ECG

Introduction

The session aims to enhance understanding of electrocardiograms (ECGs) through interpretation and problem-solving exercises. Participants are encouraged to engage actively and ask questions related to the provided ECGs.

Basics of ECG Recording

Heart Orientation and Depolarization

• The heart is located roughly in the center of the chest. Depolarization starts in the right atrium and spreads across the heart.

• Lead II Placement: Typical lead II ECG is made from the right forelimb to the left hind limb, resembling a vector from the right shoulder to the left hip.

• Positive pole of Lead II indicates that deflections toward it generate a positive ECG deflection, while those away create a negative deflection.

Electrical Activity of the Heart

• Normal electrical activities originate in the sinoatrial node, causing depolarization that travels from right to left across the atria. It results in a small positive deflection on the ECG due to atrial depolarization.

• A delay occurs at the atrioventricular node to allow atrial contraction to precede ventricular contraction, establishing a direct link between the P wave and the advent of the QRS complex, typically with a 100 ms interval in dogs.

Ventricular Depolarization

• The ventricular depolarization sequence follows through:

  1. Atrioventricular node to the bundle branches.

  2. Initial septum depolarization generates the small Q wave (negative deflection), followed by the positive R wave, and concluding with a possible small negative deflection, forming the S wave.

• The QRS complex is, therefore, the representation of ventricular depolarization regardless of its components.

Electrical Inactivity and Repolarization

• Post depolarization, the heart enters a phase of electrical inactivity that corresponds to the ST segment before repolarization occurs, represented by the T wave. It is significant that T waves are always present as they indicate the heart returning to baseline electrical function.

Analysis of ECGs

Summary of Scores

• Average performance in ECG assessments was recorded in the 70s, indicating a satisfactory understanding of the material among participants, despite its complexity.

ECG Interpretation Challenges

• ECG signals can differ based on the ECG machine's settings, including vertical calibration (5mm/mV) and horizontal paper speed (25 mm/sec). It is essential to recognize these indicators for accurate interpretation.

• Inquiries arose regarding heart rate calculation, typically done via the number of QRS complexes within a designated time frame. For example, counting complexes within a ten-second strip.

Practical Heart Rate Calculation Techniques

1. Count the QRS complexes in a 10-second interval and multiply by 6 for approximate heart rate per minute. Variations are expected, indicating that heart rates can change over time.

2. An alternative method for determining instantaneous heart rate is:
   $ext{Heart Rate} = rac{1500}{ ext{R-R interval in mm}}$
   This method calculates heart rate based on the distance between consecutive QRS complexes.

Rhythm Analysis: P Waves and QRS Associations

• For accurate interpretation, ascertain whether a P wave precedes each QRS complex and whether there is a QRS for every P.

• A spacing or absence of a P wave before a QRS complex implies that the ventricular contraction is not initiated by atrial depolarization. Such cases can lead to various explanations:

  • Ventricular Premature Beats (VPBs): occur spontaneously, not reliant on atrial depolarization.

  • Supraventricular Premature Complexes: can arise within the atrioventricular node but might not have an observable P wave.

  • Atrial Standstill or Atrial Fibrilation: typically, these conditions lead to a complete lack of P waves.

Rhythm Disturbances - Interpretation Examples

First ECG Analysis

• Segments of the ECG show variances in QRS counts leading to heart rate approximations between 120 to 140 bpm according to different segment counts.

• Key learning points are the shifts in heart rate, interpretation through mean values, and the understanding that heart rates can fluctuate.

Second ECG Analysis

• A consistent presence of P waves before QRS complexes suggests a functioning atrial rhythm concluding the heart's normal conduction pathway.

• Identification of a second-degree Atrioventricular (AV) block leads to further inquiry into potential physiological causes.

Second-Degree AV Block Explanation

• Contrasting with first-degree, second-degree can intermittently fail conduction leading to some P waves not succeeding in producing QRS complexes. This phenomenon may points towards conditions like acute therapy effects or physiological adaptations post-exercise, often considered normal in certain conditions.

Challenging Third ECG Interpretation

• The complexity of ECG in a cat highlights how subtle waveforms can alter the decision-making process.

• Interpretation strategies include magnifying particular segments to ascertain P wave presence relative to QRS complexes, which may indicate independent atrial and ventricular activity, suggesting third-degree AV block.

Last ECG Observations

• The analysis concludes by stating that the observed irregularities might relate to possible atrial fibrillation or other potential arrhythmias defined by the presence or absence of identifiable P and QRS complexes.

In summary, a solid understanding of the relationship between P waves and QRS complexes defines overall heart rhythm and can illuminate underlying cardiac issues.