Page 512 - Cardiac Nursing

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488 P A R T III / Assessment of Heart Disease
during induced hypothermia and rewarming postcardiac arrest. The TPID methods are now used primarily to calibrate the pulse
The CCO is accurate at low flow rates (0.5 to 3 L/min), 318 and contour. For thermodilution pulse contour analysis, calibration is
during tachycardia or atrial fibrillation. 319 However, at higher based on the analysis of the area under the systolic portion of the
flow rates, there is an increased difference between the absolute thermodilution curve and a coefficient characterizing vascular com-
CCO and TDCO, although the percent difference remains simi- pliance. Once calibrated, the system (PiCCO) provides a beat-to-
lar. 312,320 The CCO on average overestimates the CO from a left beat analysis of SV. The lithium-based system uses pulse power
ventricular assist device by approximately 0.5 L/min. 321 analysis, which uses a series of approximations regarding the rela-
A limitation of the CCO system is the delay between a change tionship between radial artery pressure, aortic pressure, aortic flow,
in CO and display of the change. 322,323 The displayed CO is up- and CO. 332 The LiDCO is then used to calibrate the system and
dated every 30 seconds and represents the average CO over the convert the derived CO into a patient-specific CO. There have been
previous 3 to 6 minutes. Although newer technology has de- concerns raised regarding the ability of these systems to accurately re-
creased the response time, when the two systems most commonly flect the absolute or relative change in SV during periods of marked
used in practice were exposed to a 4-L/min CO change, they de- hemodynamic instability, although the data remain equivocal. 333,334
tected 20% of the change in 5.3 to 6.5 minutes, 50% change in The FloTrac/Vigileo System (Edwards Lifesciences, Irvine, Cal-
7.6 to 8.8 minutes, and 80% change in 10.8 to 11.1 minutes. ifornia) also provides continuous SV monitoring. The system uses
When flow was changed by 1 L every 2 minutes, neither system a proprietary sensor that can be attached to any arterial line. This
detected the change. 324 The observed changes in CO also lag be- system does not require external calibration, rather the SV is de-
hind changes in MAP, HR, and and TDCO. 315,325 The use rived using the following equation (SV K pulsatility), where
of the STAT mode may be an option. In contrast to the concern the calibration constant (K ) characterizes the patient’s vascular re-
that more frequent measurements may result in increased “noise,” sistance and arterial compliance based on their sex, height, weight,
the bias ( 0.04 to 0.18 L/min) and precision (0.61 to 0.84 and age and the pulse pressure waveform characteristics, and pul-
L/min) of STAT mode versus TDCO were comparable to values satility is derived from continuous analysis of the arterial pressure
comparing the TDCO with normal CCO. 326 waveform. 328,335 The early evaluation of this device found only
Several other factors may affect the accuracy and repeatability moderate agreement with other CO methods; 316,336–343 however,
of CCO measurements. The infusion of a cold solution may cause derived CO measurements using a new algorithm (version 1.10),
overestimation of CCO measurements, although CCO measure- which updates the calibration every minute, is generally compara-
ments are minimally affected by fluctuations in PA tempera- ble to other CO measurement devices. 344,345 However, in hemo-
tures. 304,327 In addition, fluid boluses cause an underestimation of dynamically unstable patients there was an unacceptable difference
CO in low flow states (CO 4 L/min). Intracardiac shunt, tri- (56%) in FloTrac CO compared with thermodilution CO. 346,347
cuspid regurgitation, and incorrect catheter placement (thermal A concern regarding these noninvasive indices is that changes in
filament in the vena cava or in contact with the heart) decrease the vascular resistance may necessitate recalibration of the system. The
accuracy of CCO measures. manufacturers recommend recalibration of the LiDCO and
PiCCO systems every 8 hours or with marked changes in hemo-
dynamic status. 348,349 Unfortunately, there is no standard defini-
LESS INVASIVE METHODS FOR tion of what a “marked change” in hemodynamics is. One study
CO MONITORING found that while small changes in SVR ( 20%) do not affect the
pulse contour measurements, a greater than 50% increase in SVR
Over the past decade, there has been increased emphasis on de- increases the bias between PiCCO and TDCO measurements. 350
veloping less invasive methods for CO monitoring. Intermittent In a recent study using the PiCCO system, while the TPTD and
2
CO measurements using the TPID technique involve injection of the derived CO correlated over an 8-hour period (r 0.68) the
r
r
an indicator into the venous circulation with a sensor in the sys- difference between the value was less than 30% for only the first
temic arterial circulation. There are currently two TPID CO tech- hour after calibration and when the SVR changed more than 15%
niques that have been validated against other CO measurement the error was 36%. 351 In another study, the LiDCO CO was
methods. 328 The Transpulmonary Thermodilution (TPID) within acceptable levels of agreement for only 4 hours after recali-
method uses a 10 to 15 mL injection of iced D5W or saline via a bration 352 and in an animal model of acute hemorrhage the
central catheter (subclavian or jugular) as the indicator with a 4-Fr LiDCO overestimated CO, suggesting the need to recalibrate with
thermistor-tipped arterial catheter placed in the femoral, brachial, acute changes in preload, afterload, or contractility. 353 An algo-
or axillary artery (PiCCO; Pulsion Medical Systems, Munich, Ger- rithm for the PiCCO system that accounts for vascular compliance
many). Recently, a study suggests that the thermistor-tipped and resistance was studied in postoperative cardiac surgery patients
catheter may also be placed in the radial artery. 329 Injection of the with CO changes greater than 20% ( CO 40% 27%) and a
–5
cold solution through the femoral vein also produces reliable esti- wide range of SVRs (450 to 2,360 dynes/s/cm ). The PiCCO CO
mates, but absolute CO values are higher than those obtained using was closely correlated (r 0.88) and similar to TDCO measures
the jugular vein due to increased transit time. The LiDCO system (bias 0.2 1.2 L/min), although there was increased variabil-
(LiDCO; London, UK) uses a subtherapeutic bolus of lithium in- ity in the CO in contrast to hemodynamically stable patients. 349
jected via a peripheral or central catheter as the indicator. The These data suggest that the systems need to be recalibrated when
lithium bolus is detected and a time curve, which is used to derive there is a greater than 15% to 20% change in SVR or CO.
the CO, is created by a lithium-sensitive electrode attached to an ar- Another concern about using these less invasive methods is the
terial pressure monitoring line. 330 The dose of lithium is too small lack of information about the risk for pulmonary edema.
to create a pharmacological effect; 331 however, LiDCO measure- Transpulmonary indicator thermodilution methods allow for the
ments cannot be performed in patients receiving lithium and neu- measurement of both intrathoracic blood volume and extravascu-
romuscular blocking agents may also interfere with calibration. lar lung water (EVLW). 225 The EVLW is normally between 3 and