D12 Cardiovascular function

Topic

Ergometry, or exercise stress testing, evaluates cardiovascular and respiratory responses during progressively more intense exercise. The maximum oxygen consumption achieved serves as a fundamental benchmark for cardiorespiratory capacity; according to the Fick principle, it depends on cardiac output and the amount of oxygen tissues extract from the blood. According to the text, oxygen consumption below 14 mL/kg/min indicates severe functional limitation and may serve as a criterion for heart transplantation in patients with heart failure, whereas values ​​above 20 mL/kg/min generally allow for the performance of daily activities.

During exertion, the "double product"—calculated by multiplying heart rate by systolic blood pressure—can also be determined as an indirect estimate of myocardial oxygen consumption. The level at which ischemia occurs is usually reproducible in a given patient, allowing treatment to be adjusted based on their individual threshold.

Exercise-induced ischemia can also be detected through characteristic electrocardiogram changes. According to the text, a depression of at least 1 mm in a specific segment measured after ventricular depolarization is considered a positive result. However, the test identifies approximately 68% of individuals with significant coronary obstruction and correctly classifies about 77% of those without it as negative. Its performance is inferior to that of stress echocardiography or cardiac scintigraphy. Furthermore, results cannot be adequately interpreted in cases of certain conduction disorders, left ventricular hypertrophy, the use of medications such as digoxin, or an inability to perform sufficient exercise. In such cases, cardiac stress can be induced using drugs that accelerate the heart rate or alter coronary blood flow.

Doppler echocardiography allows for the non-invasive assessment of blood flow and pressures within the heart. Using the simplified Bernoulli equation, the velocity of blood passing through a narrowed valve enables the calculation of the pressure difference across the valve, a value that increases with the square of that velocity. This estimate approximates the one obtained via catheterization.

Echocardiography also makes it possible to determine the fraction of blood ejected by the left ventricle with each heartbeat. The biplane Simpson’s method mathematically divides the ventricle into a series of discs, calculates their volumes, and determines the extent of volume reduction during contraction. According to the text, it has an error rate of less than 10% compared to cardiac magnetic resonance imaging and is one of the most widely used indices of systolic function.

The tilt-table test examines blood pressure regulation by the autonomic nervous system. The patient is tilted between 60 and 70 degrees for 20–45 minutes while blood pressure and heart rate are continuously recorded. A drop in systolic pressure of more than 20 mmHg or diastolic pressure of more than 10 mmHg during the first three minutes, without an adequate heart rate compensatory response, defines orthostatic hypotension and may indicate autonomic dysfunction. Another pattern is vasovagal syncope, in which both blood pressure and heart rate drop simultaneously due to a sudden predominance of cardiac-inhibitory activity.

A 24-hour electrocardiographic recording also allows for the analysis of overall heart rate variability. The dispersion of inter-beat intervals reflects the autonomic modulation occurring throughout the day. According to the text, values ​​below 50 ms indicate a high risk of sudden death following a heart attack or in cases of heart failure. There are also geometric indices that are less sensitive to artifacts in prolonged recordings.

Taken together, these tests assess cardiovascular function under various conditions. Exercise testing quantifies exercise capacity and the ischemic threshold; the electrocardiogram detects signs of ischemia; the echocardiogram evaluates valvular gradients and ventricular function; the tilt-table test analyzes autonomic regulation during position changes; and prolonged recording examines heart rate variability. Combined, they make it possible to characterize how the heart responds at rest, during exercise, to postural changes, and over the course of a full day.

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