1、The Cardiac Vector and Electrical Axis of the Heart1.2.3.4.Deplaorization Sequence of a “Strip” of 5 Myocardial CellsSlide 1Depolarization progressing from left to rightSlide 2The Electrical System of the HeartVetricular Depolarization and the Inscription of the QRS complexVentricular Repolarization
2、 and the Inscription of the T-wave+-RARALL+-LALLLALEAD IILEAD ILEAD IIIRemember, the RLis always the ground By changing the arrangement of which arms or legs are positive or negative, three unipolar leads (I, II & III ) can be derived giving three pictures of the hearts electrical activity from 3 an
3、gles.The Concept of a “Lead”Leads I, II, and IIIIIIIIIThe Concept of a “Lead”+-RARA & RLLL & LA+-LLRA & LALALEAD AVR-LEAD AVLLEAD AVF By combining certain limb leads into a central terminal, which serves as the zero-potential electrode, other leads can be formed to fill in the gaps in terms of the a
4、ngles of directional recording. These leads require augmentation of voltage to be read called augmented voltage (aV) leads.Augmented Voltage leads aVR, aVL, and aVF0oLEAD AVRLEAD AVLLEAD AVFLEAD IILEAD ILEAD III60o90o120o-30o-150o Each of the limb leads (I, II, III, aVR, aVL, aVF) can be assigned an
5、 angle of clockwise or counterclockwise rotation to describe its position in the frontal plane. This arrangement called hexaxial array.The Concept of a “Lead”Summary of the “Limb Leads”Introduction:Genesis of Normal ECG ComplexFrom: http:/ of Normal ECG Complex0oLEAD AVFLEAD IILEAD ILEAD III60o90o12
6、0o-30o-150oSee handout showing sequential activation of atria and ventricles, with corresponding ECG recording in 6 different frontal plane leads. Note the amplitude and direction of the ECG waves vary with the electrode location on the body.Hexaxial Array for Axis Determinationdetermination of the
7、angle of the main cardiac vector in the frontal plainVector Vector represents magnitude & direction of force; polarity if electrical force.+Vectors Summed to Single Resultant Vector Vectors may be translated in space to common origin without changing magnitude or direction. Now called Component Vect
8、ors, and may be summed by parallelogram method to produce Resultant Vector.=Translate=SummedResultant VectorHexaxial array and ECG vectors from various leads.1. Find net + or QRS in lead 12. Find net + or _ QRS in Lead aVF3. Resultant Vector. This is Mean Electrical Axis of Heart or Cardiac Vector.E
9、lectrical axis is about +60oLead IIf lead I is mostly positive, theaxis must lie in the right half ofof the coordinate system; the main vector is moving mostly toward the leads positive electrode.Axis Determination Quick Locate Step 1If lead aVF is mostly positive, theaxis must lie in the bottom hal
10、f ofof the coordinate system; the main vector is moving mostly toward the leads positive electrodeLead aVFAxis Determination Quick Locate Step 2Axis Determination Quick Locate Step 3IaVFCombining the two plots, we seethat the axis must lie in the bottomright hand quadrant I aVF aVLAxis Determination
11、 Quick Locate Step 4Once the quadrant has been determined, find the most equiphasic (smallest net deflection) or smallest limb lead. The axis will lie about 90o away from this lead. Example above; aVL is the most equiphasic lead. Axis must be about 90o from this lead; here shown to be approximately
12、60o.Axis Determination Quick Locate Step 5Since QRS complex in aVL is a slightly more positive, the true axis will lie a little closer to aVL (the depolarization vector is moving a little more towards aVL than away from it). A better estimate would be about 50o (normal axis). Accuracy + or 15o. I aV
13、F aVLAxis Determination Example 2Lead IIf lead I is mostly negative, theaxis must lie in the left half ofof the coordinate system.Axis Determination Example 2Lead aVFIf lead aVF is mostly positive, theaxis must lie in the bottom half ofof the coordinate systemIaVFCombining the two plots, we seethat
14、the axis must lie in the bottomright quadrant (from heart perspective). Axis Determination Example 2Axis Determination Example 2 I aVF IIOnce the quadrant has been determined, find the most equiphasic or smallest limb lead. The axis will lie about 90o away from this lead. Given that II is the most e
15、quiphasic lead, the axis here is at approximately 150o.Axis Determination Example 2 I aVF IISince the QRS in II is a slightly more negative, the true axis will lie a little farther away from lead II than just 90o (the depolarization vector is moving a little more away from lead II than toward it). A
16、 better estimate would be 160o.Since Lead III is the most equiphasic lead and it is slightly more positive than negative, this axis could be estimated at about 40o.Precise calculation of the axis can be done using the coordinate system to plot net voltages of perpendicular leads, drawing a resultant
17、 rectangle, then connecting the origin of the coordinate system with the opposite corner of the rectangle. A protractor can then be used to measure the deflection from 0. Net voltage = 12Net voltage = 7Precise Axis CalculationNormal AxisNormal Axis = -30 to +120RAD =+120 to +180LAD = -30 to -90LADAn
18、terior HemiblockInferior MIWPW right pathwayEmphysemaRADChildren, thin adultsRVHChronic Lung DiseaseWPW left pathwayPulmonary emboliPosterior HemiblockNo Mans LandEmphysemaHyperkalemiaLead TranspositionV-TachNo Mans Land Axis = -90 to +- 180Practice Exercise Use personal ECG to determine axis. Fill in the ECG Interpretation worksheet for axis.
