hello and welcome back to chapter 17 part two electrophysiology of the cardiovascular emergency chapter okay so let's get started talking about electrophysiology the mechanical pumping action of the heart so the action can only occur in response to an electrical stimulus this impulse causes the heart to beat because of a series of complex chemical changes within the myocardial cells depolarization is the process of discharging resting cardiac muscle fibers by means of an electric impulse that stimulates contraction myocardial cells are bathed in electrical solution chemical pumps inside the cell maintain the concentration of ions which creates an electrical gradient across the cell membrane a polarized cell normally has a net charge of negative 90 millivolts when the myocardial cell receives a stimulation from conduction the permeability of the cell wall changes to allow sodium ions in this makes the cell more positive calcium ions also enter which helps maintain the depolarized state of the cell membrane this depolarization spreads along the cell until it is completely depolarized causing a mechanical contraction repolarization begins when the closing of the sodium and calcium channels to stop the inflow of these ions potassium channels open to allow the escape of potassium ions to help restore a negative charge to the inside of the cell sodium ions are pumped out of the potassium ions are pumped back into the cell re-establishing the proper electrolyte distribution after the potassium channel is closed the sodium potassium pump helps move sodium and potassium ions back to their respective locations which maintains the polarity of the cell membrane so let's talk about the cardiac action potential the action potential of a typical myocardial cell can be divided into five phases phase zero to phase four so we'll start with phase zero phase zero this phase begins when the cardiac muscle cell receives an impulse sodium moves into the cell through sodium channels causing the interior of the cell to become electrically positive relative to its exterior this results in a change in the trans membrane potential from negative 90 to about negative 70. at threshold still more sodium channels open allowing a rapid influx of sodium and a rapid raise in membrane voltage to about positive 30. at the same time calcium enters more slowly through calcium channels the influx of calcium causes the sarcoplasmic reticulum to release calcium from muscle contraction the cell depolarizes and begins to contract on the ecg the qrs complex represents phase zero phase one during this phase inward sodium channels close and the cell begins to repolarize negatively charged calcium i or chlorine chloride ions enter the cell outward potassium channels open briefly allowing potassium to leave the cell and resulting in decrease in the trans membrane potential phase two this phase is called the plateau phase it is the longest phase of the action potential during this phase sodium and calcium slowly enter the cell while potassium flows out of the cell the presence of calcium prolongs depolarization of the membrane it creates a plateau contraction ends when the outward flow of flow of potassium exceeds the inward flow of sodium and calcium phase two corresponds to the st segment on the ecg then you have phase three this is the final phase of repolarization slow calcium channels gradually close and calcium is transported out of the cell potassium channels open and a rapid movement of potassium out of the cell causes the trans membrane potential to become increasingly negative by the end of this phase the membrane potential has been restored to its resting value with repolarization complete the cell can now respond to a new stimulus on the ecg the t wave represents phase three and then you have phase four this phase called the resting phase represents the normal working myocardial cell at its resting membrane potential of negative 90. this figure shows the four phases of that stimulation okay so let's talk about refractory periods next this is the period during which the cell is depolarized or in the process of repolarizing it consists of two phases first half represents the absolute refractory period it's a period of time from phase zero to the middle of phase three in which the ventricles have not sufficiently repolarized to enable another depolarization the second half represents the relative refractory period and that's middle of phase three uh two about the beginning of phase four and which indicates that some cells have repolarized sufficiently to depolarize again so let's talk about the conduction system it's a specialized conduction tissues made up of specialized pacemaker cells this tissue conducts electrical impulses to the muscle muscular tissue in the heart the pacemaker is the area of conduction tissue in which the electrical activity arises it sets the pace for contraction the dominant pacemaker is the sa node the sa node lies at the junction of the superior vena cava and the right atrium it receives blood from the rca in about 0.08 seconds electrical impulses generated in the essay spread across the the atria and advanced through three international pathways the anterior international pathway is a branch that forms a pathway between the sa and av nodes the middle internodal tract is formed by a winky block track and the third is represented by the posterior inner nodal pathway the av node is a group of cells located in the floor of the right atria between the tricuspid valve near the opening of the coronary sinus when the impulse from the sa node enters the av node it is delayed for about 0.12 seconds before is relayed through the rest of the conduction system this allows the atria to empty blood into the ventricles about 70 to 80 percent of the blood in the atria fills the ventricles by gravity the remaining 20 to 30 comes from the atrial contraction the av junction including the a v e node surrounding tissue and the bundle of hiss also called the av bundle conducts impulses from the av junction to the right and left bundle branches the av junction is a gatekeeper to the ventricles the figure on this slide shows the electrical conduction system of the heart you can see that the sa node up here and what we just talked about the electrical impulses going down to the av node then slowly coming down to the right and left bundle branches normally impulses pass through the av junction into the bundle of hiss and then move rapidly into the right and left bundle branches on both sides of the intraventricular septum if the atrial rate becomes very rapid then the av junction can regulate the number of impulses that reach the ventricles they then spread into the purkinje fibers an electric impulse spreads across the ventricles at about 0.08 seconds while the ventricles simultaneously contract okay so you also have secondary pacemakers and any conduction system component can act as a secondary pacemaker if the sa node becomes damaged or suppressed the farther removed from the sa node the slower the intrinsic rate of firing will be if the sa node is damaged the av junction might begin firing at its own rate if the sa and av nodes are not working the purkinje fibers will initiate an impulse accessory conduction pathways so the accessory pathway or bypass track is extra heart muscle tissue that connects the atria and ventricles by bypassing the av node you have james fibers and that's in the atrial internal pathway it extends into the ventricles by bypassing the av node then you have the maham fibers in the av node the bundle of his and the bundle branches they extend into the ventricles and provide a common pathway for reentry dysrhythmias you also have the bundle of kent this is an accessory pathway typically located between the left atrium and left ventricle although it is sometimes found between the right atrium and the right ventricle it enables the depolarization wave to bypass the av node and trigger early depolarization of a second section of the ventricle tissue the accessory pathway can trigger abnormally fast heart rates and these are tachydysrhythmias medical intervention is often required the autonomic nervous system of the heart so effect on the heart by the sympathetic and parasympathetic divisions of the autonomic nervous system so first we're going to talk about the sympathetic and that's an accelerator now nerve supply specific areas of the heart's electrical system the atrial muscle and the ventricular myocardium nerves transmit commands by releasing norepi norepi travels to the sa node the av node and the ventricles by spreading the signal from the sympathetic nerves to prevent a buildup of lactic acid the heart speeds up increasing co and thereby distributing more oxygen and nutrients throughout the body an accelerated heart rate shortens all phases of the cardiac cycle so the cardiac cycle is the period from one cardiac contraction to the next when the ventricles have less time to relax less time is available for these chambers to fill adequately with blood less blood is then sent to the coronary arteries and less blood is pumped out of the ventricles co decreases and signs of myocardial ischemia may appear ischemia usually occurs because of narrowing or occlusion of an artery this figure shows the sympathetic and parasympathetic nerve fibers and their end organs now let's talk about the parasympathetic nerve fibers they're simply they supply the sa node atrial muscle and the av junction of the heart by way of the vagus nerve the vagus nerve it's also called the cranial nerve it decreases the heart rate and can be stimulated in a number of ways including increased pressure on the carotid sinus or force exhalation against a closed glottis or distension of a hollow organ such as a bladder or stomach if the brain senses that the heart should slow its pace a message in an electrical impulse travels down the vagus nerve to where the nerve inhibits the sa node the electrical impulse stimulates the release of acetylcholine acetylcholine crosses over the sa node and signals the node that the brain wants the heart to decelerate another acetylcholine molecule travels to the av node of the heart as a reminder to the sa node and ensuring that no additional impulses get through acetylcholine is then broken down by acetylcholiners atropine opposes the action of a cetacola nurse thereby accelerating the heart rate the figure shows an example of an electrical impulse that stimulates the release of a chemical baroreceptors and chemoreceptors are what we're going to talk about next and baroreceptors are sensors composed of specialized nerve tissue also known as or presser receptors they're found in internal carotid arteries and on the aortic arch they detect changes in blood pressure and they generate a reflex response in either the sympathetic or the parent sympathetic divisions of the ans sympathetic or adrenergic responses if the blood pressure falls the sympathetic division orchestrates compensatory responses so it constricts peripheral blood vessels increases the heart rate and increases the force of myocardial contraction the parasympathetic old cholinergic response so if blood pressure rises the sympathetic stimulation is decreased and the response by the parasympathetic division is increased chemoreceptors chemoreceptors are located in the internodal carotid arteries the aortic arch and the medulla detect changes in the concentrations of the blood hydrogen ions oxygen and carbon dioxide the ans may mount either a sympathetic or a parasympathetic response to such changes okay so let's talk about causes of cardiac dysrhythmias cardiac dysrhythmias is a disturbance in the normal cardiac rhythm which may or may not be clinically significant cardiac rhythm disturbances or dysrhythmias may be from various causes a paramedic needs to evaluate dysrhythmias in context with the overall condition which should determine if treatment is necessary the electrocardiogram so this is a graphic record of the changes in voltage that occur in the heart muscle during depolarization and repolarization and it can be used to continuously monitor cardiac rhythm during transport print out a rhythm strip for dysrhythmia interpretation and print out a 12 lead ecg for specific diagnosis three standard limb leads are used so leads one leads two and leads three they're used during transport to identify changes in the heart rhythm tracings from lead 2 are usually most useful when monitoring limits to areas of the heart that can be viewed with three leads now more detailed information from a 12-lead ecg can be taken and lead wires with electrodes placed on the patient each lead creates an electrical snapshot of a part of the heart the monitor records an ecg tracing for each lead which is then reviewed for findings okay so let's talk a little bit about the electrode placement the electrodes must be placed in a consistent predetermined place to get a reliable reading electrodes in the pre-hospital setting are usually adhesive with a gel center for better skin contact although some have a diaphoretic electrode to better stick to patients who are sweating basic principles should be followed for best skin contact and to minimize artifact in the single it may be necessary to shave the patient's body hair at the electrode site to remove oil and dead tissue from the surface of the skin rub the electrode site briskly with a dry gauze pad attach the electrodes to the ecg cables before placement and confirm the appropriate electrode attached to the cable is placed at the correct location on the patient once the electrode is in place turn on the monitor and print a rhythm strip to check for artifact artifact on the monitor can give false readings and to properly perform a cardiac monitor refer to skill drills 17-1 okay so let's talk a little bit about the leads the two main groups of leads include limb leads and precordial leads the augmented limb leads or avr avl or avf contain only one true pole the other is a combination of information from the other leads a standard 12 lead ecg is made up of three standard limb leads three augmented limb leads the six pericordial leads a lead wire is the electrical cable that attaches an electrode to the ecg monitor a lead is an image of the heart that measures the difference in electrical potential between two electrodes a lead axis is an imaginary line joining the positive and negative poles of a lead the position of the positive electrode determines which area of the heart is viewed by each lead frontal plane leads or leads one two and three if you the heart from the front of the body the precordial leads v1 to v6 are called unipolar leads and anterior leads or v-leads they view the heart in the horizontal plane provide images of the heart from the front or the interior wall of the heart and from the side or the anterior lateral view limb leads were initially discovered by wilhelm endhoven and hovind discovered that the heart emits electrical energy every time it contracts these energy waves could be recorded plotted and assigned the letters p q r s t to the ecg deflections he initially recorded three leads lead one is attached to the right and left arms lead two running between the right arm and the left leg and lead three running between the left arm and the left leg leads one two and three are bipolar leads bipolar leads are those that contain a positive and a negative pull with the standard limb leads each lead measures the difference in electrical potential between electrodes placed on two extremities the augmented voltage or av leads are you also created using the four limb electrodes leads avr avl and avf are created by combining two of the limb leads and using the other lead as the other pole example the lead avr is created between the right arm and the combination of the left arm and the left electrodes the augmented leads are unipolar that is they contain only true one true pole while the other end of the lead is referred against a combination of other leads example the lead avr is at the right arm it's reference against a combination of the left arm and the left leg if you were performing continuous cardiac monitoring then place four electrodes on the patient's torso so the white is the right upper chest near the shoulder the black is the left upper chest near the shoulder red is the left lower abdomen and green is the right lower abdomen if you are acquiring a 12 lead then place the four electrodes on the patient's limbs the white is on the right wrist black is on the left wrist red is on the left ankle and green is on the right ankle placing these four electrodes allows the ecg device to record all six limbs leads using enthovens theory the green leaf serves as a ground and all cases and is electrically neutral precordial leads view the heart in the horizontal plane the perichordial leads v1 through v6 are unipolar they are referred or referenced against a calculated point known as the wilson central terminal the wilson central terminal is created by bisecting the limb leads in in hovan's triangle the electrode for each unipolar lead is the positive terminal for that lead so v1 and v2 view the septum v3 and v4 look at the anterior wall of the left ventricle v5 and v6 view the lateral wall of the left ventricle the electrode for each unipolar lead is the positive terminal for that lead compare the ecgs you take with previous ecgs it is critical that these leads are placed consistently v1 is goes to the fourth intercostal space to the right of the sternum v2 fourth intercostal space to the left of the sternum v3 directly between v two and four v4 is the fifth intercostal at the left mid clavicular line and v5 at the level of lead four at the left anterior axillary line and v6 is at the level of the lead v4 at the mid axillary line for contiguous leads refer to leads that view geographically smaller areas of the myocardium which can be useful for localizing areas of ischemia injury or infarction leads 2 3 and avf are contiguous leads v1 and v2 v2 and v3 v3 and v4 v4 and v5 and v5 and v6 are pairs of contiguous leads leads 1 in avl and avl and 5 are also contiguous so posterior leads are used to evaluate left ventricular posterior wall electrical activity there are three precordial leads and they are placed in the left posterior thorax and that's a v7 v8 and v9 uh v7 is between the v6 and v8 fifth intercostal the eight is going to be mid-scapular fifth intercostal and then v9 is just above the left just to the left of the spine fifth intercostal space okay so use 15 and 18 lead ecgs as follows so the 15 lead ecg uses the standard 12 lead ecg plus leads v4 r1 and v7 and v8 so this allows the paramedic to view the right ventricular and posterior wall of the left ventricle it's used to detect ischemia or infarction and it involves recording a second tracing containing the additional leads the 18 lead ecg uses a 12 lead tracing plus leads v4r through v6r and v7 through v9 to obtain an 18 lead record a standard 12 and record the right sided pericardial leads and then the record the posterior leads so let's talk about some of these ecg concepts the ecg baseline is generally flat straight horizontal line that reflects a period of electrical silence in the myocardium think of the baseline as a period of electrical neutrality neither positive nor negative this is referred to as the isoelectric line the tp segment or the isomeric line an electrical impulse moving in the direction of a negative electrode produces a deflection below the baseline and an electrical impulse moving towards a positive electrode produces a deflection above the baseline so perpendicular movement of an impulse towards a positive electrode produces either the following a perfectly flat line or a waveform with both a positive and negative component biphasic waves is what we call those okay so next we're going to talk about ecg paper and ecg's are recorded on graph paper moving past a stylus at a constant speed and one millimeter box equals 0.046 seconds or 40 milliseconds while one large box and this the large box consists of five small boxes and they equal 0.20 seconds or 200 milliseconds the vertical axis represents the amplitude and the standard amplitude calibration is 10 millimeters per millivolt okay and so this figure shows the electrocardiogram paper and as we talked about on the last slide you could see the small box and then five of the small boxes make up the big box a calibration box is printed at the beginning of the 12 lead ecg it forms a paramedic about the paper speed and the amplitude and measures 5 millimeters wide and 10 millimeters tall so let's talk about the ecg components the electrical conduction event in the heart can produce a record on the ecg as a series of waves segments intervals or complexes a p wave represents the atrial depolarization it's characterized by a smooth round upright shape normal duration of less than 0.11 seconds and an amplitude less than 2.5 millimeters tall so we're going to go back to this slide and i'm going to show you guys this is the p wave that's what we're talking about right there okay the pr interval is the distance from the beginning of the p wave to the beginning of the qrs complex and there's a great photo on the slide right there of that the distance represents the time required for an impulse to transverse the atria in the av junction normally it's 0.12 to 0.2 0.20 seconds the pr segment represents the amount of time the av node delays transmission of atrial activity to the ventricles when the av node is disease or hypoxic the pr segment begins becomes elongated the qrs complex consists of three waveforms and represents ventricular depolarization it is measured from the beginning of the q wave to the end of the s wave and should follow each p wave consistently it is narrow in healthy people with a duration of less than 0.11 seconds or less it indicates that conduction of the impulse has proceeded normally from the av junction through the bundle of his left and right bundles and the purkinje fibers if the impulse conduction is abnormal then the complex has a bizarre appearance and the duration of 0.12 seconds or longer i'm going to show you guys the qrs complex and so this is the qrx complex that we're talking about right there the first negative deflection is the q wave which represents conduction through the intraventricular septum it should not last more than 0.04 seconds and it should be less than one-third the height of the qrs complex it's considered abnormal or pathologic if it does not mean meet normal criteria and it could indicate an ami the first upward deflection is the r wave the s wave is any downward deflection after the r wave a second upward deflection is called an r prime wave the r and s waves represent depolarization of the right and left ventricles as the current moves towards the lead it creates a positive or upright deflection on the ecg lead of that tracing lead lead avr will be inverted because the impulse is moving away from it an example if you and a friend are facing each other at opposite ends of a football field then a ball thrown towards your friend will appear bigger as it approaches the friend however the ball the same ball will appear smaller to you the j point is the point on the ecg where the qrs lead complex and the st segment begins there's a good picture on the slide right there it represents the end of depolarization and the apparent beginning of repolarization often depresses or elevates as an ischemic myocardium the st segment begins at the j point and ends at the t wave and represents early ventricular repolarization any elevated st segment may indicate myocardial injury any depressed st segment may indicate myocardial ischemia the t wave represents ventricular repolarization it should be asymmetric less than half the overall height of the qrs complex and oriented in the same direction as the overall qrs complex very large or hyper acute t waves may indicate myocardial ischemia injury and infarction tall peaked t waves may be seen with hyperkalemia and that is excess potassium in the blood deeply inverted t waves may be seen with acute cns events such as intracranial hemorrhage or a massive stroke the u wave most likely represents the final stable stage of ventricular repolarization a u wave taller than two millimeters is considered abnormal and may be a sign of hypokalemia or a low concentration of potassium in the blood or cardiomyopathy the qt interval represents all the electrical activity of one completed ventricular cycle it begins with the onset of the q wave and ends with the t wave it varies in age sex and heart rate it normally lasts between 390 to 460 milliseconds a long qt interval may lead to ventricular dysrhythmias and sudden cardiac arrest the tp segment is generally flat straight horizontal line between and at the end of the t wave and ending at the start of the p wave the baseline is a reference part to compare with the j point the rr interval represents the inter interval between two ventricular depolarizations it can be used to calculate heart rate and determine regularity of the patient's cardiac rhythm okay so this concludes part two of the cardiovascular emergencies lecture join us for part three and that's going to consist of and pick up on the dysrhythmia interpretation and treatment and we hope that you've enjoyed this lecture and thank you