Conduction delays across the specialized conduction system of the heart: Revisiting atrioventricular node (AVN) and Purkinje-ventricular junction (PVJ) delays

Figures

• 

  Figure 1. Optical mapping of conduction delay through the AV node. The AV node is a three-dimensional structure consisting of multiple layers of myocytes with different electrical properties and delays in firing; as a result, three distinct upstrokes are detected from each pixel, in the optical voltage-recordings from the AV node region. (A) A photo of the Triangle of Koch. The mapped area is marked with a white dotted box (5 s mm × 5 s mm). (B) Representative optical mapping traces recorded from the AVN region of a rabbit heart under S1–S2 stimulation protocol. The first derivative (red traces) shows three distinct upstrokes associated with atrial (A), nodal/His (AVN, blue arrows), and ventricular (V) AP. The atrial tissue was paced at S1–S2 (basic CL = 300 ms followed by S2 at 140 ms). The upstroke of the AVN AP appeared substantially later in the S2 beat than the S1 beat (see the middle trace) and propagated toward His-bundle with a diminishing magnitude of the AP upstroke compared tothe S1 beat (blue arrows for S2 beat), indicating decremental conduction. (C) Activation maps of atrial and nodal-His upstrokes. The atrial and nodal upstrokes were mapped separately, then are superimposed…

• 

  Figure 2. Conduction delay and velocity measurements across the AVN during spontaneous rhythm and pacing. (A) AV- delay of spontaneous and pacing at a CL (cycle length) = 200 ms. The step-delay between the atrial and the AV node APs (A-AVN) is responsible for a major AVN delay. (B,C) CVs of AVN conduction were measured using two methods, local vector using a 5 × 5 window (B, 0.074 ± 0.021 m/s for the spontaneous beat and 0.063 ± 0.02 m/s for the paced beat at 200 ms CL) or along the line from the initial AVN AP to the His bundle area (C, 0.096 ± 0.019 ms for the spontaneous beat and 0.074 ± 0.017 m/s for the paced beat at 200 ms CL).

• 

  Figure 3. Two distinct locations of AV block. (A) Sample traces of rapid pacing at CL = 150 ms from the SA node region. In this example, the trace shows a 2:1 AV block and the conduction block occurred at two different locations (beat 1 and beat 3 conducted while beat 2 and beat 4 were blocked, see arrows). (B) Activation maps of b1–b4 beats. AV nodal signals were present in b1, b2, and b3 but absent in b4, indicating that the AV block of b2 occurred between the AVN to His pathway while the AV block of b4 occurred between the atrium and the AVN. The darker color of AVN maps in b2 and b3 indicate an increase of the step-delay between the atrium and the AVN occurred before the block. (C) Overlapped traces from the AV nodal region. The first derivative traces from the atrial to the His locations are overlapped to visualize AV nodal propagation. The red arrows indicate AV nodal propagation. AV nodal delay increases progressively from beat 1–3 but beat 4 failed at His and absence of AVN signal in this beat. (D) AV-delay of beat 1–beat 4. Most of the AVN delay was caused by the step delays between the atrium and the AVN (A-AVN, 20–89 ms) while the delay within the AVN to His bundle remains…

• 

  Figure 4. Detection of conduction delay through PVJs. (A) Two distinct upstrokes on optical mapping traces in Purkinje/endocardial region. The image on the left shows the location of mapping and trabecular muscle in the center. The optical mapping trace in one side of the trabecular muscle showed two distinct upstrokes (dotted green box), an initial small upstroke followed by a large upstroke (right panel, Supplementary Movie S4). The rest of regions in the field of view did not show two distinct upstrokes (the right trace). (B) Sample traces recorded over a free-running Purkinje fiber that is anchored to the trabecular muscles. Two distinct upstrokes are visible over the free-running Purkinje fiber (the left trace). The superimposed activation map shows early excitation representing the free-running Purkinje fiber (right panel, Supplementary Movie S5). (C) AP propagation toward papillary muscle (PM) of the right ventricle septum. High resolution mapping shows that free running Purkinje fibers anchor and activate at the base of the PM. The sample traces showed two distinct peaks; the free-running Purkinje bundle (red) compared to the rest of PM and endocardium (blue), indicating that the…

• 

  Figure 5. Activation patterns from purkinje-papillary-myocardium. Image of the papillary muscles from the RV septum from a 5 mm × 5 mm region. Activation map of the papillary muscle (PM) under sinus rhythm. An example of activation isochrones (1 msec apart) is shown for a right ventricular papillary muscle in a perfused rabbit heart. A fast wave of depolarization appears first along the PFs lying along the papillary (base to apex), followed by the firing of papillary APs before the spread of electrical activity along the endocardium (ENDO). The Supplementary Movie S7 depicts the sequence of activation demonstrating a wave of PF depolarization followed by the PM depolarization preceding the activation of the endocardium (ENDO) by 2–5 msec.

• 

  Figure 6. Endocardial activation time before and after cutting the free-running purkinje fibers. (A) Image of the PF network on the RV apex. The white dotted line indicates the location of cut. (B) Activation map before the cut. The activation of the endocardium in the field of view (4 mm × 4 mm) occurred within a short period of time (Supplementary Movie S8). (C) Activation map after cutting a PF branch indicated in panel B. After cutting, the activation pattern changed to a simpler unidirectional propagation (Supplementary Movie S9). (D) Summary of total activation time in the field of view (4 mm × 4 mm) before and after cutting a major PF branch. E,F panels are associated with the activation maps before and after the PF cut, respectively.