Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3849_Библиотеки_им_академика_М_И_Перельмана

The action potentials of both the sinus and AV node are depen­dent on sodium “funny” currents [10]. These channels are recog­nized as funny because they open as the cell repolarizes rather than reaching a more positive threshold to evoke an action potential. The sinus node traditionally depolarizes and repolarizes most rapidly and therefore is responsible for setting the heart rate in the normal heart. Sympathetic (adrenaline) and parasympa­thetic (vagal) signals from the autonomic nervous system have strong inuences on heart rate due to their primary inputs to the sinus and AV node. Historically, a “normal” heart rate ranges from 60–100 beats per minute (bpm); however, it is not uncommon for those with high vagal tone (well-conditioned athletes, or during sleep) to have rates as low as 40bpm [11]. Electrocardiograms may show sinus bradycardia or a junctional escape rhythm (an impulse originating from the AV node). Bradycardia in and of itself is not problematic unless it is associated with symptoms such as dizziness, fatigue, shortness of breath, chest pain, or syn­cope. Placing these patients on a treadmill may reveal chrono­tropic incompetence, which refers to the inability of the sinus node to increase the heart rate appropriately with activity. It is important to make the distinction between physiologic bradycar­dia and symptomatic pathologic bradycardia to avoid unnecessary testing or treatments which may only cause harm and or provide little to no benet. Furthermore, the etiology of the arrhythmia must strongly be considered. Electrolyte imbalances, ischemia, infection, hypoxia, vagal tone, hypothermia, hypothyroidism, post-surgical state, as well as other causes may be transient, and treating the underlying cause may very well resolve the cardiac conduction disturbance. Temporary pacemakers may be consid­ered in specic emergent situations as deemed necessary.

0.2s. A long PR interval in the setting of a normal QRS duration usually represents conduction delay within the AV node. If the QRS duration is prolonged, the most common site of conduction delay is still in the AV node; however, the possibility of infra- Hisian conduction disease needs to be considered, especially in the pres­ence of left bundle branch block. It is important to recognize that a normal PR interval does not itself rule out the possibility of more advanced conduction disease. In general, rst-degree AV block is clinically well-tolerated and is not associated with an increase in overall mortality. However, in rare cases a PR interval exceeding 0.3 s can produce pacemaker syndrome-like symp­toms, where atrial activation begins as ventricular systole contin­ues, and atrial contraction occurs across closed atrio-ventricular valves.

patients with 2:1 AV nodal conduction as shown in Fig. 1.2, maneuvers like carotid sinus massage, which increases vagal tone, can be useful for differentiating between Mobitz Type 1 and Type 2 block [12]. In Mobitz Type 1 block, higher vagal input to the AV node will increase the amount of blocked p waves. However, in Mobitz Type 2 block, higher vagal input slows the sinus rate which provides the diseased infra-Hisian tissue time to recover allowing for a 1:1 AV nodal conduction (Fig.1.3).

an objective measurement of the cardiac conduction system. Catheters are placed into the heart to evaluate for dysfunction of either impulse formation or propagation. Following percutaneous venous access, catheters are typically advanced to the right atrium, the coronary sinus, the bundle of His, and into the right ventricle. These catheters have the ability to both sense electrical potentials and pace. Spanning the catheters into each of the locations facili­tates accurate diagnosis and location of potential conduction dis­ease. Baseline measurements which are noted on the surface ECG are recorded, including the PR interval, QRS duration, and QT interval. The intracardiac catheters provide electrograms that cor­respond to the ECG recordings. The PR interval is represented by the activation from the atria (A) to the His bundle (H) and the ventricular (V) signal. Of particular importance is the assessment of the HV interval. An HV measurement greater than 100msec is highly abnormal and may warrant pacemaker placement.