Lecture 17

The Heart

• The Electrical Event of a Heart Beat
  
  • First, the SA Node fires – the excitation sweeps across the atria but it will be moving the fastest through Bachmann's Bundle (the Preferential Pathway).
    
  • Bachmann's Bundle carries excitation to the left atrium.
    
  • Bachmann's Bundle carries excitation 3x faster than other parts of the atria.
    
  • Electrical activity sweeps down Inter-Atrial septum, it got there via Bachmann's Bundle. There is no connection to the AV node, it just sweeps slowly down there.
    
  • The AV node then fires
    
  • The AV node is connected to Bundle of His and the impulse is carried through the skeleton of the heart (this goes through the only hole in the heart which is insulating the 2 atria from the ventricles) through the Bundle of His – this is the beginning of the Perkinji system
    
  • The Bundle of His only conducts in one direction (that is down into the ventricles) – and is the only electrical connection between the atria and the ventricles. Anything that starts in the ventricles will not go up to the atria.
    
  • The electrical excitation branches into bundle branches.  Coming off the bundle branches are Perkinji Fibers, which moves the excitation quickly into the ventricle so that we get a uniformcontraction of the ventricle.
    
  • Bundle Branches and Perkinje Fibers are all sub-endocardial – They are under the endocardium layer and they are lining the inner parts of the ventricles.
    
  • When the electrical activity fires in the ventricles, it is going to move from the endocardium towards the epicardium. It will be firing from the inside out.
    
  • Repolarization of the ventricles will start at the apex and sweep back towards the base of the heart (look at a picture of the heart to see this)
    
• EKG or ECG – The Electrocardiogram: (monitors electrical activity of the heart)
  
  • The EKG was developed by Einthoven, he was the first one able to monitor the electrical event and name the waves. He invented the machine that we use today to monitor them (he won the Noble Prize).
    
  • Einthoven determined and used 3 leads – I, II, III
    
  • A professor at the University of Michigan, named Wilson, developed Unipolar Leads.
    
    • Wilson called the Unipolar leads VR, VL, and VF
      
    • Wilson also did Chest Leads but he never standardized them (he was always changing V1)
      
  • Standard EKG uses 12 Leads – called a 12 Lead EKG.
    
    • These are Bipolar Readings
      
    • We have Limb Leads, which are I, II, and III that Einthoven worked with, and they are termed Bipolar Limb Leads. We term them as positive or negative electrodes.
      
      • Now the pen is moving up and down – when it sees a positive charge it moves up (upward deflection) – this is referred to as the Positive electrode. The left arm does not have a physically positive charge.
        
        • The pen doesn't move left and right
          
      • I – is monitoring the Right Arm vs. the Left Arm – and the left arm is positive.
        
      • II – is monitoring the Right Arm vs. the Left Leg – and the left leg is positive
        
      • III – is monitoring the Left Arm vs. the Left Leg – and the left leg is positive
        
  • Einthoven said that if we take an equilateral triangle and put an electrical event in the center of the triangle, and we monitor the corners of the triangle for an electrical event, they will sum to zero.
    
    • Zero electrical activity means that would be a ground said Wilson
      
  • Wilson said if that is true, we can do Unipolar Reading if we add all the readings (heart in center of triangle) and compare them to a probe that is positive.
    
    • Anything with a V is a Unipolar reading (V leads = Unipolar leads). The probe would always be the positive one.
      
    • VR (Right Arm) – the positive probe is on the Right Arm, and that was compared to the other 3 – Left Arm, Left Leg, Right Arm – you get a reading wherever the probe is put.
      
    • VL (Left Arm) –the positive probe is on the Left arm, that is vs. Left Leg, Right Arm, Left Arm
      
    • VF (Femoral) – the positive probe is on the Left Leg and that is vs. the Right Arm, Left Arm, Left Leg – all these will be positive in this case, they are the probe which is always positive.
      
      • The probe is always positive.
        
    • When Wilson did chest leads he would do these 3 vs. wherever he put the positive probe on the leg.
      
    • Think of what a Bipolar recordings is -  if we saw a po sitive we move the pen in one direction, and if the other one sees that same positive it is saying move the pen in the other direction
      
    • If we put a probe on the right arm and if we put another probe on the right arm, they are seeing the same thing.  One is saying go in this direction in some amount, and the other one on the arm is saying the same thing but to go in the other direction with the same amount.  But that means that the pen doesn't move.
      
    • 1/3 of all of all of these trials are going to be 1/3 smaller than they could be. 
      
    • 1/3 of the input into the recording machine is saying don't move and it does not move. The other probes are the ones causing the movement.  So people in the 1930's and 1940's thought that they should augment the Leads (change the system).  Now we are just comparing Right Arm to the Left Arm and the Left leg.  We will do the same with the Left Arm, where we take out the comparison to the Left Arm, and compare it to the Right Arm and Left Leg.  Then we will do the same thing with the Left Leg, where we take out the comparison to the Left Leg, and compare it to the Right Arm and Left Arm.
      
    • These will give identical recordings but now we will get a larger blip and it will be easier to read.
      
      • The machine switches by comparing this aV one to the other two.
        
    • These are called Augmented Leads (a = augmented lead): these are called Unipolar Limb Leads (b/c you don't need a probe, they are called Limb Leads)
      
      • aVR (Augmented Unipolar Right Arm) – compared to Left Arm, Left Leg
        
      • aVL (Augmented Unipolar Left Arm) – compared to Left Leg, Right Arm
        
      • aVF (Augmented Unipolar Femoral – Left Leg) – compared to Right Arm, Left Arm
        
  • Wilson added Chest Leads:
    
  • V1 – in the 4^th intercostal space to the Right of the sternum.
    
  • V2 – in the 4^th intercostal space to the Left of the sternum.
    
  • V3 – is halfway between V2 and V4. You have to find V4 first but you never run V4 first, always V3 first
    
  • V4 – in the 5^th intercostal space and it is Mid-clavicular line (if you go to your clavicle to the manubrium to the Mid-clavicular line down to the 5^th intercostals space)
    
  • V5 – anterior axillary line (armpit) at the corner of the chest, and it is at the same level as V4.
    
  • V6 – is the mid-axillary line, directly under the arm, and it is at the same level as V4 (the same level as V4 doesn't mean that it is in the 5^th intercostals space – the same level as V4 means (when someone is lying down) it is going straight down the side of their chest in a straight line, like you have a plum line)
    
  • V5 and V6 are not in the 5^th intercostal space, because the 5^th intercostal space rises up – you want to be at a plum level, straight down to the ground.
    
    • Most books will say that V4, V5, V6 are in 5^th intercostals space, but that's NOT true, V5 & V6 are not in 5^th intercostals space, it's a plum bob level.  The 5^th intercostals space rises up, but V4 and V5 don't (THIS WILL BE ON THE TEST)
      
      • Read the American Heart Journal – Volume 15, page 277 (1935 Journal).  The announcement in the journal said that the American Heart Association and the Cardiac Society of Great Britain and Ireland were the ones who set the definition for V1 through V6.  When they did the setting for V4, they said "V4 is at the outer boarder of the Apex beat" – you were supposed to listen for the Apex beat with a stethoscope.  V5 and V6 were said to be at the same level as the Apex beat, the Apex beat doesn't rise up (the 5^th intercostals space does), but the Apex beat stays at the same level all the way done. It went on to say that if the Apex beat cannot detected, then we V4 should be considered  mid-clavicular line 5^th intercostal space, (but they didn't correct V5 and V6) - So the same line as V4 is a straight line down the side
        
  • Always record them in proper numerical order, but they are not necessarily found in order.
    
• There are Experimental Leads that are used for people with non-normal conditions or different orientation of the heart. Just know that they exist.
  
  • V7 – Posterior axillary line on the back edge of thorax
    
  • V8 – Inferior angle of the left scapula – V3R (Right) goes through V8R (went around the right side of the body to get there).
    
  • V9 – Spinous process of the 8^th thoracic vertebrae
    
  • Don't have to know thesehh
    
    • There is a man from Russia who did a 12 Lead EKG (in this class) – it was all messed up – he said that his aorta ascends on the right (normal to ascend on the left) – his heart was in backwards essentially – he is someone who you might want to use those leads so you could be able to rotate the leads around to make sense of the EKG
      
• Typical EKG Waves:
  
  • Einthoven (Nobel Prize – 1927) named the waves and invented the machine that could record them.
    
    • P-Wave, R-Wave, S-Wave, T-Wave, U-Wave
      
      • P = round and up and small
        
      • Q = small point and down
        
      • R = big point and up
        
      • S = small point and down
        
      • T = bigger hump up
        
      • U = tiny little bump up
        
  • P wave – 1^st upward deflection in the normal EKG – represents Atrial Depolarization
    
  • Q wave – is the downward deflection that occurs prior to the R wave. It is always followed by an R wave
    
  • R wave – (gets smaller as you move across the chest leads)
    
  • S wave – is the downward deflection/wave the follows the R wave (picture on the board is this wave)
    
  • This is called the QRS complex – represents the Depolarization of the Ventricles. The Repolorization of the Atria is probably buried somewhere in this pattern/area (it gets lost in this QRS complex), because it does not appear anywhere else. 
    
    • Want to write it like this qRs – this means q wave was short, R was tall, and s was short.
      
  • T wave (very large) – are the Repolarization of the Ventricles, they are massive.
    
    • Repolorization starts at the apex and sweeps back over to the base.
      
  • U wave – they think it is the Bundle of His Repolarization or papillary muscle repolarization.
    
  • Once in a while we see a QRS complex that has the Q the R and the S, and then the S comes up and there is another wave up and that is called an R prime (R'). And then we have another one down and that is called S prime (S'), and then if we have another one up is an R prime prime (R''), and then if we get another one down that is called an S prime prime (S'') (you rarely go beyond a double prime – you will be dead)
    

 
 

• Mechanical Contractions of the Heart
  
  • Systole – is the contraction of the heart
    
  • Diastole – is the relaxation of the heart
    
  • Highest blood pressure is the systolic, lowest is the diastolic.
    
  • Late in Diastole – the AV valves are open, and the ventricles are filling passively.  Also, the semi-lunar valves of the pulmonary trunk and aorta are closed.
    
    • The blood is coming in, but it is going down into the ventricle, it is filling passively.
      
    • During diastole 70% of all ventricle filling is passive.
      
  • Atrial Systole starts – it is the contraction of atria
    
    • This completes the filling of ventricles
      
    • Emptying auricles – are the ear like appendage on each atria (these auricles are important b/c your blood is a fluid system and you can't stop the flow.  We are shut valves, and that is going to stop the blood from flowing, but you don't want the blood to stop flowing (or else you get tragic results).  We need a place for the blood from the veins to go.  When the valve is closed between the Right Atrium and the Right Ventricle that ear like appendage will fill with blood, and the blood will continue to flow into the atria (this is a place to flow).  When it beats it empties the blood out, and it gets ready to accept blood when the valves close again.  This will keep the blood flowing)
      
  • Ventricular Systole starts – the AV valves shut (between the atria and the ventricle). The ventricles are kind of full of blood. 
    
    • We have valves between the aorta and the pulmonary trunk, and they are shut too.  Now we have a chamber with all valves shut, and you can't compress a fluid. The tension builds up in this until finally it creates enough pressure in there to pop open the aortic valve and the pulmonary trunk valve. 
      
    • The semi-lunar valves have not opened yet and pressure builds up, you cannot compress a fluid (cannot compress blood) – the volume is not changing because there is not place for it to go.  The time that the blood spends here where all 4 valves are shut (in this closed chamber where you are increasing your pressure) is called the Isovolumetric Contraction Phase.
      
    • Finally the pressure exceeds the pulmonary trunk and the aorta and the semi-lunar valves are forced open (ejecting the blood), and they stay open (the pressure is going to drop), but both of these are expandable b/c they are elastic arteries, so they expand with the pressure.
      
      • Semi-lunar valves close when pressure drops, but there is a hesitation (especially in aorta) because a small amount of the blood will go into the aorta because it has momentum.
        
      • The Semi-lunar valves close due to the drop in pressure of the ventricles. The pressure continues to drop and diastole beings.
        
      • Even though the pressure has dropped, it is not low enough to open the AV valves – this is called the Isovolumetric Relaxation Phase (the heart it is relaxed and all 4 valves are closed.
        
        • Isovolumetric = all 4 valves are closed
          
        • Then the pressure drops and passive filling will begin.
          
  • At rest, at the end of Diastole, the ventricles are each filled with 130ml of blood.
    
  • At the end of systole, approximately 50ml will remain,
    
  • 80ml were ejected during the beat – this is 62% of the total blood in the ventricle that was pumped out
    
    • This percentage is called the Ejection Fraction. The fraction will change with heart rate.
      
  • The highest pressure that was reached in the aorta at rest was approximately 120mmHG – we call this the Systolic Pressure
    
  • The lowest pressure that was reached in the aorta at rest was approximately 80mmHg – we call this the Diastolic Pressure
    
    • 120/80mmHG (Systolic over Diastolic)
      
    • This is the blood pressure of the aorta.
      
      • If we had this pressure in the lungs we would be dead (fluid would get into our lungs immediately)
        
    • A Pulmonary Trunk Pressure would be far lower otherwise we would drown in our own fluids – 25/12mmHg (Systolic over Diastolic). Sometimes this is referred to as the Hidden Blood Pressure (so that you don't drown in your own fluids)
      

 
 

• Cardiac Output – the volume of blood pumped per one minute.
  
  • Cardiac Output is dependent upon 2 variables:
    
    • Heart rate
      
    • Stroke volume
      
  • Heart Rate:
    
    • If increase heart rate that increases cardiac output
      
    • This works until you get to 180 beats/minute
      
    • The heart rate will rarely go over 200.
      
    • Heart rate peaks at 180 beats per minute because the veins cannot return blood faster than that. They cannot fill the ventricles quick enough.
      
    • The pacemaker of the heart is the SA node. 
      
    • The SA node is being stimulated tonally by the sympathetic and parasympathetic fibers firing at the same time. They are both sides of the SA node telling it what to do.
      
    • The parasympathetic system is saying slow down
      
    • The sympathetic system is saying speed up. 
      
    • The normal resting heart beat is about 70 beats per minute.
      
  • If we cut Vagal stimulation (thereby blocking the sympathetic tonal firing) then the heart rate goes up to 150 to 180 beats per minute – the sympathetic is telling this thing to beat at 150 to 180 beats per minute all the time.
    
  • If we block both the parasympathetic and the sympathetic system, so the SA node is on its own, then the heart beats at 100 beats per minute.
    
  • The parasympathetic system is winning b/c your normal heart rate is only 70 beats per minute – this is tonal firing of the parasympathetic system, it adjusts for different activity levels.
    
    • As the professor walks around he is not in fight or flight, so when stands up his heart rate changes. But it is the Parasympathetic system that changes the firing rate, which then allows the Sympathetic system to win to increase your heart rate (under normal circumstance of non flight or fight the heart rate is controlled by the parasympathetic system)
      
• The Vagal Innervation of the SA node (we should immediately think Parasympathetic system b/c it is the Vagus Nerve) is from the Right Vagus – it slows the heart by decreasing cyclic AMP – this slows the opening of the Calcium – L Channels.
  
  • It also decreases the slope of the Pre-potential by opening potassium channels.
    
  • The Stellate Ganglion (is Sympathetic) is innervating to the SA node is from the Right Stellate ganglion – it increases cyclic AMP (but it doesn't do anything to the slope of the Pre-potential – if the slope does change it is due to the lack of parasympathetic stimulation).
    
• The Vagal Innervation of the AV node is from the left and the Stellate Ganglion of the AV node is from the left.  They are fighting over the AV node.
  
  • The AV node speeds or slows down the rate of conduction; it determines how quickly it passes the electrical activity to the Bundle of His
    
  • The AV node is always beat to the punch by the SA node – under parasympathetic control the SA node will slow the conduction rate of the AV node and under sympathetic it will increase the conduction rate of the AV node.
    
  • There is Parasympathetic and sympathetic innervation of atrial cardiac muscle cells.
    
  • Only the sympathetic system innervates the ventricular cardiac muscle cells.
    
    • In ventricular muscle cells it Catecholamines will increase cAMP.
      
      • This activates a Protein Kinase.
        
      • The activated Protein Kinase phosphorylates:
        
        • Calcium channels in the sarcolema
          
          • This increases the calcium intake into the cell – in cardiac muscle cell, some calcium comes in from the outside and some comes in from the sarcoplasmic reticulum + terminal cisterns – increasing available calcium.
            
        • Troponin – Tn-I – inhibits Tn-C from binding calcium – shortens length of time Calcium stays on Tn-C.
          
        • Phospholamban – increase calcium uptake by the sarcoplasmic reticulum so terminal cisterns have more.
          
      • The faster the heart beat, the more calcium that is going to come out, the faster the Calcium will be on the thing so you will have a shorter contraction.  This shortens the action potential and quickens the beat. It does not change the rate.
        
  • Parasympathetic fibers don't innervate the ventricles, any lengthening of the beat is just the lack of sympathetic stimulation.