Fig 1.
Location of the cavotricuspid isthmus (CTI) within the heart.
(A) Illustration of the CTI within the right atrium (Modified with permission from [26]). (B) CTI in a tissue preparation from a rabbit heart. RAA … right atrial appendage, RV … right ventricle, TC … terminal crest, VB … vestibule, SCV … superior caval vein, IVC … inferior caval vein, OF … oval fossa, CSO … coronary sinus ostium, RVOT … right ventricular outflow tract, STV … septal attachment of the tricuspid valve.
Fig 2.
Cavotricuspid isthmus (CTI) and proposed nomenclature for the pectinate muscle network.
(A) Image of the specimen with incident light (left) and transillumination (right). (B) The CTI is delimited by the TC, VB, ILI, and PSI. (C) Ramifications of PMs were identified and assigned to levels c, p, x, y, a, and v from TC to VB and numbered from inferolateral to paraseptal. Note that in this particular preparation no level y was present. One PM (orange) connected ramifications at the same level, i.e. ramifications R.a1 and R.a2 (D) Abstract representation of the CTI. Solid lines are single PM segments, dashed lines represent multiple parallel connections between ramifications. PM segments are designated by their start- and endpoints, e.g. P.x1.a1. Note that ramifications c0 and p0 are outside of the ROI but are added for labeling of the corresponding PM segments within the CTI. PM … pectinate muscle, TC … terminal crest, VB … vestibule, ILI … inferolateral isthmus, CI … central isthmus, PSI … paraseptal isthmus, TV … tricuspid valve, ICV … inferior caval vein.
Fig 3.
Ramification levels and types of PM segments.
Depending on the bridged levels, 15 different types of segments can be assigned. PM segment yv was not found in any preparation. Three sectors can be defined: posterior between level c and p, central between level p and a, and anterior between level a and v. Connections between same levels are referred to as type ii (not shown here, compare Fig 2C and 2D).
Fig 4.
Bilinear transformation from image coordinates to normalized coordinates.
The contour of the CTI can be described by 6 points, i.e. the intersections of the 3 isthmi with the TC and the VB (left). These 6 points in image coordinates x/y are transformed by a bilinear mapping function onto 6 points of a unit square in normalized coordinates l/m where TC and VB are the top and bottom edges, ILI and PSI are the left and right edges and CI divides the unit square into two halves. We refer to the transformed contour as “normalized CTI” (NCTI).
Table 1.
Summary of included specimens.
Table 2.
CTI morphology.
Fig 5.
Morphometric analyses of the CTI.
(A), (B), (C) Total area of the CTI, muscular area, and non-muscular area are increasing with animal weight. (D), (E) Muscular and non-muscular area within the CTI is increasing with total CTI area. (F) Non-muscular area is increasing with muscular area at a rate of 0.35 within the CTI. For all panels linear regression was performed (grey line) and Spearman’s ρ with associated p-value is given. Grey area shows the 95% regression confidence interval.
Table 3.
Trapezoidal shape of the CTI.
Table 4.
Morphometric analyses of area, length and diameter of PM segments.
Fig 6.
Distribution of morphological parameters grouped by PM segment type.
(A) area of PM segments, (B) length of PM segments, (C) diameter of PM segments. Segment area was normalized to total muscular area within the CTI and length and diameter were normalized to the square root of total muscular area.
Fig 7.
Pairwise comparison of scaled area, length, and diameter for PM segments.
Each box shows the p-value of a two-sided Mann-Whitney-U test with a p-value of 0.05 considered as statistically significant. NS … not significant.
Table 5.
Ramifications within the CTI.
Fig 8.
General ramification topology of the CTI.
(A) Number of ramifications within the CTI versus total CTI area shows an increasing relationship with larger CTIs containing more ramifications. (B) Ramifications in the TC (starting sites) versus ramifications in the VB (ending sites). Marker area is proportional to number of observations, number of observations is additionally given next to markers. In (A) linear regression was performed (grey line) and Spearman’s ρ with associated p-value is given. Grey area shows the 95% regression confidence interval.
Fig 9.
Location and complexity of ramifications within the NCTI.
Grey box in all panels shows the NCTI. For better visual orientation, the NCTI is divided by white lines into three vertical domains of equal size and into left and right part. (A)-(F) Location of ramifications within the NCTI grouped by ramification level c—v. (G) Distribution of ramification index RI and complexity index CXI within the NCTI. CXI is color coded and marker area corresponds to RI. Ramifications with large RI and negative CXI (merging sites) are located between 20% and 50% of the CTI height mainly in the left half. Ramifications with large RI and positive CXI (branching sites) are located between 60% and 90% of the CTI height, again mainly in the left half. In (F) the anatomical correlate of the edges and the center line is shown (compare Fig 4).
Fig 10.
Histogram bars show the probabilities of RI and CXI, i.e. counts are normalized so that the sum of bar heights equals 1. Analysis was performed on ramification levels c to v. CXI and RI are identical for ramification level c (A) as there are no afferent PM segments. CXI and RI at level v (F) have identical height but reversed signs as there are no efferent PM segments. CXI range was [-5:4], RI range was [1:8].
Fig 11.
Consolidated topology of the paraseptal half of the NCTI.
A description is given in Table 6. (A) Plain, (B) Simple, (C) Simple with crossover IL-PS, (D) Simple with crossover PS-IL, (E) Complex but no crossover, (F) Complex with crossover.
Table 6.
Classification of network complexity in the paraseptal half of the NCTI.
Fig 12.
(A) Macrograph showing a typical CTI structure. (B) Micrograph cut parallel to the CTI plane. (C) Ramification in the TC, i.e. an exit site from the TC into the PM network. (D), (E) Central section of PMs. (F) Merging site of two PMs and transition from the PM network into the VB. (G) Center of VB. (B) and (C) show densely packed and parallel oriented fibers, whereas (A), (D), and (E) reveal a highly complex microstructure. Extracellular signal recordings in these areas can be expected to be fractionated and of small amplitudes. TC … terminal crest, VB … vestibule.