Normal Sinus RhythmEach P-wave (caused by atrial contraction) is followed by a QRS-complex (caused by ventricular contraction) which is followed by a T-wave (ventricular repolarization).
Ventricular Premature BeatsVPB's are wide premature beats (QRS greater than 0.14 sec) arising from either ventricle that are not proceeded by P-waves. Much knowledge has been gained about why the heart suddenly flips from a normal to a lethal rhythm; however, in many cases, the exact mechanism remains unknown. Many of these deaths could be prevented with a device called an implantable cardioverter defibrillator (ICD).
The National Institute for health and Clinical Excellence (NICE) has specified certain markers that can help in this decision. Dr Nicolson, Clinical Research Fellow at the Department of Cardiovascular Sciences,  has been working under the supervision of Dr G. The research creates a "LifeMap": an electrical map of sudden cardiac death risk using an ECG and combines it with a cardiac MRI anatomical map of arrhythmia risk.
At the moment, the team is looking towards collaboration with Industry on a clinical version of the device. This electricalconduction system adjusts the timing of the contraction patterns of the two atria and two ventricles to provide optimal pumping of the heart to maximize cardiac output.
The maximum heart rate for adult patients at peak exercises typically 180 beats per minute.A  Only in young patients during extreme exercise does the heart rate go higher. These cells, referred to as the sinoatrial node,emit a regular pulsed electrical signal which triggers the heart beat. Typically your heart rate and blood pressure diminish at night during sleep, lowering the work of the heart and permitting it to recover.
Experiments in laboratory animals show that if high heart rates averaging 150 beats per minute are maintained continuously by an electric pacemaker for weeks, the heart begins to weaken and heart failure ensues. Extremely low heart rates reduce cardiac output further and can result in low blood pressure and eventually loss of consciousness. Normally the two atriae contract first, filling the two ventricles which are the main pumps of the heart. This delay provides time for the atria to completely fill the ventricles before ventricular contraction. This electrical signal passes quickly through the right and left atriae activating the AV node, which slows electrical conduction, providing time for atrial contraction to fill the ventricles. Normal sinus rhythm is usually very regular with equally spaced intervals between heart beats.

Ectopic beats arising from the atrium are referred to as premature atrial beats (PABa€™s), and ectopics beats arising from either ventricle are referred to as premature ventricular beats (VPBa€™s).
Note that the configuration of the ectopic P-wave differs from the configuration of the other P-waves.
These early P-waves are usually followed by a QRS complex, resulting from ventricular contraction.
Occasionally patients with very frequent PABa€™s will develop symptoms due to the palpitations caused by multiple PABa€™s and will require treatment with antiarrhythmic agents which suppress the PABa€™s. VPBa€™s are characterized by early wide QRS complexes with a T-wave usually directed in the opposite direction from the QRS. In patients suffering an acute myocardial infarction, VPBa€™s are an indication that the heart may fibrillate or arrest. So, before inserting an ICD, we need to balance the risks and benefits of inserting the ICD in a particular patient. It has led to the development of a new marker for SCD: the Regional Restitution Instability Index or R2I2. The electrical conduction system also automatically adjusts the heart rate to provide faster heart rates when the body needs more blood flow anda slower heart rate when at rest.
The normal heart rate at rest ranges from 60 beats per minute to 100 beats perminute, but wide variations are common.
Trained athletes frequently have slower heart rates at rest because their left ventricles enlarge to pump more blood during exercise. The sinoatrial node, influenced by neuro-humeral factors and by pressures in the rightatrium, adjusts the heart rate up or down to adjust the cardiac output to match the needs of the body for blood flow and oxygen requirements. Next the electrical signal travels to the HIS bundle, where it is transmitted down the right and left bundles which activitates contraction of the right and left ventricles, respectively. Occasionally, however, the heart rate will vary slightly with respiration causing a slight variation in the intervals between beats. Occasionally, however, the premature P-waves arrive so early that the ventricles are still recovering from the previous beat and are refractory to the premature atrial beat. VPBa€™s can be associated with any type of heart disease but are also common in patients with completely normal hearts. Studies have shown that in individuals with normal hearts, based on echocardiography and cardiac catheterization, VPBa€™s are harmless and are not associated with a poor cardiac prognosis. Since only the normal beats produce a pulse, detection at the wrist will be one-half the patienta€™s actual heart rate.A Weak peripheral pulses resulting from VPBa€™s can cause some patients with frequent VPBa€™s to feel fatigue and asensation of loss of energy.

We know that some people, for example people who have had heart attacks, are at high enough risk to benefit from an ICD. The resulting increase in stroke volume means that their hearts pump more blood with slower heart rates. For example older individuals in their eighties rarely elevate their heart rates over 150 beats per minute during exercise. When the heart rate increases during exercise, the AVinterval shortens to maintain optimal filling of the ventricles. The bundles are strands of specialized conduction cells, which transmit theelectrical signal rapidly and simultaneously to the right and left ventricles to trigger simultaneous contraction of the two ventricles. This variation of the normal heart rhythm is referred to sinus arrhythmia and is more common in younger individuals. VPBa€™s are commonly seen in normal individuals who consume large quantities of caffeinated beverages or who became exhausted from lack of sleep.
Individuals who are out of shape due to sedentary lifestyles often have faster heart rates during lowlevel exercise, whereas, highly trained individual have slower heart rates during high levels of exercise. Abnormal function of the AV node results in desynchronized or independent contraction of the atriae and ventricles which is referred to as AV dissociation, and causes suboptimal performance of the heart. The large electrical signal resulting from contraction of the ventricles produces the R-wave on the electrocardiogram. These very early premature atrial beats, not followed by ventricular contraction, are referred to as blocked premature beats.
The most important determinant of the clinical significance of VPBa€™s is the structural condition of the heart. After left ventricular contraction, the T-wave is generated during recovery of the ventricles. Often PABa€™s are asymptomatic,but occasional patients, especially individuals with thin chests, will experience each and every PAB as an early beat interrupting the normal regularly sequenced heart rhythm.

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