Page 215 - Basic Monitoring in Canine and Feline Emergency Patients

connected to the other end of the three-way stop- The primary indication historically cited for CVP
cock. Finally, the water manometer is connected monitoring is to predict the need for fluid therapy
VetBooks.ir to the last connection of the three-way stopcock, in a critically ill patient (i.e. the patient’s predicted
fluid responsiveness). The CVP has also been used
ensuring that its orientation is vertical.
6. The water manometer should be zeroed at the
altered in a critically ill patient. Even those who
level of the patient’s right atrium. This involves to determine if fluid therapy should be stopped or
placing the water manometer in a set location currently subscribe to the belief that CVPs can still
such as attached to the patient’s cage, an IV pole, be used in certain situations to guide clinical deci-
or a wall, to ensure that it does not move dur- sions limit use of the CVP to when the values are
ing the measurement, and noticing which reading really high or really low. That is to say, a ‘really
on the manometer lines up with the level of the low’ CVP reading is a reason to initiate fluid ther-
right atrium – this is considered the zero point. apy and a ‘really high’ CVP reading is a reason to
Typically, the thoracic inlet is used as the external stop fluid therapy. There are no real guidelines as to
marker for the right atrial location when in lateral what constitutes high or low enough to dictate
recumbency. alterations in fluid therapy, but the author would
7. Turning the stopcock off to the manometer, the typically consider lower than −5 to −10 cmH O
2
syringe is used to flush about 5–10 mL of saline very low and higher than +15 to +20 cmH O very
2
into the catheter. The stopcock is then turned off high. Please note that as with IAP monitoring,
to the patient and the manometer is filled to about serial CVP measurements are recommended, in
10–20 cmH O of saline higher than the patient’s concert with consideration of the clinical condition
2
expected CVP. of the patient. Therefore, one single CVP reading is
8. The stopcock is finally turned off to the syringe, typically not a reason to change treatments, espe-
to allow the fluid in the manometer to equilibrate cially if the patient is not giving any other indica-
with the patient’s right atrial pressure. The level of tions of fluid overload or hypovolemia.
the saline in the manometer will fall as it flows into Many factors serve to complicate the ability of
the patient, and then eventually stabilize as the fluid CVP to predict the patient’s fluid requirements.
level equilibrates with the CVP. The level in the For example, any increase in thoracic, pericardial
manometer where the water level stabilizes should or abdominal pressures will be transmitted through
be compared to the previously determined zero the wall of the veins. This can increase the CVP by
point to yield the CVP reading. For example, if the increasing pressure on the vessels; at the same
zero point was determined to be 4 cmH O in the time, increased pressure on the vessels will also
2
manometer, if the water level stops at 10 cmH O, partially or fully occlude them, causing a decreased
2
the recorded CVP would be +6 cmH O. Please note in volume within the vessels. Since increased CVP
2
that CVP readings can also be negative. readings are supposed to represent more volume in
9. About three to five consecutive measurements the vasculature and eventually returning to the
should be performed and the average should be right atrium, in the setting of increased body cavity
recorded. pressures, the CVP is not able to predict the vascu-
10. To disconnect the assembly, turn the stopcock lar volume. Similarly, changes in sympathetic tone
off to the manometer and disconnect the extension that lead to venoconstriction can increase the CVP
set from the central venous catheter in an aseptic but actually reduce the blood volume present in
manner. The CVP measurement setup should be the vessels. Alternatively, venodilation will decrease
stored aseptically prior to its use again. the CVP but often indicates more volume in the
vessels rather than less. Any structural disease
affecting the heart will lead to aberrant blood flow
through the heart. Hence, more volume can be
Comments on CVP Monitoring
present in a heart chamber due to regurgitant
Due to the controversies associated with the value blood flow or incomplete emptying of the cham-
and interpretation of the CVP, many human and bers during each cardiac cycle. This can have the
veterinary facilities have discontinued the use of effect of increasing the CVP but does not indicate
CVP in critically ill patients. The author does not that this increased right atrial volume is also leav-
recommend using CVP measurements to guide ing the heart. For example, in the case of the right
treatment in veterinary patients. atrium, tricuspid valve regurgitation or decreased

Manometer-based Monitoring 207