Standard limb leads

The electrocardiogram can be obtaining using standard limb leads and/or precordial leads. A lead should be selected that is stable over time and that has a sharp demarcation at the end of the T wave to facilitate the measurement of the QT interval duration. One can also position a monophasic action potential electrode catheter through the femoral or carotid artery to obtain endocardial monophasic action potentials (see below Modification of the Method). 

Rabbits are anesthetized with methohexital and alpha-chloralose and ventilated mechanically. After establishing an intravenous port, the animals receive the alphai-agonist methoxamine at 15 fig/kg/min, a dose that should have only a slight effect on systemic arterial blood pressure. The ECG is measured using standard limb leads and an monophasic action potential... 

Q5 How does the ECG recorded from a standard limb lead correlate with the excitation processes in the heart ... 

To record a 12-lead ECG, place electrodes on the patient s arms and legs (with the ground lead on the patient s right leg). The three standard limb leads (I, II, and III) and the three augmented leads (aVp, aV, and aVp) are recorded using these electrodes. Then, to record the precordial chest leads, place electrodes as follows ... 

Fig. 3.10 Left Standard limb lead position. Right Modified limb lead positions...

To record a 12-lead electrocardiogram, place electrodes on the patient s arms and legs and a ground lead on the patient s right leg. The three standard limb leads (I,... 

Diuretics and their mechanisms of action will be discussed in detail in Chapter 21. Loop diuretics, such as furosemide (Lasix), block the Na" -K" -2CLsymporter in the ascending limb of the loop of Henle.The resultant effect is delivery of more Na" to the distal tubule and enhanced urinary loss of Na" and water. Unfortunately, the resultant increase in urinary excretion of and K+ can lead to arrhythmias. The potential for arrhythmias is exacerbated by the loss of Mg++ and Ca++ and an underlying vulnerability of the myocardium in CHF. However, loop diuretics are still part of the mainstay of therapy for CHF despite these potential problems and the absence of well-controlled multicenter clinical trials. The rationale for their use is so compelling that placebo-controlled studies appear unethical. Moreover, furosemide was accepted as the standard of care in all of the clinical trials that form the basis for recommended therapy for CHF. The use of the potassiumsparing diuretic spironolactone has been shown to improve survival and is discussed below. 

Heparin-induced thrombocytopenia (platelet count <150,000/ml or a 50% decrease from the pretreatment value) occurs in about 0.5% of medical patients 5 to 10 days after initiation of therapy with standard heparin. The incidence of thrombocytopenia is lower with low-molecular-weight heparin. Thrombotic complications that can be life threatening or lead to amputation occur in about one-half of the affected heparin-treated patients and may precede the onset of thrombocytopenia. The incidence of heparin-induced thrombocytopenia and thrombosis is higher in surgical patients. Venous thromboembolism occurs most commonly, but arterial thromboses causing limb ischemia, myocardial infarction, and stroke also occur. Bilateral adrenal hemorrhage, skin lesions at the site of subcutaneous heparin injection, and a variety of systemic reactions may accompany heparin-induced thrombocytopenia. The development of IgG antibodies against complexes of heparin with... 

Figure 10.4 ECG waveforms of the six standard extremity leads shown with the augmented leads interlaced between the bipolar limb leads (Cabrera sequence).

Einthoven s electrode placement has been adopted and standardized into the ECG Lead system known as the Einthoven triangle and is shown in Fig. 17.38. Electric potential differences are measured between the three limb electrodes along the line between the electrode placements, and their potentials are called lead I, n, and III such that... 

Other Lead systems are also possible and have been standardized into what is known as the 12-lead system that constitutes the clinical ECG. In addition to leads I, II, and m, there are three others known as the augmented leads aVR, aVL, and aVF. These leads use an electric addition process of the limb-electrode signals to create a virtual signal reference point in the center of the chest known as the Wilson central terminal (WCT). From the WCT reference point, the augmented lead vectors point to the right arm, left arm, and left leg, respectively. 

Normal 12-lead ECG recorded with standard lead positioning Same ECG with limb leads swapped over. Note positive aVR while lead I, II and aVL are now negatively deflected Misplaced chest leads causing a change in R wave progression Somatic muscle tremor in multiple leads, seen predominantly in leads II and III... 

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