CARAT Tutorials: Myocardial Jeopardy

Exploring the quantitative and spatial utility of angiography

Although coronary angiography is primarily a descriptive tool, its ability to define more quantitative myocardial-coronary disease relationships continues to be explored.  These explorations have been only modestly success as they arise from empiric anatomic assumptions that fail to capture the significant anatomic variability seen between patients. The CARAT software has attempted to capture this anatomic heterogeneity, hopefully leading to greater utility in small cohort and individual patient assessments.

Direct myocardial imaging techniques are reliable when anatomic or distinct biochemical or functional myocardial boundaries are present, but they are of little value in the absence of these delimiters and angiography may then play a role. Clinical situations in which angiography may have a quantitative advantage include:

  • Assessment of the relative amount of myocardium subtended by disease vessels in the absence of ischemia or myocardial injury

  • Quantitative assessment of completeness of revascularization after revascularization procedures

  • Providing a descriptive link between arterial configurations and the 17-segment ventricular model adopted by most other imaging techniques

17 Segment Model

Unlike the anterolateral, obtuse marginal and posterolateral myocardial region designations used in most published quantitative angiographic models and the previously published APPROACH Jeopardy score (MM Graham et al Am Heart J 2001;142:254), CARAT has adopted the 17 spatial designations proposed by the Cardiac Imaging Committee of the AHA. (See figure 1 below). In this model the LV is divided into three slices of equal thickness from base to apex. Each slice contains free wall segments and septal segments that are separated by the insertion points of the RV free wall.  There are 11 free wall segments, 5 septal segments and a distinct apical (non-cavitary) segment.

Until direct precise measurements of the size of each segment are available (under study), it will be assumed that the 16 free-wall and septal segments are of equal size (6% of the LV myocardium) and the non-cavitary apex is 4%.

The most effective graphic presentation of this model is one using a circumferential polar plot as indicated to the right of the diagram with the rings representing the basal, mid and apical layers and the center (area 17) the non-cavitary apex.


Figure 1: 17-Segment Model.

LV Free Wall Branch Assumptions

  • Number and size of free wall branches. . A major goal of CARAT is to link each side branch location and size to specific segments in the above 17-segment model. The reporting conventions of the CASS registry with respect to branch configurations have stood the test of time and have been modified only slightly in CARAT (Circulation 1981;63:I-1). In particular the specification of a maximum of 3 branches for the anterolateral region has been adopted as it fits nicely with basal, mid and apical segment designations. Within the marginal region, however, only two marginal branches are included to coincide with the anterolateral and inferolateral LV segments in the marginal territory. Also, only two branches are included in the posterolateral region to coincide with inferior and inferolateral LV segments. A basic starting CARAT assumption is that a Size 1 branch supplies the majority of one myocardial segment, a Size 2 branch supplies two segments and a Size 3 branch supplies three.

  • Location assumptions for anterior and lateral free wall branches. Diagonal, marginal and ramus side branches are assumed to follow an oblique course consistent with the usual direction of superficial myocardial fibers. Diagonal branches are designated as proximal (or basal), mid or distal (or apical) with respect to their points of origin along the LAD. We avoided ordinal numbering of branches to emphasize their spatial association with specific myocardial segments. Marginal branches are designated as anterolateral or inferolateral to reflect a proximal versus a distal marginal distribution. These associations are shown on the charts below. In addition to the Normal oblique configuration of free wall branches, distribution variants are occasionally seen in which the oblique direction is not followed. The most common variants follow either axial or transverse directions and these options are highlighted in yellow along with their corresponding myocardial region assignments.

Figure 2: Quantitative LV Assignments - Diagonal and Marginal Branches.

Posterolateral Branch Assumptions

The table in figure 3 below specifies segmental myocardial assignments for the PDA and posterolateral branches. The posterior interventricular artery (PDA) assignment procedure is unique. Unlike the anterior interventricular artery (LAD), which makes its contribution to the anterior LV free wall through prominent diagonal branches, the PDA makes its important contributions to the adjacent inferior segments through channels that are very much less distinct. For this reason we have had to rely on the classic pathologic observations of Kalbfleish and Hort (Am Heart J 1977;94:183) to specify the quantitative and spatial contributions of the PDA to in the inferior and septal segments.

The posterior region of the LV, referred to as the posterolateral region in classic CASS terminology, can be variably supplied by:

  • branches from the PDA,
  • PL branches that are easily distinguished by their axial pathway after originating in a perpendicular fashion from the posterior AV artery (either from the RCA or circumflex), and
  • distal marginal branches that have an initial curvilinear followed by an axial trajectory along the more lateral aspect of what was known as the posterolateral region.

This variability presents a challenge to the specific branch assignments to the 17-segment model segments in this region. Rightward PL-configuration branches are assigned to inferior segments whereas leftward PL-configuration branches are assigned to inferolateral segments. As a single PL branch is frequently seen in  the middle of the PL territory, CARAT divides the value of this single branch equally to the inferior  and inferolateral segments. (See table below)


Figure 3: Quantitative LV Assignments - PDA and Posterolateral Branches.

Septal Branch Assumptions

The picture in figure 4 below, from Bertho and Gagnon (Chest 1964;46:251), illustrates the usual balance between perforating arteries to the septum from the posterior (white) and anterior (red) interventricular arteries. Although there is variability, the anterior (LAD) septal branches usually supply 2/3 of the septum. As shown in the picture, there tends to be a gradient in relative inferior and anterior septal contribution between the base and apex with the LAD assuming proportionately more responsibility for the septum as one proceeds toward the apex.  CARAT assumes that the relative contribution remains the same from base to apex.

The 17-segment model proposes 5 septal segments, 2 basal, 2 mid and 1 apical. The basal and mid anteroseptal segments and the anterior 25% of the basal and mid inferoseptal segments are supplied from the anterior. The remaining inferior 75% of the basal and mid inferoseptal segments are supplied by from the PDA perforators. The apical septum blood supply is more difficult to specify as this area is determined to a large extent by the variable size balance between the LAD and PDA. We have specified that the distal LAD and PDA together supply the apical septum, 2/3 by distal LAD perforators and 1/3 by the distal PDA for a Type II LAD. With a Type I LAD and a Type III LAD, the PDA and LAD supply all of the apical septum, respectively.

Figure 4: PDA and LAD contribution to septum.

Myocardial Assignment Process

As each coronary segment or branch is constructed in CARAT, specific properties are assigned to each component (line). If a branch traverses more than one myocardial segment, myocardial values are assigned to each segment. This assignment can be seen by right-clicking on a branch of interest > right-clicking on ‘Edit Vessel’ > right-clicking on the line of interest > selecting ‘Line properties’. You will then see a view (similar to the one shown in figure 5 below) with segment number and the weighting values specified.


Figure 5: Process of LV assignment to coronary segments.

Initial Tally of Myocardial Assignments

In constructing the coronary tree, we ask operators to pick template combinations that best depict the location and distribution of branches and segments. We also ask that they designate a branch size that indicates the desired size/distribution area relationship.  It is not necessary to make template selections that yield a summed value for each segment of “1.0” in all cases, for we have included a reasonable equilibration process that adjusts for under and over-assignment of myocardial values to individual segments.

The first step in this equilibration process is to make a myocardial assignment tally based upon the tree diagram generated by the angiographer. Selecting File/Unadjusted RMA Form from the tool bar will reveal this tally. An example is shown in figure 6 below. Some segments have the optimal value of 1.0 but there are other regions that are less than or greater than unity. For these areas adjustments will be made.


Figure 6: Tally of assigned myocardial values.

Myocardial Assignments Talley Adjustments

For segments such as 1, 6, 12, 4 and 14 in the above tally illustration; the total myocardial assignment from contributing branches has exceeded 1.0. When this occurs, the contribution from each vessel is proportionately reduced to end up with a final summed assignment of 1.0. When the contribution tally for one or more regions is less than 1.0, a different strategy is needed. Specifically a default arterial segment has been assigned to each myocardial segment.  When a tally is less than 1.0, the unassigned value is given by default to pre-specified regions, as shown on the table in figure 7 below. (These defaults were derived from recent CMR-angiographic correlation work in a series of single-vessel STEMI patients. The report correlates infarct-related arteries to injured ventricular segments from the 17-aegment model (JT Ortiz-Perez et al JACCImg 2008;1:282).


Figure 7: Default artery assignments for LV segments.

Final Quantitative/Qualitative Myocardial Picture

The finished CARAT diagram provides an adjusted index of the amount of myocardium supplied by arteries that are narrowed by > 75% (or >50% in the case of the LM). The total jeopardy score is indicated in the upper right portion of the diagram. In the example in figure 8 below, an acute inferior STEMI is described. The area supplied by the occluded RCA was calculated to be 31%, a designation that can be displayed by right-clicking on the lesion of interest while in the ‘lesion’ editing mode and then select ‘Area at risk’. In this example there were other lesions greater than 70% in the LCA yielding the total jeopardy calculation of 48%. The myocardial segments affected by lesions greater than 70% are highlighted on the circumferential polar plot to the lower right.


Figure 8: Example report with jeopardy and area-at-risk scores.

Sources of Error in Model

  1. Relative size of myocardial segments – The assumption made by the Cardiac Imaging Committee of the AHA is that their myocardial segment design would yield segments of roughly similar size. For this reason we have assumed in our calculations that the size of segments 1 through 16 will each represent 6% of the LV mass with the non-cavitary apex (region 17) making up the difference of 4%. The extent of the error inherent in these estimates, given the shape of the LV and the relative size of the septum, is uncertain and requires clarification. (We are undertaking CMR measurements of the relative size of the 17 segments in 20 normal patients and will introduce these actual values in our model when this information is available.)

Figure 9: Relative size of LV segments.

  1. Branch length – myocardial volume relationship. The strongest theoretical association between branch size and the amount of myocardium subtended by a branch is based upon accurate assessment of vascular luminal area, a measurement that is not possible from invasive angiography in the presence of coronary disease. The branch length-to-myocardial volume relationship has been carefully explored by C. Seller et al (Circ. 1992;85:1987) who demonstrated a strong relationship between the summed length of secondary plus tertiary branches (easily visible on angiography) and myocardium supplied. Although this summed length value is not practical for routine clinical use, the length-volume relationship has been simplified in CARAT, by the assumption that a dominant vessel crossing most of a myocardial segment will supply most of that segment.

  2. Attention to detail in vessel template selection. As a reporting tool increases in complexity, there is increasing dependence upon due care and attention of the operator as vessel template options and lesion features are selected. A reporting tool must ‘walk the balance’ between achieving inter-observer reproducibility yet capturing important complexities in individual coronary trees. Reporting consistency in CARAT is encouraged by the use of intuitive template options of basic coronary structure and by providing immediate diagrammatic feedback of selections made. To minimize errors, sub-branching complexity has been removed from initial template options with the freedom to add this complexity at a later stage by vessel editing functions as described in other tutorials.

  3. Is CARAT too complex for widespread use?  Most published quantitative coronary models are based upon common major epicardial coronary artery configurations that rarely capture the anatomic variations seen in individual patients. For this reason the utility of published models is limited to global observations in larger populations with little precision when applying the model to individuals or small patient cohorts. The CARAT methodology should allow greater precision, but the tool is somewhat complex and is not widely available.  There will be an ongoing effort to clarify the methodology by making an interactive electronic spreadsheet available to all interested investigators. The document will allow ‘point-and-click’ selection of branch configurations and lesion locations with automatic myocardial volume calculations.

APPROACH Jeopardy Score Modification

The original APPROACH Jeopardy Score (MM Graham et al Am Heart J 2006;142:254) was calculated from Heartview TM diagrams routinely used at that time. The assumptions adopted were based upon the recurrent number “3”, with three free wall regions of equal size (anterolateral, marginal and posterolateral), a maximum of three branches allowed for each region and three sizes variations allowed for each free wall branch. Although this method did have calculation simplicity and demonstrated a strong correlation between the jeopardy score and one-year mortality in a cohort of 20,000 patients, the methodology frequently led to clinically important oversimplification. Finally, the earlier method did not lend itself to correlation with the 17-segment myocardial model used by newer imaging modalities. It is believed that the new CARAT reporting strategy and its revised APPROACH Jeopardy model will provide increased reporting accuracy.

Disclaimer

Although the regional myocardial assignment and jeopardy values in CARAT are based upon reasonable assumptions, they are not derived from direct area or mass measurements. As such their precision, in relative and in absolute terms can be questioned. Certainly it is not reasonable to make clinical decisions based upon the specific numbers generated.

Despite this important disclaimer, the CARAT calculations and renderings can give useful insights into the myocardial regions affected by significant coronary lesions thereby allowing more informed integration of data between imaging techniques.  The CARAT data may also present useful insights into the area at risk in acute coronary syndromes and into the completeness of revascularization after intervention, particularly when used in qualitative and relative terms.

 

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