Biomedical Engineering Reference
In-Depth Information
can be placed in the arterial line (for in vivo applications) to enable continuous trending of
arterial blood gases. With the advent of continuous arterial blood gas monitoring, treatment
modalities can be proactive rather than reactive, which is vital for therapeutic interventions
in ICU patients who may experience spontaneous and often unexpected changes in acid-
base status.
Intra-arterial blood gas sensors typically employ a single- or a double-fiber configuration.
Typically, the matrix containing the indicator is attached to the end of the optical fiber.
Since the solubility of O
2
and CO
2
gases, as well as the optical properties of the sensing
chemistry itself, is affected by temperature variations, fiber optic intravascular sensors
include a thermocouple or thermistor wire running alongside the fiber optic cable to moni-
tor and correct for temperature fluctuations near the sensor tip. A nonlinear response is
characteristic of most chemical indicator sensors. Therefore, the operating range of these
sensors is typically optimized to match the range of concentrations according to the
intended application.
Intra-arterial fiber optic blood gas sensors are normally placed inside a standard 20-gauge
arterial cannula that is sufficiently small, thus allowing adequate spacing between the sensor
and the catheter wall. The resulting lumen is large enough to permit the withdrawal of blood
samples, introduction of a continuous or intermittent anticoagulant (e.g., heparin) flush, and
the recording of a blood pressure waveform. In addition, the optical fibers are encased in a
protective tubing to contain any fiber fragments in case they break off. The material in contact
with the blood is typically treated with a covalently bonded layer of heparin, resulting in low
susceptibility to fibrin deposition. Despite excellent accuracy of indwelling intra-arterial
catheters in vitro compared to blood gas analyzers, when these multiparameter probes were
first introduced into the vascular system, it quickly became evident that the readings (pri-
marily
O
2
) vary frequently and unpredictably, mainly due to the sensor tip intermittently
coming in contact with the wall of the arterial blood vessel and intermittent reductions in
blood flow due to arterial vasospasm.
A more advanced multiparameter disposable probe (Figure 10.39) consisting of
p
p
O
2
,
p
CO
2
, and pH sensors was developed by Diametrics Medical, Inc. The sensor has a diame-
ter of 0.5 mm and can be inserted intravascularly through a 20-gauge indwelling cannula.
Clinical studies confirmed that the system is adequate for trend monitoring, eliminating
invasive blood sampling, potential errors in analysis, and significant delays in obtaining
results, which may affect treatment. The device has been evaluated in neurosurgical
patients for continuous monitoring in the brain and in critically ill pediatric patients.
Fiber Optic pO
2
Sensors
Various fiber optic sensors were developed to measure
O
2
in blood based on the prin-
ciple of fluorescence quenching. Quenching reduces the intensity of the emitted fluores-
cence light and is related to the concentration of the quenching molecules. For example,
quenching can result from collisions encountered between the fluorophore (a fluorescent
substance) and the quencher. For quenching to occur, the fluorophore and quencher must
be in contact. When light is absorbed by a molecule, the absorbed energy is held as an
excited electronic state of the molecule. It is then lost by coupling to the mechanical move-
ment of the molecule (heat), reradiated from the molecule in a mean time of about 10 ns
(fluorescence), or converted to another excited state with a much longer mean lifetime
p
