The jugular venous pressure, sometimes called the jugular venous pulse, is a clinical sign. It's not a symptom (a patient won't come to his doctor complaining of a raised JVP!) and it doesn't necessarily tell you what's wrong with a patient, but it's an important sign nonetheless.
Put as simply as possible, it's an indication of the pressure in the right atrium of the heart. The higher the JVP, the higher the pressure in the right atrium; the lower the JVP, the lower the pressure in the right atrium.
As blood returns to the heart in the veins, it's destination is the right atrium, and prior to that the vena cavae (either the superior vena cava or the inferior vena cava). One of the vessels feeding the superior vena cava is the internal jugular vein, which passes from the medial side of the ear, down to the lateral side of the medial head of sternocleidomastoid. If the pressure is high in the right atrium, a column of 'backed up' blood will give rise to a visible level in the internal jugular vein, and this will pulsate with the varying pressure in the right atrium.
It may be useful to think about it like a column of water leading into a big pool. If the pressure in that pool (the 'central venous pressure') is increased, the column will back up and rise up. It is a similar occurence that is going on when blood pressure is measured, which is why the JVP may be described as a natural manometer.
The JVP is usually no higher than the clavicle, so you can't see it in a normal, fit and healthy person. A raised JVP may well be a sign that something is wrong. The main cause of a raised JVP is right heart failure, as this means that the right side of the heart isn't working and therefore the pressure on this side will be very high. Another thing to consider if the JVP is raised is the possibility of fluid overload.
A normal JVP waveform is the pattern of changing pressure that occurs during the heart beat. It's often called a double waveform because, unlike the pulse, it isn't made up of a single 'bulge', but a flickering pattern. Throughout the course of a single heart beat, the rapidly altering pressures in the right atrium are translated to the jugular vein and expressed in the JVP.
The basic pattern of a JVP waveform is an a wave followed by a much smaller c wave, an x descent leading to a v wave and finally the y descent. The change in direction with each part of the wave coincides neatly with a different occurrence in the right heart activity, and can be explained simply:
From the beginning of the wave, we start with contraction of the right atrium, which inevitably pushes up the pressure (the a wave) and then of course drops as contraction slows. The pressure dips up (the c wave) as the right ventricle shuts and the tricuspid valve bows in; then we get the drop in pressure as there is no contraction and the atria are left to relax.
But the atria aren't left alone. The pressure starts to rise as the atria are filling; more and more blood comes into the right atrium as blood is returned to the heart having completed its journey around the body. Eventually we get to such a state that the pressure is higher in the right atrium than in the relaxed right ventricle (the v wave), and the tricuspid valve opens up again, allowing blood to leave the right atrium and the pressure to go down (the y descent).
The column of blood backed up from the right atrium reflects this pressure, and rises and falls to demonstrate what's going on. Of course, it's happening very quickly, and it's extremely hard to work out quite what's going on - noticing, for instance, a "slow y descent" when the whole thing lasts a fraction of a second, is a very difficult thing to achieve. However, take it from me, it's valid!
Abnormalities of the JVP are fundamentally simple - they are very logical, they just make sense. The difficult thing is trying to spot them - the JVP is hard enough to find in a normal clinical examination, let alone trying to work out if there's anything abnormal about it! Lets look at each one in turn, and think about what could be causing it:
Absent JVP - what could cause the JVP waveform to be completely absent? Well, first of all you've got to make sure this isn't simply normal (i.e. in normal, healthy people who are lying at a particular angle, you can't see the JVP). However, what if one day you see the JVP, and the next day you're looking at a patient and the JVP's completely gone - and you can't understand why. Well, you should consider that it might be that the jugular vein isn't connected up to the workings of the heart - i.e. there's obstruction of the superior vena cava. If you've got something blocking the route, there's no way the column of blood is going to translate what's going on any further than the blockage!
Dominant A wave - the A wave is the bit that corresponds to the right atrium contracting. Why would the height of the blood column be higher in this circumstance? Surely it's only because there's some kind of blockage to the outflow of the right atrium - either because there is tricuspid stenosis, or (more commonly) because something's wrong a little further on. The pulmonary valve may also have stenosis, or instead you may have pulmonary hypertension, where the blood pressure in the lungs is too high, preventing blood getting through. In all three circumstances, the right atrium is contracting against a greater pressure, so while it's contracting, the A-wave is taller and you get this dominant A-wave shape.
Cannon A wave - now, given that the A wave corresponds to the right atrial contraction (as we've said), what might cause the blood column to shoot up out of all proportion? We've already said that if the pressure it's contracting against is high then you'll get a dominant A-wave - what about if the tricuspid valve is closed altogether! Then you really see it, because there's no where else for the blood to go but back; instead of getting a mild impression of the pressure in the system, you get an impression of the full-force of the right atrium!! So, why would the right atrium be closed? Well, this basically means that the atria and ventricle are contracting at the same time rather than one after the other (i.e. beating out of time with each other or 'asynchronously'). This might be due to complete heart block, or sometimes ventricular tachycardia.
Dominant V wave - imagine you have tricuspid regurgitation. Every time the right ventricle contracts, blood leaks back into the atrium. This means that as the blood is building up in the right atrium in it's normal fashion (causing the V wave), the effect of this build up is going to be accentuated by the abnormal flow coming through the valve. Instead of just entering the atrium from the superior and inferior vena cavae (as it returns from the body), blood comes back from the right ventricle too, leading to a dominant V-wave.
Other things to bear in mind, although which are a bit more difficult to understand logically, are an absent or exaggerated X-descent, caused by atrial fibrillation or cardiac restriction (cardiac tamponade or constrictive pericarditis) respectively. The y descent may also be sharp (from tricuspid regurgitation or constrictive pericarditis) or slow (from a right atrial myxoma).