MID in blood test

Global ventricular function and volume calculation

To correctly assess cardiac performance, it is standard practice to analyze the volume of blood the ventricle is able to eject in relation to its physical dimensions. To objectively compare these data across different patients, the parameters are normalized to body surface area, which eliminates the influence of sex and age.

Key parameters of cardiac function include:

• End-diastolic volume (EDV) — reflects ventricular capacity at maximum filling.
• End-systolic volume (ESV) — the residual volume after maximum contraction.
• Stroke volume (SV) — the difference between EDV and ESV, i.e., the amount of blood actually ejected.
• Ejection fraction (EF) — the fraction of ejected blood relative to maximum filling (SV / EDV).
• Cardiac output (CO) — the product of stroke volume and heart rate.

In clinical practice, volumes are calculated in two ways. The first is using geometric models, which provides rapid results but suffers from inaccuracies when the chamber shape changes (it is especially difficult to assess the right ventricle due to its specific architecture). The second is the volumetric method, which sums up measurements from multiple slices. This yields high accuracy, but requires significantly more time for data processing.

Systolic function and contractility analysis

Normally, the heart ejects approximately two-thirds of the accumulated blood per systole. Contraction is driven by complex deformation of muscle fibers (myocardial strain), where deep and superficial layers interact with each other. This causes the ventricle to shorten in longitudinal and transverse directions, the walls to thicken, and the chamber itself to perform a twisting motion.

Wall motion is assessed according to the following criteria:

1. Normokinesia — adequate, physiological motion.
2. Hypokinesia — reduced contraction amplitude.
3. Akinesia — complete absence of motion in a segment.
4. Dyskinesia — paradoxical outward bulging of the wall during systole.
5. Hyperkinesia — excessively active contraction.

For precise quantitative analysis, strain assessment in three planes (longitudinal, circumferential, radial) is used, and twisting is studied in detail using tagged cardiac MRI.

17-segment model and coronary bed

The American Heart Association proposed a universal scheme for dividing the left ventricle, independent of the diagnostic method used. The chamber is divided into four levels along the long axis:

• Basal level (includes segments 1–6).
• Mid level (mid-level or MID) (includes segments 7–12).
• Apical level (segments 13–16).
• Apex (segment 17).

Each zone has a typical blood supply. For example, the anterior interventricular branch supplies most of the anterior wall and apex, the right coronary artery supplies the inferior regions, and the circumflex branch supplies the lateral segments. However, in myocardial infarction, this model has limitations: the necrotic zone often involves segments only partially. In such situations, a compartment model based on the actual boundaries of the lesion is more precise.

It is important to remember that a full assessment of pump function is impossible without analyzing the right ventricle, as its dysfunction during infarction serves as a formidable predictor of patient deterioration.

Class and exam questions

What else to study on this topic

• Features of coronary stent visualization using photon-counting technology
• Impact of diastolic dysfunction on the development of heart failure
• Anatomic variations of coronary circulation dominance
• Myocardial compartment model for infarction assessment
• Specifics of left ventricular twisting biomechanics

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.