3.3.6 The jugular vein
The jugular vein acts as a manometer of central venous pressure and can provide very useful information. Patency of flow should be assessed on both sides. The jugular filling time can be measured from the time taken for the vein to fill to the level of the jaw when occluded, and it can be delayed in some animals with heart failure. However, it is a very crude guide to cardiac output and can be very variable in normal animals.
Some 'pulsation' of the vein is observed at the thoracic inlet in the normal horse, because it is so close to the right atrium. The normal variation in pressure in the jugular veins is described as a series of 'waves' and 'descents' (Figure 3.1), which are related to the changes during the cardiac cycle (see section 1.4.1). The a wave marks right atrial contraction. This is followed by the x descent, when the atria relax. The C wave occurs at the time of ventricular systole, and is relatively small. It may be difficult to distinguish it from pulsation of the adjacent carotid artery, which occurs simultaneously. This is followed by the x' descent, when the ventricles pull the atrioventricular (AV) valves towards the apex, resulting in a fall in pressure within the atria. Because the c wave is often small, the x and x' descents are sometimes regarded as one phase. The next positive wave is termed the V wave. This is due to the increase in right atrial pressure caused by filling during the latter half of ventricular systole. This is followed by the y descent, which occurs when the AV valves open and the stored blood rapidly fills the ventricles in early diastole. The waves and descents are more easily seen in most horses than in other species because they have relatively superficial veins, a thin hair coat and a slow resting heart rate.
Observation and palpation of the vein may help to identify the atrial conwave. Its presence or absence provides valuable information about conduction of the electrical impulse through the heart. It is often particularly prominent in the horse because the ventricles have usually almost completely filled before atrial contraction, and the ventricular pressure at this point is relahigh. Right atrial contraction therefore contributes relatively little to venfilling at low heart rates and the pressure wave is transferred to the jugular veins. Identification of the a wave therefore indicates that atrial contraction has taken place. It will be absent in horses with atrial fibrillation, and present in the absence of the V wave and x and y descents in 20AVB (see section 7.7.2).
Some confusion exists over the term 'jugular pulse'. Pulsation may occur in three situations:
· Tricuspid regurgitation, with a raised venous pressure. · Atrioventricular dissociation. · Right-sided CHF.
Only the first two of these result in a true jugular pulse. In tricuspid regurthe contraction of the ventricles forces blood up the jugular vein in a retrograde fashion through the incompetent valve. The a wave and the V wave may merge to produce one large pressure wave. The x descent is lost and a prominent y descent is seen. Just one large rise and fall in jugular filling is therefore seen. This is almost invariably accompanied by a raised central venous pressure. In AV dissociation, pulse waves will be seen in the jugular vein when the atria contract against a closed tricuspid valve, during ventricular systole. They fail to generate sufficient pressure to open the AV valves and the contents of the atria are forced in a retrograde fashion. These are called 'cannon' or 'giant a' waves.
Right-sided CHF may result in distension of the jugular vein due to an increased right ventricular end-diastolic pressure and subsequently an increased central venous pressure. In this situation the vein is continuously distended rather than distended by a pulsation. However, pulsations may be superimposed owing to incompetence of the tricuspid valves which often accompanies the volume overload of the right side of the heart, or because the pulsation of the carotid artery is transmitted through the filled vein. The latter is not a true jugular pulse.
To check whether a jugular pulse is present, the vein should be occluded and blood milked from the point of occlusion towards the thoracic inlet. If a true jugular pulse is present, the vein will fill from the thoracic inlet.
The observation of a distended jugular vein is a very important finding. However, the position of the head will alter the degree of distension. The jugular vein will fill in a normal animal when it lowers its head because the vein is then ventral to the right atrium. One or both jugular veins may also be distended when venous return is impaired by a thoracic mass or even when there is a large pleural effusion.