Physiology Ch 11 The Normal Electrocardiogram Notes Normal ECG-P wave- caused atrial depol, just before contraction= depolarization wave -atrial repol happens during QRS complex and is not seen on ECG -(0.15-0.20sec after end of p wave) -QRS complex- technically, 3 separate waves (Q, R and S)- vent depol, before contraction= depolarization wave -T wave- vent repol, approx 0.25-0.35 sec after depol= repolarization wave - vents remain contracted from end of QRS, until end of T wave When muscle is in complete depol or in complete repol, the ECG is at baseline, the spikes and waves away from baseline show that there is movement of the impulse. Voltage and Time Calibration of ECG Standard Calib: -Horizontal lines- 10 small line divs up(+) or down(-) = 1 millivolt -Vertical lines- time; paper speed of 25mm/sec 25mm=1sec 5mm segments=0.20 sec (dark lines) further split into 5 more boxes=0.04sec -Normal Voltage-When recorded on the arms or leg: -QRS complex= 1.0-1.5mv from top of R to bottom of S -P wave= 0.1-0.3mv -T wave=0.2-0.3mv P-Q or P-R interval- the time btwn the start of the P wave and the start of the QRS complex. Btwn atrial and vent. excitation Q-T interval- period of vent contraction HR as determined by ECG- the reciprocal of the time interval btwn two successive heart beats. Methods for recording ECGs Recorders: these days, computer based systems; used to be moving paper with pen Different pen types: those connected to an inkwell on one side; those that are made hot and write on paper that turns dark when heated; others turn dark when exposed to electric current and the “pen” emits current to an electrode on the back. Seriously why do we need to waste time reading that? [/Pointless info] Flow of Current around the Heart during the cardiac cycle (+)charge on exterior, (-) charge on interior upon depol, (-) charge leaks out of cell, making exterior surface (-) -meter with (-) end in depol’d area (also –) and (+) end in non-stimulated area= reads =(+) -meter with (+)end in depol’d area(-) and (+)end in non-stim=reads (-) -meter with (-) and (+) ends in non-stim area = reads 0 (zero) Flow of Electrical Currents in the chest around the heart Heart suspended in conductive medium. The average current flow occurs with (-) toward the base of the heart and (+) toward the apex Just before depol ends, the average direction of the current flow reverses for 0.01sec -In normal, healthy heart, current flow from (-) to (+) mostly from base to apex during all of depol, expect the very end ECG leads 3 bipolar leads: “bipolar”: means from two diff sides of the heart “lead”: not a single wire, but a combination of two wires and their electodes to make a complete circuit btwn the body and the electrocardiograph. Lead I= (-) on R arm, (+) on L arm Lead II= (-) on R arm, (+) on L leg Lead III= (-) on L arm, (+) on L leg Einthoven’s Triangle- drawn around the heart… points downward Einthoven’s Law- if the electrical potentials of any 2 of the 3 bipolar leads in known, the 3rd can be figured out mathematically by summing the first 2. When diagnosing arrhythmias, it does not matter which lead is used When diagnosing damage in the vent of atrial muscle, or perkinje system- it does matter bc abnormalities change the patterns in some leads, but not others. Chest Leads (precordial leads) V1-V6 (+) electrodes at 6 places on the chest, near the heart (-) electrods connected at equal resistances in the RA, LA, and LL at the same time. Called the “indifferent electrode” Leads V1-2= mostly (-), closer to base of heart Leads V4-6= mostly (+), closer to apex of heart Augmented Unipolar Limb Leads Another system of leads, two of the limbs are connected to the (-) terminal, rd and the 3 limb is connected to the (+) terminal. When (+) on RA= aVR- this reading is inverted When (+) on LA= aVL When (+) on LL= aVF