The arterial pressure waveform is a dynamic graphical representation of the pressure changes occurring within the major arteries during a single cardiac cycle. This waveform provides a detailed, beat-by-beat picture of how the heart is pumping blood and how the blood vessels are responding to that flow. It is a fundamental tool in cardiovascular monitoring, particularly where a precise, continuous assessment of circulatory status is needed. A small, distinctive feature on the descending slope of this pressure curve, known as the aortic notch or dicrotic notch, holds particular significance. This subtle indentation marks the transition between the heart’s contraction and relaxation phases, providing insights into the mechanics of blood flow and the condition of the vascular system.
Understanding the Arterial Pressure Waveform
The arterial pressure waveform is typically divided into two main components: the systolic phase and the diastolic phase. The initial, steep upward slope of the curve, called the anacrotic limb, represents the rapid ejection of blood from the left ventricle into the aorta. This rapid pressure rise culminates in the systolic peak, which corresponds to the maximum pressure achieved during the heartbeat.
Following the peak, the pressure begins to decline as the ventricle starts to relax and blood continues to flow into the peripheral circulation. This downward slope is interrupted by the aortic notch, marking the end of the systolic ejection period. The subsequent, more gradual descent of the curve, known as the diastolic runoff, reflects the continuous outflow of blood into the distal capillary beds and veins.
The precise shape of this waveform changes depending on where it is measured due to pulse wave travel and reflection. In the aorta, the notch is sharp and is often called the incisura. In peripheral arteries, like the radial artery, the systolic peak is higher, and the notch is more rounded or delayed. Despite these morphological differences, the mean arterial pressure remains largely consistent throughout the systemic circulation. This variability highlights the complex interplay between the heart’s output and the elasticity of the arterial tree.
The Physiological Mechanism of Notch Formation
The appearance of the aortic notch is directly linked to the mechanics of the aortic valve. As the left ventricle completes its contraction and begins to relax, the pressure inside the ventricle drops rapidly. This fall in ventricular pressure eventually drops below the pressure remaining in the aorta, causing a brief, momentary reversal of blood flow back toward the heart.
This sudden, transient backflow of blood acts upon the leaflets of the aortic valve, causing them to snap shut. The abrupt closure of the valve generates a small, high-frequency pressure oscillation, which is transmitted through the column of blood in the aorta. This pressure oscillation manifests as the distinctive dip and slight rebound on the pressure tracing, marking the transition from forward ejection to the beginning of the heart’s filling phase.
In the central aorta, this sharp indentation is referred to as the incisura and precisely indicates the moment of aortic valve closure. As the pressure wave travels farther away from the heart, its shape is progressively modified by pressure waves reflecting back from points of resistance, such as the branching points of major arteries. In peripheral arteries, the feature is termed the dicrotic notch, and its precise location and shape are influenced by the valve closure event and these returning reflected waves.
The notch itself is the point where the initial pressure decline is interrupted, and the slight rise immediately following it is sometimes called the dicrotic wave. This secondary wave is often enhanced by wave reflection, where pressure waves bounce off the stiff, high-resistance walls of smaller arteries and travel back toward the aorta. The overall shape of the notch in a peripheral artery is a composite signal reflecting both the heart’s action and the physical properties of the downstream vasculature.
Clinical Interpretation of Notch Variations
The characteristics of the aortic notch are useful in clinical practice because they provide immediate insights into a patient’s cardiovascular state. The notch serves as a reliable demarcation point, signifying the end of the heart’s ejection phase and the beginning of the diastolic phase. Diastole is when the heart muscle and coronary arteries are primarily perfused. Its position on the waveform is used to accurately calculate the duration of systole and diastole, which is important for evaluating cardiac function.
The depth and position of the notch are informative about the systemic vascular resistance (SVR), which is the resistance to blood flow offered by the systemic blood vessels. A shallow notch with a steep diastolic runoff indicates low SVR, suggesting conditions of significant vasodilation, such as in states of severe infection or shock. In these cases, blood flows rapidly out of the compliant arteries, causing the pressure to drop quickly during diastole.
Conversely, a notch that is positioned higher on the descending limb and is less pronounced suggests high SVR, characteristic of vasoconstriction. When the peripheral vessels are constricted, resistance to flow is high, and blood runs off more slowly during diastole, maintaining a higher pressure in the arteries. In conditions where the arterial walls become stiff, such as with atherosclerosis, the reflected pressure waves return earlier and faster, which can cause the notch to be blunted or disappear entirely.
The notch also plays a role in identifying specific cardiac abnormalities. For example, in severe aortic regurgitation, where the aortic valve is incompetent and does not close completely, the reverse flow of blood back into the ventricle can significantly diminish or entirely eliminate the presence of the notch. Modern monitoring techniques, including Pulse Wave Analysis, use the notch as a reference point to calculate parameters like stroke volume and cardiac output, linking the visual feature of the waveform to quantitative measures of heart performance.