Healthcare and Medicine Reference
In-Depth Information
CHAPTER 19
Monitoring: Central Line
Ronan O'Leary 1 and Andrew Quinn 2
1 Yorkshire Deanery, York, UK
2 Department of Anaesthesia, Bradford Royal Infi rmary, Bradford, UK
Critically ill patients will often have a poor CO, but this can be
optimised in a number of ways.
Preload
OVERVIEW
By the end of this chapter you should be able to:
understand the use of central line monitoring in theatres and
can be altered by varying the volume of fl uid fi lling of
the heart during diastole.
Afterload
critical care settings
understand how the central venous pulse waveform is directly
can be manipulated by using vasodilators and
vasoconstrictors .
Contractility
related to the cardiac cycle
use central venous pressure as a guide to fl uid therapy to
• can be increased by the use of inotropes which act
to increase the calcium concentration within the myocyte and
increase the force of contraction.
The CVP gives us an estimation of preload. The tip of the cath-
eter should lie in a central vein, i.e. a large, intrathoracic vein close
to the heart which lacks valves. The CVP therefore gives an estimate
of right atrial pressure, since there is a continuous column of blood
between its tip and the right atrium.
If preload is increased, the stroke volume will increase. This rela-
tionship is described by the Frank-Starling law. This states that the
force of contraction is related to the initial muscle fi bre length. If
the muscle fi bres of the heart are stretched by increasing the pre-
load, the force of contraction exerted by these muscle fi bres will
increase. Therefore, when the heart rate is constant and afterload is
unaltered, CO is directly proportional to preload. This applies until
excessive end-diastolic volumes are reached when CO no longer
increases and eventually decreases: the failing heart (Figure 19.1).
optimise cardiac function
understand central venous oxygen saturations.
Introduction
Central venous catheters can be used for a number of physiological
measurements and can aid the assessment and treatment of criti-
cally ill patients.
How does central venous pressure relate
to cardiac fi lling?
Measurement of central venous pressure (CVP) is a frequently
used tool in the management of critically ill and high-risk surgi-
cal patients. CVP is a refl ection of the state of cardiac fi lling before
ventricular contraction and a means of assessing the intravascu-
lar volume status of a patient. The CVP allows optimisation of
cardiovascular function and can be used to guide fl uid therapy
during resuscitation.
Cardiac output (CO) is calculated in the following way:
Peak systolic
pressure
(mm Hg)
150
Cardiac output = Heart rate
×
Stroke volume
(CO) (L/min)
(HR)(beats/min)
(SV) (L/stroke)
100
The determinants of CO are preload, afterload and contractility.
Preload
is the degree of fi lling of the heart during diastole.
Afterload
is the force the heart has to contract against to eject
blood during systole; this is primarily due to systemic vascular
resistance (SVR) or the 'tone' of the vascular system.
Contractility
50
End diastolic
pressure
is the ability of the heart muscle itself to alter the vol-
ume of blood ejected during each beat independent of the preload
and afterload; essentially it is the inotropic state of the heart.
5
10
15
20
Figure 19.1 Frank-Starling curve: the curve shows that the relationship
between preload and stroke volume is linear until a plateau is reached where
the heart is working at peak effi ciency - further increases in preload do
not improve CO. CVP monitoring guides fl uid therapy to allow the plateau
portion of the Starling curve to be reached.
 
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