What is the Index of Microcirculatory Resistance (IMR)?
The Index of Microcirculatory Resistance (IMR) is a modern, quantitative measure used in cardiology to assess the functional status of the heart's microcirculation, or the network of tiny blood vessels. The microcirculation is responsible for over 90% of the resistance to blood flow in the coronary arteries, and its dysfunction can lead to conditions like angina, even when larger coronary arteries appear normal.
IMR's derivation is based on a simplified application of Ohm's law, where resistance is analogous to a pressure difference divided by flow. The measurement is performed under conditions of maximal hyperemia—a state of maximum blood flow achieved by administering a vasodilator drug. This ensures the measurement reflects the minimal possible resistance, providing a more accurate and reproducible result than techniques reliant on baseline values.
Unlike older methods, IMR specifically measures the resistance within the microvasculature and is independent of larger epicardial coronary artery stenosis. This makes it a crucial tool for diagnosing conditions where microvascular dysfunction is the primary issue, a situation that cannot be identified by a standard angiogram alone.
How is IMR measured?
The measurement of IMR is an invasive procedure conducted in a cardiac catheterization laboratory. It requires specialized equipment and pharmacological agents to induce the necessary conditions for an accurate reading.
The invasive thermodilution method
The standard method for measuring IMR uses a combined pressure/thermistor guidewire, such as the PressureWire (Abbott Vascular). The procedure involves several steps:
- Positioning: A guiding catheter is advanced to the coronary artery. The pressure/temperature sensor guidewire is then passed through the catheter and positioned distally within the target vessel.
- Hyperemia Induction: An intravenous infusion of a vasodilator like adenosine is administered to induce maximal hyperemia, significantly increasing blood flow and minimizing resistance.
- Data Collection: The guidewire simultaneously measures the mean distal coronary pressure ($Pd$) and uses thermodilution curves to determine the mean transit time ($T{mn}$) of a saline injection. The saline is injected multiple times to ensure consistency and reliability of the measurements.
- Calculation: The IMR is calculated as the product of the distal coronary pressure and the mean hyperemic transit time: $IMR = Pd imes T{mn}$. The unit is typically U or mmHg·s.
Calculation in the presence of epicardial stenosis
When a significant blockage (epicardial stenosis) is present, collateral blood flow can affect the measurement. In such cases, a more complex formula that accounts for the coronary wedge pressure ($P_w$) is used to ensure the microcirculatory resistance is measured accurately. Fortunately, a simplified calculation method has been validated, which relies on the relationship between fractional flow reserve (FFR) and IMR, eliminating the need for wedge pressure measurement during intervention.
Angiography-based measurement
Less invasive, angiography-derived methods for estimating IMR are also being developed and validated. These methods calculate flow based on the speed of contrast medium during an angiogram, coupled with pressure data, to provide a pressure-wire-free estimate of IMR. While offering potential convenience, they are still primarily research tools compared to the established invasive thermodilution method.
Why is IMR important in cardiac health?
The measurement of IMR provides critical information for cardiologists, influencing diagnosis, treatment decisions, and prognosis across several cardiovascular conditions. Its key clinical applications include:
- Diagnosing Coronary Microvascular Dysfunction (CMD): IMR is considered the gold standard for invasively assessing CMD, particularly in patients with symptoms of ischemia (lack of blood flow) but no obstructive epicardial coronary artery disease (INOCA).
- Predicting Outcomes after Myocardial Infarction: Studies have shown that a high IMR following a heart attack (specifically, a STEMI) is strongly associated with a larger infarct size, more significant microvascular obstruction, and higher rates of rehospitalization and mortality from heart failure.
- Guiding Treatment Strategies: By quantifying the degree of microvascular damage, IMR can guide therapeutic strategies, such as determining the need for pharmacological interventions or predicting the effectiveness of stenting.
- Assessing Heart Transplant Viability: IMR has been used to assess microvascular status in heart transplant patients, helping to evaluate conditions like transplant arteriopathy.
IMR vs. other microvascular assessments: A comparison
| Feature | Index of Microcirculatory Resistance (IMR) | Coronary Flow Reserve (CFR) (Thermodilution) | Hyperemic Microvascular Resistance (HMR) |
|---|---|---|---|
| Measurement Technique | Bolus thermodilution using a pressure/temperature wire | Bolus thermodilution or Doppler flow velocity | Doppler flow velocity |
| Parameter Measured | Pressure and mean transit time during maximal hyperemia | Ratio of resting to hyperemic flow or transit time | Ratio of hyperemic distal pressure to hyperemic Doppler velocity |
| Dependence on Hemodynamics | Independent of hemodynamic variations due to measurement at maximal hyperemia | Can be affected by systemic blood pressure and heart rate variations | Less affected by systemic hemodynamics than CFR |
| Microvasculature Specificity | Highly specific and independent of epicardial artery status | Also incorporates flow through epicardial arteries, making it less specific to the microvasculature alone | Highly specific to microcirculation, similar to IMR |
| Reproducibility | Very high reproducibility, operator-dependent in technique | Lower reproducibility due to dependence on resting values and hemodynamics | Also has good reproducibility |
| Cut-off Value | Generally >25 U for abnormal, >40 U in STEMI | Generally <2.0 for abnormal | Generally >2.5 mmHg·cm-1·s for abnormal |
What do IMR values indicate?
Interpreting IMR values is crucial for clinical decision-making. The values are typically reported in units of U (mmHg·s), and different cut-off points may apply depending on the clinical context.
- Normal vs. Abnormal: A standard reference interval for IMR in a healthy heart is typically considered to be below 25 U. A value of 25 U or higher is often used as a diagnostic criterion for microvascular dysfunction.
- Context-Dependent Cut-offs: In specific situations, such as in patients experiencing a ST-elevation myocardial infarction (STEMI), a higher IMR cut-off of over 40 U has been shown to predict adverse outcomes.
- Significance of High Values: Elevated IMR values indicate increased resistance in the coronary microcirculation, which can impede myocardial blood flow and lead to issues like myocardial damage and compromised cardiac function.
- IMR as a Prognostic Tool: The prognostic significance of IMR is particularly notable. In patients with STEMI, high IMR values measured after a percutaneous coronary intervention (PCI) can help identify individuals at higher risk for heart failure and other adverse events.
Conclusion
The measurement of IMR provides a crucial quantitative assessment of the heart's microcirculation, offering unique diagnostic and prognostic insights that are not available through traditional angiography alone. Using a pressure/temperature-sensing guidewire, this invasive procedure measures resistance in the heart's smallest blood vessels under maximal hyperemia. It is the gold standard for diagnosing coronary microvascular dysfunction, informing treatment decisions, and predicting outcomes, particularly after a heart attack. As invasive procedures become more refined and less-invasive alternatives are developed, IMR continues to be an essential tool for providing personalized, evidence-based care in cardiology.
Authority Link: Index of Microcirculatory Resistance: state-of-the-art and future perspectives - NIH
