A "small box" on the ecg strip typically has a duration of:

40ms (.04s)

A "large box" on the ecg strip typically has a duration of:

200ms (.2s)

The full length of an ecg strip represents a duration of:

10s

Describe 2 methods for estimating rate from an ecg strip:

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- The "300" method: count the number of large boxes between two successive R-waves. Divide 300 by this number to obtain the rate. Fast and easy, but imprecise and not effective for irregular rhythms
- The 10 second method: count the number of complexes in a single strip and multiply by 6. More time consuming but more precise and useful for irregualr rhythms

Assessment of the QRS axis usually only concerns its projection onto the ______ plane

frontal

*assessment of the projection of the QRS axis onto the transverse plane is described as assessing the R-wave progression or transition*

A QRS axis that is more negative than normal is referred to as:

Left Axis Deviation (LAD)

A QRS axis that is more positive than normal is referred to as:

Right Axis Deviation (RAD)

A QRS axis from +180° to +270° is referred to as:

"extreme axis" "superior axis" or "indeterminate axis"

Although not rigorously defined, a common guideline for normal range of QRS axis is:

-30° to +90°

What range of QRS axis is sometimes considered "physiologic LAD"?

0° to -30°

List and describe two non-pathologic factors which affect cardiac axis

Age: Generally rightwards axis at birth which drifts progressively leftwards with age

Body type: Tall and thin tends to have a vertical axis, short and obese more of a leftwards axis

Describe the "quadrant" approach to qualitative QRS axis determination.

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- Compare the predominant deflections of leads I and aVF.
- Use the following table to estimate the QRS axis:
- In the case of potential LAD, compare to lead II. If lead II is predominantly positive, then the LAD is physiologic and can be considered a normal axis. If II is negative, then pathologic LAD is present.

Use the quadrant method to qualitatively assess the QRS axis in the following scenarios:

A. LAD (pathologic)

B. RAD

C. Normal Axis

D. Extreme Axis

E. Normal Axis (Physiologic LAD)

Describe the Equiphasic approach to QRS axis determination

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- Find the most equiphasic (AKA most isoelectric) of the limb leads. The QRS axis will be 90° to this lead.
- Choose a lead that is 90° to the most equiphasic lead.
- If the QRS of this lead is predominantly positive, then this lead approximates the QRS axis.
- If the QRS of this lead is predominantly negative, then this lead is 180° to the QRS axis

Use the quadrant method and equiphasic method to qualitatively and quantitatively estimate the QRS axis of this ECG

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- By quadrant: I is -ve, aVF is positive, therefore RAD
- By equiphasic: Most equiphasic is I, 90° to I is aVF, aVF is positive. Therefore the QRS axis is approximately +90° (likely slightly more)

Use the quadrant method and equiphasic method to qualitatively and quantitatively estimate the QRS axis of this ECG

By quadrant: I is +ve, aVF is -ve or equiphasic, II is positive. Therefore normal or physiologic LAD.

By equiphasic: Most equiphasic is aVF, 90° to aVF is I, I is positive. Therefore the QRS axis is approximately 0° (likely slightly negative)

Use the quadrant method and equiphasic method to qualitatively and quantitatively estimate the QRS axis of this ECG

By quadrant: I is -ve, aVF is +ve. Therefore RAD

By equiphasic: Most equiphasic is II, 90° to II is III, III is positive. Therefore the QRS axis is approximately 150°.

By quadrant: I is -ve, aVF is -ve. Therefore Extreme (Superior) AD

By equiphasic: Most equiphasic is aVL, 90° to aVL is II, II is negative. Therefore the QRS axis is approximately -120° (240°).

What ECG phenomenon leads to an indeterminate axis?

All limb leads are equiphasic, often due to pulmonary disease pattern (COPD)