Page 515 - Cardiac Nursing

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C HAPTER 2 1 / Hemodynamic Monitoring 491
aspiration during placement of the probe. Additional complica- the potentially large bias in the ICG-CO measurements compared
tions include pharyngeal, laryngeal, or esophageal trauma and with other CO measurement methods. These results support the
dental damage. recommendation to follow the ICG-CO for trends and to cau-
tiously interpret absolute CO values.
Impedance Cardiography
Clinical Applications of ICG
Impedance cardiography (ICG) or thoracic electrical bioimped- Despite the concerns regarding the comparability of ICG-CO
ance (TEB) measures the electrical resistance of the thorax to a with other CO measures, ICG has been used for intraoperative
high-frequency, low-amplitude current. Bioreactance technology, monitoring and in outpatient and emergency department settings
which is a modification of the TEB technology, may improve the for the diagnosis and management of patients. For example, in he-
signal-to-noise ratio seen with TEB and decrease the variability in modynamically unstable ICU patients, the physicians were only
the measurement. 381,382 With TEB, the current is passed through accurate in their assessment of CO and thoracic fluid content in
the thorax and the voltage change with each systole is measured. 57% and 48% of the patients, respectively, compared with ICG-
This change in voltage is the result of a change in TEB, which cor- derived values. 393 A unique variable provided by ICG is a meas-
responds with systole. 383 The TEB is inversely proportional to the ure of thoracic fluid status (Zo), which is a composite measure of
content of thoracic fluids (i.e., when thoracic fluids increase the interstitial, alveolar, and intracellular fluid and is the inverse of
TEB decreases). Three factors affect TEB: (1) change in tissue thoracic fluid content. 394 A normal Zo is 20 to 30 for men and
fluid volume; (2) respiratory-induced changes in pulmonary and 25 to 30 for women, although there is individual variability and
venous volume; and (3) changes in aortic blood flow. The change the Zo values can be affected by other thoracic conditions (e.g.,
in aortic blood flow can be measured by the change in TEB, as- emphysema, pneumonia). 395 In patients with HF, there was an in-
suming that the other factors remain stable or are filtered. 384 The verse relationship between Zo and chest radiograph findings of
system consists of specialized electrodes placed laterally on the pulmonary edema, and a Zo less than 19 was highly sensitive
neck and at the lateral aspect of the lower thorax (at the level of and specific for identifying radiographic findings of pulmonary
the xiphisternal junction). The electrical voltage (2 to 4 mA), edema. Interstitial edema was present at Zo 18.5 7.1 and
which is safe and not felt by the patient, is passed longitudinally alveolar edema was present at 14.8 5 . 396 Use of ICG may
through the thorax between electrodes. 383 Whole body ICG uses aid in the differential diagnosis of shortness of breath. In patients
electrodes placed on the wrists and ankles and a different algo- with suspected HF and shortness of breath, the Zo was signifi-
rithm to estimate CO. 385 cantly different in those with radiographic evidence of car-
Factors that affect the accuracy of ICG measures include the po- diomegaly (17.5 5 ) and pulmonary edema (17.2 4.2 )
sitioning of the electrodes (i.e., the electrodes must be in exactly the than those with normal radiographs (23.4 5.4 ). 397
same position for each measurement), any factor that interferes with In patients with HF or acute coronary syndrome, ICG has
electrode contact (e.g., perspiration), an irregular heart rhythm, al- been used to measure CO and cardiac power (the product of si-
tered tissue water content (chest wall edema, pulmonary edema, or multaneously measured CO, MAP, and SVR). Measurement of
pleural effusions), aortic valve disease, abnormalities of the aorta CO, SVR, and cardiac power has been found to be useful in the
(coarctation or aortic aneurysm), and cardiac shunts. Obesity does differential diagnosis of different acute HF syndromes (e.g., car-
not appear to affect the accuracy of ICG measurements. 386 diogenic shock, hypertensive crisis, ADHF, and pulmonary
A meta-analysis published in 1997 suggested that ICG-CO edema) 398 and in the titration of vasodilator agents for patients
might be useful for trend analysis, but absolute ICG-CO meas- with acute HF. 385 However, for the detection of a CI 2.2
2
urements may not be accurate. 387 Since then, there have been L/min/m as determined by TDCO, the ICG had a sensitivity of
changes to the algorithm and technology. In a study of cardiotho- 62%, specificity of 79%, and a positive predictive value of 68%,
racic patients using the BioZ (CardioDynamics, San Diego, CA) again highlighting the limitations in using absolute ICG values
and the most current algorithms there was good agreement be- rather than following trends. 399 In patients with chronic HF, ICG
tween ICG-CO and TDCO (bias 0.17 L/min, precision measures of cardiac reserve (i.e., increased CO with exercise or a
1.09 L/min). 388 Similar results were found in post-operative car- stress test) were inversely related to exercise intolerance. 400 In addi-
diac surgery patients using the Aesculon (Osypka Medical, Berlin, tion, bioimpedance measures of CO reserve during exercise and car-
Germany) 389 and in cardiac surgery and medical-surgical cardiac diac power during dobutamine stress echocardiography identified
patients using whole body impedance. 385,390 However, in a recent patients with multivessel coronary artery disease or stress-induced
study of 15 medical and surgical ICU patients, the bias and pre- ischemia. 398,401
cision between Fick and TDCO was 1.7 3.8 L/min, Fick and ICG monitoring also has been used for the management of pa-
ICG-CO (BioZ, CardioDynamics) 2.4 4.7 L/min, and TDCO tients with HF . 394,402–404 Intrathoracic impedance monitoring de-
and ICG-CO 0.7 2.9 L/min; although there was less internal scribed below may also be beneficial for these patients. 395 In patients
agreement for TDCO ( 8%), which was measured throughout with resistant hypertension, therapy guided by ICG monitoring re-
the respiratory cycle, than for ICG-CO ( 4%). 391 In periopera- sulted in a greater improvement in control rates compared with
tive cardiac surgery the ICG SV measured with the HL-4 (Hemo- management by an experienced clinician (56% vs. 34%, p .05) 405
logic, Amersfoort, the Netherlands) and three different algorithms and ambulatory impedance monitoring, which uses ambulatory
was not comparable to TDCO. However, the ICG-CO was less ICG and BP monitoring may provide further insight into the pa-
variable than TDCO, which was measured throughout the respi- tient’s hemodynamic status (LV function, SVR, BP) during daily
ratory cycle. 392 These studies demonstrate the need to identify activity and allow for further tailoring of therapy. 406,407 The ICG
which monitor and the algorithm are used in a given study, the has also been used to optimize pacemaker therapy. 408,409 How-
limitations of using TDCO as the standard for comparison, and ever, ICG monitoring is absolutely contraindicated in patients who