Page 226 - ACCCN's Critical Care Nursing

Cardiovascular Assessment and Monitoring 203

hypertension’) is caused by pulmonary vascular disease, as the indicator substance, the calculation of cardiac
pulmonary embolism, pulmonary vasculitis or hypoxia. output is as follows:
A lowered PVR is caused by medications such as calcium
(
channel blockers, aminophylline or isoproterenol, or by CO = VO / CaO − CvO 2 )
2
2
the delivery of O 2 . 62,63
where VO 2 is oxygen consumption, CaO 2 is arterial
Contractility oxygen concentration, and CvO 2 is venous oxygen
Contractility reflects the force of myocardial contraction, concentration.
and is related to the extent of myocardial fibre stretch
(preload, see above) and wall tension (afterload, see Thermodilution methods
above). It is important because it influences myocardial Thermodilution methods calculate cardiac output by
oxygen consumption. Contractility of the left side of the using temperature change as the indicator in Fick’s
heart is measured by calculating the left ventricular stroke method. Cardiac output and associated pressures such as
40
work index (LVSWI), although the clinical use of this global end-diastolic volume can be calculated using a
value is not widespread. thermodilution PA catheter. Cardiac output can be moni-
tored intermittently or continuously using the PA cathe-
Right ventricular stroke work index (RVSWI) can be simi- ter. Intermittent measurements obtained every few hours
larly calculated. Contractility can decrease as a result of produce a snapshot of the cardiovascular state over that
excessive preload or afterload, drugs such as negative ino- time. By injecting a bolus of 5–10 mL of crystalloid solu-
tropes, myocardial damage such as that occurring after tion, and measuring the resulting temperature changes,
MI, and changes in the cellular environment arising from an estimation of stroke volume is calculated. Cold injec-
acidosis, hypoxia or electrolyte imbalances. Increases tate (run through ice) was initially recommended, but
in contractility arise from drugs such as positive studies now support the use of room temperature injec-
inotropes. 64
tate, providing there is a difference of 12° Celsius between
67
Cardiac Output injectate and blood temperature. Three readings are
taken at the same part of the respiratory cycle (normally
As discussed earlier in the chapter, the cardiac output end expiration), and any measurements that differ by
(CO) refers to the blood volume ejected by the heart in more than 10% should be disregarded (see Table 9.4 for
one minute. Stroke volume (SV) is the blood ejected by normal values). Since the 1990s, the value of having con-
the heart in one beat. Therefore cardiac output can be tinuous measurement of cardiac output has been recog-
49
calculated as the heart rate multiplied by stroke volume. nised and this has led to the development of devices
Stroke volume is determined by the heart’s preload, after- which permit the transference of pulses of thermal energy
load and the contractility. to pulmonary artery blood – the pulse-induced contour
method. 61
The variety of cardiac output measurement techniques
65
has grown over the past decade since the development Pulse-induced contour cardiac output
of thermodilution pulmonary artery catheters, pulse-
induced contour devices and less invasive techniques Pulse-induced contour cardiac output (PiCCO) provides
such as Doppler. As many critically ill patients require continuous assessment of CO, and requires a central
mechanical ventilation support, the associated rises in venous line and an arterial line with a thermistor (not a
68
intrathoracic pressure, as well as changing ventricular PAC). A known volume of thermal indicator (usually
compliance, make accurate haemodynamic assessment room temperature saline) is injected into the central vein.
difficult with the older technologies. Therefore, volumet- The injectate disperses both volumetrically and thermally
ric measurements of preload, such as right ventricular within the cardiac and pulmonary blood. When the
end-systolic volume (RVESV), right ventricular end- thermal signal is detected by the arterial thermistor, the
diastolic volume (RVEDV) and index (RVESVI/RVEDVI) temperature difference is calculated and a dissipation
69
as well as measurements of right ventricular ejection frac- curve generated. From these data, the cardiac output can
tion (RVEF) are now being used to more accurately deter- be calculated. These continuous cardiac output measure-
mine cardiac output. The parameters RVEF, CO and/or ments have been well researched over the past 10 years
CI, and stroke volume (SV) are generated using thermo- and appear to be equal in accuracy to intermittent injec-
dilution technology, and from these the parameters of tions required for the earlier catheters. 65,70,71 The para-
68
RVEDV/RVEDVI and RVESV/RVESVI can be calculated meters measured by PiCCO include:
10
(see Table 9.4 for normal values). The availability of ● Pulse-induced contour cardiac output: derived normal
continuous modes of assessment has further improved a value for cardiac index 2.5–4.2 L/min/m .
2
clinician’s ability to effectively treat these patients. 10 ● Global end-diastolic volume (GEDV): the volume of
blood contained in the four chambers of the heart;
The Fick principle assists in the calculation of intrathoracic blood
Several cardiac output measurement methods use the volume. Derived normal value for global end-diastolic
Fick principle. In 1870, Fick proposed that ‘in an organ, blood volume index 680–800 mL/m .
2
the uptake or release of an indicator substance is the ● Intrathoracic blood volume (ITBV): the volume of
product of the arterial-venous concentration of this sub- the four chambers of the heart plus the blood volume
66
stance and the blood flow to the organ’. Using oxygen in the pulmonary vessels; more accurately reflects