Twelve-Lead Electrocardiography phần 3

Hình . Ba dấu hiệu của nhồi máu cơ tim cấp tính, bao gồm cả độ cao đoạn ST, sóng T đảo ngược, và sự hình thành sóng 2,9. Vô cùng cao, sóng T chỉ nhìn thấy tăng kali máu. Làn sóng T tương ứng với tái cực của tâm thất. | 28 Chapter 5 Electrical Axis Shifted Toward the Taller of the Two Tallest R Waves But what if the two tallest R waves are not of equal height what if one is slightly taller than the other In this instance we need to shift our estimation of the axis slightly more toward the taller of the two R waves. For example if our patient above had an axis of 70 degrees instead of 75 degrees then the vector would be going slightly more toward II than toward aVF and lead II would therefore have a slightly taller R wave than aVF Figure . A word of caution here. This method works only when the axis is in the right lower quadrant between 0 and 90 degrees leads I and aVF . The reason for this can be seen by examining the tracing in Figure . Note that lead aVL has the tallest R wave and lead I has the next tallest R wave. If we use only the method of placing the axis between the two tallest R waves and then shifting toward the taller of the two we would estimate the axis to be approximately -20 or -25 degrees. However if we look for an isoelectric lead we find that lead aVR is equally biphasic. That would place the axis at -60 degrees. In this case both leads I and aVL would be expected to be positive with lead aVL taller than I because the vector is going more directly toward lead aVL than toward lead I indeed when we look at the tracing we find that to be the case. Thus when outside the right lower quadrant of the hexa-xial reference system we cannot reliably use alone the method of placing the axis between the two tallest R waves and then shifting it toward the taller of the two. Putting It All Together After a little practice in determining axis you will find yourself able to do it without consciously thinking about it and naturally using all of the above methods in a very rapid sequence. A very quick glance for the tallest R wave establishes the general direction of the axis. Looking for an equally biphasic QRS is usually the next natural step followed by a quick .

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