Page 391 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Technical Aspects of Fluid Therapy 381

3. One 30-inch intravenous extension tubing set if
needed
4. Three-way stopcock if needed
5. 20-mL syringe filled with saline solution
6. 20-gauge needle
The manometer and tubing are primed with saline
solution, and the column is filled to a level well above
the anticipated CVP of the patient. The animal is posi-
tioned in sternal or lateral recumbency with lateral recum-
bency preferred for accuracy. The stopcock at the bottom
of the manometer column should rest on the table or cage
floor. When the stopcock is turned to connect the column
of saline with the catheter, the hydrostatic pressure in the
column forces fluid through the catheter. The saline col- Figure 15-13 Measurement of central venous pressure (CVP)
umn continues to fall until the hydrostatic pressure of the using a saline column manometer. The dog is positioned in lateral
column reaches equilibrium with the hydrostatic pressure recumbency, and the manometer rests on the table surface. The
of the blood at the end of the catheter. When it has location of the patient's midline was estimated to be level with the
reached equilibrium and has stopped decreasing, the “0” mark on the column, and the marker ring has been slid to
height of the saline column above the catheter tip, that point (large arrow). The saline column has stopped falling, and
the ball floating on the top of the saline column rests at 4.5 cm at
expressed as centimeters of water, reflects the blood pres- the end of expiration (small arrow). Therefore this patient's CVP is
sure within the vessel at the catheter tip. Therefore it is read as 4.5 – 0 ¼ 4.5 cm of water.
important to know approximately where the catheter
tip lies in relation to the manometer fluid column. When
the animal is in lateral recumbency, the cranial vena cava the beginning of diastole (Figure 15-14). The response
lies near the midline, and the sternum is a good reference of the fluid column in the manometer is too slow to show
point. In sternal recumbency, the cranial vena cava is all of the peaks and valleys of these pressure changes accu-
approximately level with the point of the shoulder rately. When using a water column manometer, the best
(scapulohumeral) joint. method is to measure the CVP just before inspiration
When the appropriate external anatomic landmark is and at the lowest diastolic swing. This value correlates
found, the manometer column is positioned with the best with real CVP. 11 If the rhythmic fluctuations are
stopcock resting on the table surface immediately next absent, malpositioning of the catheter should be
to the landmark, and the centimeter mark nearest that suspected: either it is too short or too long and the tip
point is labeled. This mark is now the zero reference point is not within the thoracic cavity, or the tip is butted up
on the manometer, and all subsequent measurements are against a vessel wall or the right atrial wall. Obstruction
read as the distance from that mark. Measurements can be of the catheter tip can be confirmed by aspirating blood
made with the manometer located anywhere nearby that from the catheter: blood flows rapidly and with little resis-
is convenient, as long as the stopcock rests on the same tance if it is floating freely within the lumen of the vessel.
horizontal surface as the animal (Figure 15-13). If the A high CVP or the presence of large fluctuations synchro-
animal is in a cage, the manometer may be taped to nous with the heartbeat suggests that the catheter tip is in
the wall of the cage and used there. the lumen of the right ventricle; if this is the case, it should
be partially withdrawn to the proper level. Dorsal recum-
INTERPRETATION bency, abdominal compartment syndrome, or pleural
When obtaining a CVP measurement, rhythmic effusion will increase the CVP and may lead to erroneous
fluctuations in the height of the saline column meniscus assumptions about the cardiovascular system if the
are usually seen. These oscillations are caused by two pressure influence of those syndromes is not taken into
factors: large ones occur with respiration, and smaller consideration. 19,29
ones occur with each heartbeat. Fluctuations in the col- Water manometers tend to overestimate CVP by 0.5 to
umn synchronized with respirations are usually easily 5cm H 2 O; this overestimation varies from patient to
seen. As the patient inhales, the intrathoracic pressure patient and from measurement to measurement in the
and CVP decrease; the reverse occurs during exhalation. same patient, even when positioning is done as carefully
These excursions are exaggerated in animals with upper as possible. 11 This variation can be important when
airway obstruction and are reversed by positive pressure following a critically ill animal that requires aggressive
ventilation. With regard to the cardiac cycle, CVP fluid support and in animals that are hyperventilating,
increases steadily until atrial contraction, jumps up a bit dyspneic, or being treated with positive pressure
during atrial contraction, and then decreases rapidly at ventilation. A calibrated electronic pressure transducer