Biomedical Engineering Reference
In-Depth Information
4.1.11.2 Fluid status monitoring
to detect sustained decreases in impedance which may
be indicative of acutely worsening thoracic congestion.
Figure 4.1-14b shows an example of impedance re-
duction before heart failure hospitalization (arrow) for
fluid overload and impedance increase during intensive
diuresis during hospitalization. Label indicates reference
baseline (initial reference impedance value when daily
impedance value consistently falls below reference im-
pedance line before hospital admission). Magnitude and
duration of impedance reduction are also shown. Days in
hospital are shaded.
Some commercially available implantable devices for
the treatment of CHF and/ or ventricular tachyarrhyth-
mias now continually monitor intrathoracic impedance
and display fluid status trends. This information is then
provided to the clinician via direct device interrogation or
by remote telemetry. Recent reports based on actual
clinical experience with this feature have attested to
critical reliability and utility (Vollmann et al., 2007) and
good correlation with other traditional tools (Luthje
et al., 2007).
However, besides lung fluid, other physiological pa-
rameters might explain device measured changes in in-
trathoracic impedance. Some of these factors include
ventricular dilation, atrial or pulmonary vascular dilation,
anemia, hyper or hypovolemia, right and left ventricular
preload, hematocrit, electrolyte balance, pocket in-
fection, kidney dialysis, pneumonia, bronchitis; weight
change (not related to fluid accumulation), lymphatic
fluid changes, etc.
Externally measured transthoracic impedance tech-
niques have been shown to reflect alterations in in-
trathoracic fluid and pulmonary edema in both acute
animal and human studies (Fein et al., 1979). The elec-
trical conductivity and the value for transthoracic im-
pedance are determined at any point in time by relative
amounts of air and fluid within the thoracic cavity
(Gotshall and Davrath, 1999). Additional studies have
suggested that transthoracic impedance techniques pro-
vide an index of fluid volume in the thorax (Pomerantz
et al., 1969; Ebert et al., 1986). Wang et al. (2005)
employed a pacing-induced heart failure model to dem-
onstrate that measurement of chronic impedance using
an implantable device effectively revealed changes in left
ventricular end-diastolic pressure in dogs with pacing-
induced cardiomyopathy. Several factors were identified
that may influence intrathoracic impedance with an im-
plantable system. These include: (1) fluid accumulation
in the lungs due to pulmonary vascular congestion, pul-
monary interstitial congestion and pulmonary edema;
(2) as heart failure worsens, heart chamber dilation and
venous congestion occur and pleural effusion may de-
velop; (3) after implant, the tissues near the pacemaker
pocket swell and surgical trauma can cause fluid buildup
(Wang et al., 2005).
Yu et al. (2005) also showed that sudden changes in
thoracic impedance predicted imminent hospitalization
in 33 patients with severe congestive heart failure
(NYHA Class III-IV). During a mean follow-up of
20.7 8.4 months, 10 patients had a total of 25 hospi-
talizations for worsening heart failure. Measured im-
pedance gradually decreased before admission by an
average of 12.3 5.3% ( p < 0.001) over a mean duration
of 18.3 10.1 days. The decline in impedance also
preceded the symptom onset by a mean lead-time of
15.3 10.6 days (p < 0.001). During hospitalization,
impedance was inversely correlated with pulmonary
wedge pressure (PWP) and volume status with r ¼ 0.61
( p < 0.001) and r ¼ 0.70 ( p < 0.001), respectively.
Automated detection of impedance decreases was 76.9%
sensitive in detecting hospitalization for fluid overload
with 1.5 false-positive (threshold crossing without hos-
pitalization) detections per patient-year of follow-up.
Thus, intrathoracic impedance from the implanted
device correlated well with PWP and fluid status, and
may predict imminent hospitalization with good sensi-
tivity and low false alarm rate in patients with severe
heart failure ( Fig. 4.1-14 ). Figure 4.1-14a shows the re-
sults from an algorithm for detecting decrease in im-
pedance over long time. Differences between measured
impedance (bottom; circles) and reference impedance
(solid line) are accumulated over time to produce fluid
index (top). Threshold values are applied to fluid index
4.1.11.3 Cardiac pacemakers
Pacing of the heart may be done transcutaneously, but
this is accompanied by pain. The usual method is with
two epicardial electrodes and leads out through the
chest to an external pacemaker, or with an implanted
pacemaker.
The implanted pacemakers are of many models. Let
us consider a demand pacemaker, with special recording
ring electrodes on the catheter for the demand function.
If QRS activity is registered, pacing is inhibited. The
pacemaker housing may be of metal (titan) and function
as a large neutral electrode. Pacing is done with a small
catheter tip electrode, either unipolar with the neutral
electrode or bipolar with a catheter ring proximal to the
tip electrode.
As can be seen from Fig. 4.1-15 , the chronaxie is less
than about 500 m s, so there is an energy waste choosing
the pulse duration much larger than 100 m s.
A pacemaker may be externally programmed by mag-
netic pulses. Also because of this a pacemaker is to certain
degree vulnerable to external interference. Typical limits
are: static magnetic field < 1 gauss, 40 kVarcing > 30 cm
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