By Dr. Thomas Burnell and Dr. Bethany Turner
Next lesson - Control of Cardiac Output
Core
Blood flow within a vessel can be described in two ways, either laminar or turbulent:
- Laminar flow is where all the blood is flowing in one direction within a blood vessel. The blood flowing in the centre of the blood vessel is flowing faster than the blood closer to the edge (a parabolic profile). This can be thought of as circular layers of blood in a vessel flowing at different speeds.
- Turbulent flow involves chaotic mixing rather than orderly layers. High flow velocity, low blood viscosity (blood that is less thick and sticky) and disturbed vessel geometry favour turbulence. A narrowing can create a fast jet, while an irregular surface or branch point can disturb the flow. Increased resistance alone, including in hypertension, does not establish that flow is turbulent. Turbulence increases energy loss, so a greater pressure difference is needed to maintain a given flow.
Diagram - Diagrammatic depiction of both laminar and turbulent blood flow - the arrows represent separate blood flow streams. In the second diagram, after the obstruction the flow becomes turbulent
SimpleMed original by Dr. Thomas Burnell and Dr. Bethany Turner
When feeling a pulse, we feel the pressure wave being transmitted before the blood arrives.
Type of pulse rhythm:
- Regular - lub dub lub dub...
- Regularly irregular - the beat is abnormal but the pattern is repeated at set intervals. E.g. second-degree heart block of the Mobitz type 1 kind (also called Wenckebach), where the electrical signal from the atria (upper chambers of the heart) to the ventricles (lower chambers) is delayed or sometimes fails to get through, and ectopic beats (extra early beats) that repeat in a pattern, such as every other beat (bigeminy).
- Irregularly irregular - the beat is abnormal and does not follow a pattern. E.g. atrial fibrillation (a common rhythm problem in which the atria beat in a disorganised way) and ectopic beats that occur on their own, without a repeating pattern.
Type of pulse rate:
- Bradycardia - where the heart rate is less than 60 beats per minute.
- Tachycardia - where the heart rate is more than 100 beats per minute.
Strength of the pulse:
- Pulse strength is influenced by pulse pressure, the difference between peak systolic arterial pressure and diastolic arterial pressure. E.g. a blood pressure of 120/80 mmHg has a pulse pressure of 40 mmHg (i.e. 120-80=40).
- In atrial fibrillation, disorganised atrial electrical activity replaces effective coordinated atrial contraction. Irregular impulse transmission through the atrioventricular (AV) node (the relay between the atria and the ventricles) gives unequal intervals between ventricular beats. The varying filling time contributes to changes in stroke volume and pulse strength from beat to beat.
- In ventricular ectopics there is an early beat in the cycle caused by the ventricle spontaneously depolarising. This means there is a longer pause before the next beat. The ventricle has more time to fill during that pause, so the beat after the pause pumps out more blood and causes a stronger pulse.
Pulse Character/Quality:
- Thready pulse - a weak pulse that may accompany reduced stroke volume and a narrow pulse pressure.
- In severe left ventricular (LV) failure the main pumping chamber of the heart is weak, so it cannot beat as strongly and the systolic pressure can be lower.
- Bounding Pulse - a strong, forceful pulse. It is felt when the pulse pressure is wide, for example when the diastolic pressure is lower than normal.
- Bounding pulse is associated with low peripheral resistance (little resistance to blood flow in the small arteries). A dilated vessel gives less resistance to blood flow so diastolic pressure is lower.
- In bradycardia the time between beats is longer, so there is more time for diastole (when the ventricles relax and fill). The diastolic pressure drops further, which widens the pulse pressure.
Raising the arm above the level of the heart lets gravity empty the blood out of the arm more quickly, so a stronger pulse pressure can be felt. This can be used to exaggerate a bounding pulse, as in Watson's water hammer pulse (a bounding, forceful pulse that rises rapidly and then collapses).
Locations of Common Pulses
Radial Pulse - felt on the radial side of the wrist.
Brachial Pulse - felt on the medial side of the biceps tendon at the inside of the elbow.
Carotid Pulse - in the neck, beside the larynx (the voice box) and anterior to the sternocleidomastoid muscle (the large muscle at the side of the neck). Palpate one side at a time only.
Femoral Pulse - felt just inferior to the mid-inguinal point (the middle of the groin). This point is halfway between the anterior superior iliac spine (ASIS, the bony point at the front of the hip) and the pubis (the pubic bone at the front of the pelvis).
Popliteal Pulse - felt deep in the popliteal fossa (the hollow behind the knee). It is a hard pulse to find.
Pedal (foot) pulses:
- Dorsalis Pedis Pulse - lateral to the extensor hallucis longus tendon (the tendon that lifts the big toe).
- Posterior Tibial Pulse - behind the medial malleolus (the bony bump on the inner side of the ankle).
The mean arterial pressure can be calculated by either of the following equations:
If the mean arterial pressure falls below around 70 mmHg then organ perfusion can be impaired, depending on the clinical context.
The jugular venous pressure (JVP) is used to measure the pressure in the venous system as the blood is returned to the right atrium. The JVP is read on the right side of the neck, from the right internal jugular vein (a large vein running down the neck), because it has a straighter course to the right atrium and more reliably reflects right atrial pressure than the left.
The JVP can be seen as a pulsation in the neck when the patient lies at a 45° angle and looks to the left. It should be seen between the two heads of the sternocleidomastoid muscle, just above the clavicle. The JVP is measured as the vertical height, in centimetres, from the sternal angle (the bony ridge on the front of the breastbone) up to the top of the pulsation. It may also be measured using a central line (a long thin tube placed in a large vein). The line is inserted into either the internal jugular vein or the superior vena cava (the large vein that carries blood from the upper body to the right atrium), and it allows a waveform (the pressure trace over time) to be measured.
Normally the JVP should not be more than about 3-4 cm above the sternal angle.
Conditions where the JVP can be raised include: right-sided heart failure, cardiac tamponade and volume overload.
- Right-sided heart failure has a raised JVP because the right side of the heart is not working as an effective pump. In cardiac tamponade (fluid in the sac around the heart squeezes it, so the right side of the heart cannot fill properly) the raised JVP comes from this impaired filling. In both, blood builds up in the venous system and raises the venous pressure.
- Volume overload causes an increased volume of blood in the venous system, which raises the venous pressure.
Diagram - The measurement of jugular venous pressure in a clinical setting. Note the 45 degree angle the patient is placed at
SimpleMed original by Dr. Bethany Turner
Reviewed by: Dr. Marcus Judge
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