Types of arterial blood pressure waves
When pressure is recorded using a direct method, the curve has a complex shape. It shows periodic rises and falls, which are classified into three orders.
1. First-order waves (pulse waves) These are the most frequent oscillations strictly tied to the cardiac cycle.
- Pressure increase: occurs during the rapid ejection phase of the left ventricle. At this moment, the inflow of blood into the aorta significantly exceeds its outflow to the periphery, causing the pressure to rise from 80 to 120 mmHg.
- Pressure decrease: is observed when blood actively flows out of the aorta into smaller arteries and the microvasculature.
2. Second-order waves (respiratory waves) These are peripheral changes in both systolic and diastolic pressure. They have a lower frequency and longer duration than pulse waves because they are synchronized with the respiratory rate. Their mechanism is related to chest physics:
- During inspiration, the volume of the chest increases, and the pressure in the pleural cavity drops to -6 mmHg. This reduces the external pressure on the large vessels of the systemic circulation (aorta and arteries), their configuration changes, the resistance to blood flow decreases, and arterial pressure drops slightly.
- During expiration, the pressure in the pleural cavity rises to -3 mmHg. The external mechanical impact on the vessels increases, leading to an increase in pressure inside them.
3. Third-order waves (central waves) The slowest oscillations, with a frequency of only 1–3 waves per minute. On the graph, they look like a smooth change in the baseline against which faster pulse and respiratory fluctuations occur. These waves arise due to periodic changes in the tone of the vasomotor and respiratory nerve centers.
Key parameters of arterial blood pressure
Several pressure indicators are used to assess hemodynamics:
- Systolic BP (SBP): the maximum pressure recorded in the arteries during ventricular systole. When measured at the brachial artery, the normal range is 110–130 mmHg.
- Diastolic BP (DBP): the minimum value during diastole. The normal range is 70–80 mmHg.
- Pulse pressure (PP): the difference between systolic and diastolic values. On average, it is 40–50 mmHg (the acceptable normal range is 35–55 mmHg).
The physiological significance of pulse pressure is that it shows the excess of systolic pressure over diastolic pressure, which is critically important for opening the semilunar valves of the aorta. PP is viewed as the increment in arterial volume. Its value is equal to the difference between the volume of blood ejected by the heart in the rapid ejection phase and the volume that managed to flow into the capillaries during the same phase.
Mean arterial pressure (MAP)
This is the pressure that reflects the amount of average energy ensuring the continuous movement of blood in both systole and diastole. This indicator is fairly constant for a specific vessel and the body as a whole, and it also reflects the degree of elasticity of the arterial wall.
Mean pressure depends on four main factors:
- The pumping work of the heart.
- The total circulating blood volume (CBV).
- Blood viscosity.
- Total peripheral vascular resistance.
Features of MAP calculation It is important to understand that MAP is not a simple arithmetic mean between systolic and diastolic pressure. To determine it accurately, one must measure the area under the BP curve and divide it by the length of this curve.
Since the duration of the systolic pressure phase is shorter than the diastolic phase in the cardiac cycle, the MAP value is always shifted closer to the diastolic pressure.
For an approximate calculation, the formulas are used: PP = SBP - DBP MAP = DBP + (PP / 3)
Briefly: answers to topical questions
What hemodynamic factors determine the isolated values of systolic and diastolic pressure?
Sources indicate the following hemodynamic factors affecting the value of blood pressure:
- Systolic pressure increases due to an increase in cardiac output; the pressure value is directly proportional to the stroke volume.
- Diastolic pressure depends on vascular resistance: with a decrease in total peripheral vascular resistance and vasodilation, it decreases.
- The pressure value is inversely proportional to the distensibility of the arterial bed.
What are the direct and indirect methods for measuring human blood pressure?
There are invasive (direct) and non-invasive (indirect) methods for determining blood pressure.
- Direct method — insertion of a needle with a manometer directly into the vessel (most accurate).
- Indirect methods — require the use of a sphygmomanometer:
- Riva-Rocci method — based on palpating the pulse while deflating the cuff (determines systolic pressure only).
- Korotkoff method — based on arterial auscultation; listening to sound tones makes it possible to determine both systolic and diastolic pressure.
To avoid confusing the orders of BP waves, remember the frequency of physiological processes: the heart beats most frequently (1st order waves), we breathe less frequently (2nd order waves), and the tone of autonomic centers changes very slowly (3rd order waves).
Class and exam questions
Why is mean arterial pressure not equal to the arithmetic mean of systolic and diastolic pressure?
Because systole lasts shorter than diastole. Consequently, blood remains under a lower (diastolic) pressure longer, and the final value of the mean pressure shifts toward the diastolic value.
What causes second-order arterial blood pressure waves?
They are associated with the act of respiration. During inspiration, intrathoracic pressure drops, and vascular resistance decreases. During expiration, pressure rises, the arterial walls experience external pressure, and BP increases.
What does pulse pressure reflect?
It reflects the increment in arterial volume during the ejection phase. This is the difference between how much blood the heart ejected into the aorta and how much blood flowed into the microvasculature.
What else to study on this topic
- Mechanisms of vasomotor center regulation
- Hemodynamics during the rapid ejection phase of the cardiac cycle
- The effect of negative intrapleural pressure on blood circulation
- Factors determining vascular resistance and blood viscosity
- Direct methods of arterial blood pressure recording
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