Page 214 - Basic Monitoring in Canine and Feline Emergency Patients

The right atrium is unique among the chambers of exist so the clinician has to rely on indirect meas-
the heart in that the primary determinant of the ures such as blood pressure and other perfusion
VetBooks.ir pressure within that chamber is the volume in the parameters (see Chapters 1 and 2).
chamber. Therefore, the CVP (and right atrial pres-
sure) can be used as an estimate of the right ven-
tricular preload (i.e. the volume presented to the How the monitor works
right atrium prior to emptying into the right ven-
tricle). The right ventricular preload is largely CVP is generally measured with a water manome-
determined by the venous return and the ability of ter or a pressure transducer. This chapter focuses
the right ventricle to eject blood in a normally func- on the use of a manometer for the measurement of
tioning heart. The venous return is in turn affected the CVP. Normal CVP for dogs and cats is gener-
by venous tone, venous wall compliance, and circu- ally around 0–5 cmH O, although values of up to
2
lating blood volume; right-sided cardiac output is 10 cmH O can be normal in critically ill patients. If
2
determined by the heart rate, afterload, preload, used, a pressure transducer will report values in
and contractility. Any variables that affect these mmHg. Measurement can be converted to cmH O
2
factors may ultimately affect the CVP. by multiplying the mmHg value by 1.36.
The CVP has been used historically to assess pre- Items required:
load and volume status in critically ill patients, to ● ● a central venous catheter, which can be single-
guide fluid resuscitation, as well as to aid in the lumen or multi-lumen;
diagnosis of right-sided heart failure. The basis for ● ● three-way stopcock;
using CVP to guide fluid management originates ● ● 20–35 mL syringe filled with saline;
from the dogma that CVP reflects intravascular ● ● a 250 mL bag of sterile saline; and
volume. It is thought that patients with a low CVP ● ● specialized noncompliant tubing (extension set)
are volume-depleted and need fluids, while patients if available.
with a high CVP are volume-overloaded and will
not respond to fluid therapy. However, there is a Steps for obtaining a CVP using a water
tremendous amount of evidence in critically ill manometer:
human patients that CVP does not reflect venous
volume and that a low CVP does not necessarily 1. Place the patient in lateral or sternal recum-
mean the patient requires fluid therapy. bency and place an indwelling central venous cath-
The current thought is that CVP is a misleading eter using aseptic techniques. The tip of the central
tool for guiding fluid therapy, although some venous catheter should be in the cranial vena
research in humans does suggest ‘extreme’ CVP cava, just before it enters the right atrium (verified
values may be able to guide response to fluids radiographically).
while intermediate values cannot. In human medi- 2. The central venous catheter should be secured
cine, values below 8 cmH O (6 mmHg) and more in place, flushed to maintain patency, and lightly
2
than 20 cmH O (15 mmHg) are considered wrapped to keep it clean.
2
extreme values. Typically, approximately twice as 3. To obtain a CVP, the patient should be placed
many human patients with low CVPs (below in lateral recumbency (ideally, right lateral recum-
8 cmH O) respond to fluids (i.e. had an increase in bency but either side is acceptable). Right lateral
2
their cardiac output after fluid administration) recumbency places the physical location of the right
than those with high CVPs (above 20 cmH O). atrium close to the thoracic inlet for use in zero-ing
2
Therefore, in a clinical setting it may be wise to (see step no. 6). No matter the recumbency selected,
refrain from administering fluids when CVP values as with IAP measurements, CVP should be done the
are extremely high and to give fluids when CVP same way every time.
values are very low. However, besides the fact that 4. The extension set (flushed with sterile saline)
there are no similarly determined extreme values should be connected to the central venous catheter.
in veterinary medicine, the CVP is influenced by If a multi-lumen catheter is being used, connect the
many factors, which makes its interpretation com- extension set to the central lumen and clamp off the
plicated (see Comments on CVP Monitoring sec- other lumens.
tion). Also, the ability to directly measure cardiac 5. The three-way stopcock is connected to the
output in a clinical veterinary patient does not extension set, while the saline-filled syringe is

206 A. Odunayo