular nde (AV nde). The AV nde cntains the nly pacemaker cells that lead ut f the ventricles, thus nrmally electrical signals riginating in the SA nde and passing thrugh the atria can nly be cnducted t the ventricles thrugh this structure. The pacemaker cells in the AV nde have very lw cnductin velcities, thus electrical signals pass thrugh this regin very slwly. Once the signal passes thrugh the AV nde, it is transferred t a structure called the p.1

2 Fig 10.2 An electrcardigram fr a single cardiac cycle. Nte the three distinctive wavefrms: the P wave, QRS wave, and the T wave. Three imprtant diagnstic intervals (P-R, R-T, and S-T) are als nted. atriventricular bundle (AV bundle) r Bundle f His, which cnducts the signal thrugh the interventricular septum twards the apex f the heart. Sn after entering the interventricular septum the AV bundle bifurcates int tw separate branches. The cnductin f the electrical signal thrugh the interventricular septum, cupled with the slw cnductin velcity f the AV nde, causes a delay frm when actin ptentials frm in atrial mycardium and when they frm in the ventricular mycardium (and subsequently in when the tw sets f chambers cntract) called the atriventricular delay. This delay ensures that atrial systle is cmplete at the nset f ventricular systle. Once the signal reaches the apex f the heart it is cnducted up the lateral walls f the ventricle thrugh branched tracts f pacemaker cells called Purkinje fibers, which distribute the electrical signal t the ventricular mycardium. Electrical changes ccurring during the cardiac cycle can be mnitred frm the surface f the bdy in a recrding called an electrcardigram (ECG, r EKG). A nrmal ECG recrding assciated with a single cardiac cycle cntains three distinctive wavefrms (Fig 10.2). The P wave is generated when the atria deplarize as the actin ptential wave spreads ut frm the sinatrial nde. The QRS cmplex (which cnsists f the Q, R, and S waves) is triggered by the deplarizatin f the ventricles just befre ventricular systle. During the QRS wave the atria are replarizing, but the small electrical disturbance caused by this is masked by the massive change in extracellular charge caused by the ventricles deplarizing. The last wavefrm, the T wave, is triggered by the replarizatin f the ventricles at the end f ventricular systle. A number f imprtant intervals can be measured frm an ECG recrding. A simple measure f the duratin f the cardiac cycle can be measured simply as the time that elapses between a particular pint in ne cardiac t that same pint in the next cardiac cycle (e.g., frm R wave t R wave). The P-R interval, (which here we will measure as frm the start f the P wave t the peak f the R wave), indicates the duratin f time that the atria are deplarized, which is rughly equal t the duratin f atrial systle. In additin, the P-R interval indicates hw lng it takes electrical signals t travel frm the atria t the ventricles (i.e., the AV delay). During the R-T interval (here measured as the duratin frm the peak f the R wave t the start f the T wave), the ventricles remain in a deplarized state. The duratin f this interval is rughly the duratin f ventricular systle, thus the amunt f time that bld is being frced ut f the heart and int the arteries. Cnversely, the T-R interval (here measured as the duratin between the start f the T wave f ne cardiac cycle t the peak f the R wave f the next cycle) indicates hw lng the ventricles remain in a plarized state between deplarizatins, crrespnding t the duratin f ventricular diastle and thus hw lng the ventricles refill with bld fllwing cntractin. Finally, the S- T interval (the segment f baseline between the end f the S wave and the start f the T wave) is an imprtant diagnstic interval, in that this sectin may becme elevated as a result f a recent mycardial infarctin ( heart attack ) r depressed in individuals with crnary ischemia. ECGs are imprtant diagnstic tls fr evaluating cardiac abnrmalities. Sme examples f ECGs assciated with particular pathlgies are presented in Fig p.2

3 Fig 10.4 Events assciated with the cardiac cycle. The tp f the figure represents changes in bld pressure in the arta, left ventricle, and left atrium. The mid prtin f the figure displays changes in the vlume f the left ventricle. The bttm indicates assciated events in an ECG as well as the generatin f sunds assciated with the cardiac cycle. Ventricular systle cnsists f phases 2-4 in the cardiac cycle. The tw nrmal sunds generated by the heart are I and II Illustratin frm Heart%20Disease/HD002.htm The Cardiac Cycle and Heart Sunds Fig 10.3 Examples f Abnrmal ECGs. Images (as well as infrmatin n the pathlgies that cause these abnrmal ECGs) are frm /mmhe/ The electrical signals recrded n an ECG are caused by intermittent perids where the mycardium f the heart underges actin ptentials. These actin ptentials trigger the mycardium f the ventricles t cntract fr a perid f time and then relax. The resultant cycle f cntractin and relaxatin f the heart is called the cardiac cycle. During the cntractin phase f the cardiac cycle (systle), the walls f the ventricles cntract n the bld within these chambers. This elevates the pressure f this bld abve that f the bld in the arteries, thus frcing bld ut f the ventricles and int the p.3

4 Fig One-way valves f the heart. Image frm Cardivascular/Cardivascular.htm Fig Changes in bld pressure in different regins f the systemic circuit. Frm Fx, S.I. Human Physilgy, 8 th ed. McGraw Hill. arteries (Fig 10.4). During the relaxatin phase (diastle), the bld pressure in the ventricles falls belw venus pressure. Thus bld drawn frm the veins fills the ventricles, and the vlume f the ventricles expands. A series f ne-way valves prevents backflw f bld frm the ventricles int the atria during systle and frm the arteries int the ventricles during diastle (Fig 10.5). The clsure f these valves can be heard during the cardiac cycle (Fig 10.4). The first sund prduced, the lub sund, is caused by the clsure f the atriventricular valves at the beginning f ventricular systle when pressure in the ventricles exceeds atrial pressure. The secnd sund, the dub sund, is generated at the beginning f ventricular diastle when ventricular pressure falls belw arterial bld pressure. The Cardiac Cycle and Arterial Bld Pressure The flw f bld thrugh the cardivascular system is driven by pressure differences between ne segment f a bld vessel circuit and the next. Bld pressure drps sequentially thrughut the circuit (Fig 10.6), and thus the bld at ne pint will flw t the next where the pressure is lwer. The cntractins f the heart elevate bld pressure high enugh s that it can be prpelled thrugh the entire circuit. Arteries have particularly imprtant rles in ensuring adequate bld flw thrugh the cardivascular system. Arteries serve as pressure reservirs their elastic walls expand during ventricular systle t accmmdate the influx f fresh bld, and then cmpress back n the bld during ventricular diastle, maintaining relatively high bld pressure even when ventricular bld pressure has drpped t near 0 (Fig 10.6). This ensures that bld flws cnstantly thrugh the cardivascular system thrughut the cardiac cycle. Bld pressure in the arteries scillates during the cardiac cycle. Systlic bld pressure (i.e., the pressure in the arteries during ventricular systle) is ~120 mmhg, similar t that f bld in the ventricles during this perid. Diastlic bld pressure (pressure in the arteries during ventricular diastle) is smewhat lwer at ~80 mmhg, althugh nt nearly as lw as the pressure in the ventricles at this time. The difference in pressure between systle and diastle is called the pulse pressure, which is a useful diagnstic measure fr cardivascular health health. Anther derived measurement is the mean arterial pressure, which is the average bld pressure in the arteries thrughut the cardiac cycle. Mean arterial pressure is calculated as fllws: MAP (mmhg) = Diastlic pr + 1/3 (Pulse pr) p.4

5 Mean arterial pressure is an imprtant diagnstic measurement in identifying chrnic hypertensin. Bld pressure can change based n activity levels and n bdy psitin. Fr example, when a persn is standing, bld will tend t be drawn int the extremities (particularly the legs) with the frce f gravity. Thus the heart will need t pump harder in rder t recver bld and t deliver bld t the brain against the frce f gravity, thus bld pressure will becme elevated. In cntrast, if a persn is reclining, bld tends t pl in the abdmen and thrax, and the effects f gravity becme less, thus the heart des nt need t pump bld as rigrusly t ensure adequate circulatin, thus bld pressure will tend t becme lwer. Cardivascular Fitness Bld flw thrugh the cardivascular system is adjusted in rder t meet the demands f the tissues. During high levels f activity, cardiac utput (the rate that bld is pumped int circulatin by the heart) becmes elevated. This is typically due t an increase in bth cmpnents f cardiac utput: heart rate (hw frequently the heart beats) and strke vlume (hw much bld is ejected frm the ventricles with each beat). Althugh bth nrmally elevate during exercise, the relative cntributins f each can differ substantially based n cardivascular fitness. If an individual exercises regularly, they tend t increase the number f myfibrils in their cardiac muscle cells, and thus the ventricles can cntract mre frcefully during systle and increasing the strke vlume. As a result, heart rate des nt need t increase as much during exercise t generate the same degree f cardiac utput. This enables individuals wh exercise regularly t sustain the same level f exercise fr lnger perids, t recver mre quickly frm exercise, and t be able t cmpensate fr changes in circulatin (e.g., psitinal changes) mre effectively. p.5

6 Experiment I. Electrcardigram. Yu will be recrding tw different ECG tracings. First, recrd the ECG f the subject in yur grup per the instructins belw. Then unsnap the cable leads frm the adhesive electrdes. Have the subject exercise (run arund in the hallway r utside, up / dwn staircases, d jumping jacks, etc.) t get their heart rate elevated. Then quickly recnnect the cable leads t the electrdes and recrd a secnd tracing. Fr the Subject 1. Remval any metal jewelry frm wrists and left ankle. 2. Apply an adhesive disk electrde t the skin n the inside f each wrist and t the left ankle. 3. Attach the ECG electrde cables t the electrde disks in the fllwing manner: White (-) left arm Red (+) right arm Black (grund) left ankle The disks snap n t the cables 4. Attach the clip n the ECG cable t a belt lp f the edge f yur pcket t help keep the cables frm mving. 5. Attach the pulse plethysmgraph transducer (Fig 10.7) t the tip f yur index r middle finger s that the white tp f the electrde is in cntact with the pad f the finger. It shuld be secured firmly (but nt s hard that it cuts ff circulatin) with the Velcr strap. 6. Sit dwn n a stl and rest yur arms n tp f yur legs. Be sure that the ECG cables are slack, and that yu are relaxed. Sit quietly during the recrding, and avid mving, talking, etc. electrical signals frm yur skeletal muscles will distrt the recrding. Fr the Operatr Fig Pulse plethysmgraph transducer. 1. Once the subject is ready, left-click n START in the upper right crner f the screen. 2. Yu shuld see the ECG being traced n the upper part f the screen, and the pressure pulse f the finger being recrded n the lwer part f the screen (See Figure 7A). If yu d nt see a distinctive ECG pattern r a regular pulse pattern, ask the instructr fr help. 3. Recrd fr ~15-20 secnds, then left-click n STOP in the upper right crner f the screen. Data Analysis Click n the tw marker buttn (the secnd t last buttn in the tp rw f buttns in yur windw) t bring up the measuring tls. The tw measuring markers (blue vertical lines) will appear ver yur tracings. Yu can mve the psitin f either blue line by mving the arrw ver a line, hlding dwn n the left muse buttn, and dragging the line. Ntice that as yu mve the lines a cuple f sets f numbers change: p.6

7 Time between markers Vltage difference between markers fr ECG EGC Tracing Vltage difference between markers fr Pulse Pressure Pressure Pulse Tracing Fig ECG tracing with pressure pulse recrding. T2-T1 (tp left crner f the windw, Fig 10.8). This number is the difference in time measured between the tw blue lines n the recrding. The number is expressed as HOURS: MINUTES:SECONDS, with secnds given in fractins dwn t sec (r 1 msec) V2 V1 (tp right hand crner f each tracing, Fig 10.8). This number is the difference in vltage recrded between the psitins demarcated by the tw blue lines. Yu can use these t measure changes in signal strength t make precisin estimates f when particular events ccur (e.g., when the R wave reaches its peak, etc.) Fig Psitin f reference lines fr determinatin f cardiac cycle duratin. Using the blue lines and their assciated values, determine the fllwing: 1. Duratin f a single cardiac cycle. Measure frm the peak f ne R wave f the QRS cmplex t the peak f the next R wave (See Fig. 10.9). D this fr 4-5 intervals, and nte any differences that may ccur in the duratin f these ECG cycles.. Using this value, calculate the heart rate () by dividing 60 by the cardiac cycle duratin. Fig Psitin f reference lines fr determinatin f the P-R interval p.7

8 Heart Rate (beats/min) = 60 sec/min cardiac cycle (sec) 2. Duratin f the P-R interval. Measure frm the peak f the P wave t the peak f the R wave in the QRS cmplex (See Fig ). This is an indicatin f the time it takes fr an electrical signal t travel frm the SA nde t the ventricles. 3. Duratin f the R-T interval. Measure frm the peak f the R wave t the peak f the T wave in the same ECG cycle (See Fig ). This is a measure f hw lng the ventricles are deplarized when they underg an actin ptential, and is rughly the same duratin as ventricular systle. Fig Psitin f reference lines fr determinatin f the R-T interval. 4. Duratin f the T-R interval. Measure frm the peak f the T wave f ne cardiac cycle t the peak f the R wave in the same ECG cycle (See Fig ). This is a measure f hw lng the ventricles remain replarized after underging an actin ptential, and is rughly the same duratin as ventricular diastle. 5. Duratin f a single heart beat. Measure the amunt f time that elapses between ne heartbeat n the pulse pressure recrding and the next (See Fig ). Yu can use either the start f the heartbeat r the peak r the heartbeat as yur reference pint. Cmpare this value with that f the duratin f a single cardiac cycle. 6. Bld cnductin time. Measure the duratin between the R wave f a cardiac cycle and the peak f the crrespnding pressure wave in the pulse recrding (See Fig ). This is rughly hw lng it takes bld t travel frm yur heart t the tip f yur finger thrugh arteries. Fig Psitin f reference lines fr determinatin f the T-R interval. Fig Psitin f reference lines fr determinatin f heartbeat duratin based n pressure pulse. If yu assume that the distance frm yur heart t yur finger is ~1 meter, calculate the velcity that bld is traveling by dividing 1m by the bld cnductin time. Bld cnductin velcity (m/s) = 1 m cnductin time (sec) Fig Psitin f reference lines fr determinatin f bld cnductin time. Nte the first reference line is psitined n the ECG tracing, but the secnd is n the pulse tracing. p.8

9 Experiment II. Heart Auscultatin and Bld Pressure Measurement Heart Auscultatin Place the earpieces f the stethscpe int bth ears and psitin the bell f the stethscpe at the varius psitins indicated in Fig t hear the clsures f the different valves. Arterial Bld Pressure Measurement With the subject seated, apply the cuff f the sphygmmanmeter arund the upper arm f the subject s that the hsing fr the cuff is psitined ver the cubital fssa (Fig 10.16). Apply the bell f the stethscpe t the skin ver the brachial artery in the cubital fssa. Clse the screw valve n the hand pump and pump the cuff t a pressure f ~160 mmhg. D nt exceed 180 mmhg. Open the screw valve n the pump t slwly release the pressure, listening t the brachial artery thrugh the stethscpe and nting at what pressure the sunds f Krtkff (the sunds generated by bld turbulence in a partially ccluded artery) begin (systlic pressure) and end (diastlic pressure). Recrd these values. Calculate the pulse pressure fr the subject as fllws: Pulse Pressure = Systlic BP Diastlic BP Fig Apprximate psitins where the clsure f specific heart valves are best heard. Image is frm anatmy.med.umich.edu/ surface/thrax/hsunds.html Calculate the mean arterial pressure fr the subject as fllws: Mean Arterial Pressure = Diastlic BP + 1/3 (Pulse Pressure) Experiment III. Cardivascular Fitness. In this exercise, we are ging t evaluate yur cardivascular fitness using an ld but very reliable index called the Schneider index (develped by E.C. Schneider and published in the Jurnal f the American Medical Assciatin in 1920). The activities in this exercise evaluate the ability f the Fig Measurement f bld pressure with a sphygmmanmeter and stethscpe. Image frm cardivascular system t cmpensate fr changes in bdy psitin (which alter the effects f gravity n circulatin) and changes in activity (a brief amunt f exercise). This is a lw intensity exercise. Hwever, if yu have any ptentially serius cardivascular cnditins (e.g., chrnic severe hypertensin, heart disease, etc.) that culd be aggravated by these activities, please have smene else in yur grup serve as the subject fr this activity. p.9

10 Fig Tw techniques fr mnitring pulse. Images frm Activity 1. Reclining Heart Rate 1. Have the subject recline n the lab table fr a perid f 5 min 2. Recrd reclining heart rate by measuring either radial r cartid pulse (See Fig 10.17) fr 30 secnds and multiplying that value by tw. Recrd this value 3. Measure the subject s bld pressure with a sphygmmanmeter and recrd this value. 4. Scre pints fr the individual based upn their reclining heart rate: Activity 2. Standing (Nrmal) Heart Rate, 1. The subject shuld stand up and their pulse shuld be immediately measured fr 30 secnds then multiplied by Tw minutes after standing up, measure and recrd the subject s bld pressure. 3. Scre pints fr the individual based upn a) hw much systlic bld pressure changed upn standing b) standing pulse rate, and c) hw much pulse rate increased upn standing. Reclining Pulse () Rate Pints SCORE Change in Systlic Pr frm Reclining t Standing Change in Pr Pints Increase 8+ mmhg 3 Increase 2-7 mmhg 2 N change (±1 mmhg) 1 Fall f 2-5 mmhg 0 Fall f 6+ mmhg -1 SCORE Standing Pulse () Difference in Pulse btw Standing and Reclining Rate Pints Increase in heart rate Reclining Rate () SCORE SCORE p.10

11 Activity 3: Changes in Heart Rate with Exercise 1. Have the subject step up nt an 18 stl, right ft first, then bring up the left ft and place it next t yur right. Step dwn with the left ft and then bring the right ft dwn t the flr next t it. Repeat this exercise five times, allwing three secnds ttal time fr each repetitin. 2. Immediately after cmpletin f the fifth repetitin, measure the subject s heart rate fr 15 secnd and multiply the number f pulses fr 15 secnds. Recrd this value. 3. Repeat the 15-secnd pulse measurements at 30, 60, 90, and 120 secnds pst-exercise. Recrd the time it takes the pulse t return back t nrmal rate. 4. Scre the subject based upn a) the difference between heart rate immediately pst-exercise and nrmal standing heart rate and b) The time needed fr heart rate t return t nrmal standing rate after cessatin f exercise. Time fr pulse t return Difference btw nrmal and immediate pst-exercise heart rates t nrmal Elevatin in heart rate ver nrmal rate Time (sec) Pints Standing Pulse () > > SCORE SCORE TOTAL SCORE (sum f all scres): Cardivascular Fitness Rating Ttal Scre Rating Excellent Gd 8-13 Fair 0-7 Pr Negative value Please check yurself int the hspital after class p.11