ECG Lab Electrocardiography- electrical changes that accompany the cardiac cycle Today you will use Bio Pac to record an electrocardiogram (ECG) and: ▫Analyze the results by measuring and calculating the amplitudes of the deflections, time intervals, and segments. ▫Determine heart rate.

Cardiac Cell Types Contractile myocardial cells (99% of cardiac muscle) are responsible for the mechanical pumping of the heart. Autorhythmic cells (1% of cardiac muscle tissue) of the conduction system are responsible for electrical stimulation of the myocardial cells. These cells spontaneously depolarize.

Intrinsic Conduction System SA node = 70 BPM AV node = 50 BPM AV Bundle (Bundle of His) = 35 BPM Bundle branches = 35 BPM Purkinje Fibers = 30 BPM

Action Potential in Autorhythmic Cells

Action Potential of Contractile Myocardial Cells

Electrocardiogram Deflections of the ECG: P wave: atrial depolarization from SA node toward the ventricles QRS complex: ventricular depolarization. Atrial repolarization also occurs during this phase but is masked by the large QRS T wave: ventricular repolarization

Time Intervals and Segments of the ECG: P-R interval: interval between the activation of the SA node and AV node P-R segment: interval between atrial depolarization and ventricular depolarization S-T segment: interval between end of S wave and start of T wave R-R interval: interval of time between successive cycles

Electrode Placement A lead is an arrangement of electrodes used in recording the electrical changes in the heart during a cardiac cycle. Bipolar Limb Leads: records changes in electrical potential between two electrodes placed on two limbs. ▫Lead I: Right arm (-) and Left arm (+) ▫Lead II: Right arm (-) and Left leg (+) ▫Lead III: Left arm (-) and Left leg (+)

Electrode Placement Unipolar Limb Leads: records changes in electrical potential in reference to a single exploratory (+) electrode placed on the limbs. ▫aVR: Right arm(+) and Left arm/Left leg(-) ▫aVL: Left arm(+) and Right arm/Left leg(-) ▫aVF: Left leg(+) and Left arm/Right arm(-)

Electrode Placement Unipolar Chest Leads: : records changes in electrical potential in reference to a single exploratory (+) electrode placed on an intercostal space. ▫V1 - V6 which are placed on various intercostal spaces

Abnormalities of the ECG Enlarged Q wave = myocardial infarction Enlarged P wave = enlargement of atria Lengthened P-R interval = scarring or inflammation of the heart muscle (as in rheumatic fever) Enlarged R = enlarged ventricles Enlarged ST segment = acute myocardial infarctions Flattened T wave = heart muscle is “ischemic”

Experimental Procedure Each group needs a 3-lead cable, 3 electrode pads, and alcohol swab. Clean the anterior surface of right wrist and the medial aspect of both ankles. Start the software by clicking on the “ECG” file on the desktop. Record a resting ECG for approximately 15 seconds. Measure the duration of the indicated waves, segments and intervals and record the time values in the first column (“Resting”) on the data sheet.

Measurements Sample selection for measuring P wave duration (Delta T) and amplitude (P-P)

Measurements Sample selection for measuring P-R Interval duration (Delta T).

Calculation for Heart Rate Given paper speed: 2.5cmMeasured R-R: 1.5cm 1 sec 2.5cm= 1.5cm 1 sec x sec 2.5x = x= 0.6 sec 1 beat x 60 sec = 60beats = 100beats/ min 0.6 sec 1 min0.6 min

Experimental Procedure Extending Your Knowledge Have your subject perform another activity, record an ECG, measure and record your data in the column labeled “Activity ”. Run a Lead I and Lead III ECG ECG practice measurements