The T Wave

After an electrical impulse is initiated and conducted, there is a period of time during which cells and fibers cannot be depolarized again. This period of time is referred to as the absolute refractory period (Fig. 6-2),2 and corresponds to phases 1,2, and approximately half of phase 3 repolarization on the action potential. The absolute refractory period also corresponds to the period from the Q wave to approximately the first half of the T wave on the ECG (Fig. 6-2). During this period, if there is a premature stimulus for an electrical impulse, this impulse cannot be conducted, because the tissue is absolutely refractory. 

Figure 13.4 Electrocardiogram. The electrocardiogram (ECG) is a measure of the overall electrical activity of the heart. The P wave is caused by atrial depolarization, the QRS complex is caused by ventricular depolarization, and the T wave is caused by ventricular repolarization.

Second, the area under the curve of the P wave is small compared to that of the QRS complex. This is related to the muscle mass of the chambers. The ventricles have significantly more muscle than the atria and therefore generate more electrical activity. Furthermore, although it may not appear to be the case given the spike-like nature of the QRS complex, areas under the QRS complex and the T wave are approximately the same. This is because these recordings represent electrical activity of the ventricles even though one is caused by depolarization and the other by repolarization. Either way, the muscle mass involved is the same. 

The earliest ECG change (serum potassium 5.5 to 6 mEq/L) is peaked T waves. The sequence of changes with further increases is widening of the PR interval, loss of the P wave, widening of the QRS complex, and merging of the QRS complex with the T wave resulting in a sine-wave pattern. 

The QT interval (measured from the beginning of the Q wave to the end of the T wave of the surface electrocardiogram) reflects the duration of individual action potentials in cardiac myocytes (Figure 3.1) indeed, a prolongation of the action potential duration (APD) of myocytes will result in a prolonged QT interval. 

Changes in heart rate require correction different formulas may optimize correction in different species Definition of the end of the T wave is problematic in some species such as the dog, having a variable morphology of the T wave... 

Measurement of QT interval Definition of the end of the T wave. Changes in T wave morphology and occurrence of U waves (these may be important warning signs and precede the occurrence of TdP) Errors in manual measurement in QT interval Variability in the heart rate (need to correct the QT value for heart rate) Lack of reliable correlation between readings from Holter recordings and standard ECG Lack of standardization of automated ECG readings (computerized methods are often unreliable) Need for a central core laboratory to analyze data... 

Figure 4.2 Cartoon representation of an ECC trace and ventricular cardiac action potential, (a) A representation of an ECC trace with its five typical deflections (PQRST) arising from the spread of electrical activitythrough the heart. The QRS wave denotes the ventricular depolarization, while the T wave represents ventricular repolarization. The QT interval therefore estimates the duration of a ventricular action potential, (b) Schematic of the five phases of a ventricular action potential. Phase 0 is the rapid depolarization phase due to a large influx of Na+ ions (Ina). Phase 1 occurs with the inactivation of Na+ channels and the onset of transient outward (repolarizing) currents (/to)...

ECG changes Rarely, a large negative amplitude of the T wave may encroach upon the ST segment, but the ST segment is not independently altered. These changes usually disappear with continuance of treatment and revert to the pretreatment state if therapy is discontinued. 

Benign flattening of the T wave on the electrocardiogram occurs in 20%-30% of patients taking lithium (Bucht et al. 1984). In addition, lithium may suppress the function of the sinus node and result... 

Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG). Sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje cells display pacemaker activity (phase 4 depolarization). The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart. The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization, and the T wave by ventricular repolarization. Thus, the PR interval is a measure of conduction time from atrium to ventricle, and the QRS duration indicates the time required for all of the ventricular cells to be activated (ie, the intraventricular conduction time). The QT interval reflects the duration of the ventricular action potential.

The ECG consists of the P-wave, the QRS complex, and the T-wave. These components, represented in Figure 4.2, are associated with different aspects of the cardiac cycle atrial activity, excitation of the ventricles, and repolarization of the ventricles, respectively. 

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). 

Atrial depolarization results in the P wave of the ECG, the QRS complex denotes ventricular depolarization and the T wave represents ventricular repolarization. 

Disequilibrium in the eleetrolyte balanee ean provide diagnostic clues. For example, hyperkalemia causes tail T-waves in leads II, III, V2 to V4, when the potassium balance exceeds 5.5 mmol/1. In conjunction, the amphtude of the P wave is reduced and QRS is widened. Hyperkalemia is usually present when the amphtude of the T-wave is higher than that of the R-wave. With increasing potassium concentration, P-waves widen and eventually disappear. Accentuated hyperkalemia results in asystole. 

These are common after myocardial infarction. Their particular significance is that the R-wave (ECG) of an ectopic beat, developing during the early or peak phases of the T-wave of a normal beat, may precipitate ventricular tachycardia or fibrillation (the R-on-T phenomenon). About 80% of patients with myocardial infarction who proceed to ventricular fibrillation have preceding ventricular premature beats. Lignocaine (lidocaine) is effective in suppression of ectopic ventricular beats but is not often used as its addition increases overall risk. 

Fig. 19.8 Torsade de pointes The QT interval represents the phase of myocardial spread of stimulus and repolarization. Excessive QT lengthening may be caused by certain drugs or electrolyte imbalance. In addition, a U wave can occur, whereby its amplitude exceeds the T wave in V4-Vg. Subsequently, a potential life-threatening arrhythmia of type torsade de pointes may develop. Clinical symptoms include vertigo and syncopes. This arrhythmia can spontaneously disappear, but also pass into ventricular fibrillation and thus end fatally...

Because of the continuous presence of seismic background noise, only explosions whose yield exceeds a certain value are detected. This detectability limit varies with the distance from the explosion point and increases with the amplitude of the background noise. In the case of the Pacific Test Centre, the station at Rarotonga (Cook Islands), located at approximately 2000 km on a direct uninterrupted line from Mururoa, receives the T waves emitted by weak explosions and thus has a detectability close to one kiloton. The other stations are much further away and, apart from a few particularly sensitive stations such as the Yellowknife network in Canada, they have a much higher detectability limit. 

---