General
anaesthesia requires proper monitoring to avoid the potential complications producing
irreversible changes leading to death of the patients. Some of the adverse
effects are occurring
gradually with progressive depression of respiratory and circulatory system.
These problems can be prevented by continuous monitoring of the patients. Now a
days, a variety of methods and devices are available to assess the functional
status of patients. Some of the techniques are too invasive and require
technical expertise but some others are relatively easy and non invasive. The
most popular non-invasive methods include electrocardiography,pulse
oximetry, capnography and sphygmomanometry.
These devices provide informations regarding
the electrical activity of heart, patient's blood oxygenation, respiratory
activity and arterial blood pressure directly, though there are some
limitations with each procedure. This article is indented to make the
practitioners familiarize with the principles of pulse oximetry, its clinical applications
and limitations in veterinary practice. It is a most popularly employed non
invasive monitoring procedure adopted in human anaesthesia and critical care
patients since 1980s. Pulse oximeter is a compact, portable device used for
monitoring oxygen saturation in the peripheral
circulation. The colour of blood is a reflection of its oxygen saturation and
it changes with oxygen saturation due to the optical properties of the haemoglobin
molecule, more specifically, the heam. Before pulse oximeters were available,
the arterialoxy haemoglobin saturation was assessed with the use of blood gas
analyzers. This technique is expensive and requires invasive sampling of
arterial blood and provides only intermittent monitoring.
ie.,pulse
oximetry is an excellent alternative tool for measuring PaO with a blood gas
analyzer. As pulse 2 oximetry provides
a continuous estimate of the oxygen saturation of haemoglobin in arterial blood(SpO
) with an early warning of desaturation, it enables clinicians to respond with preventive or
corrective measures before severe problems occur. In pulse oximetry, oxygen
status at the tissue level will be indicated before clinical signs are evident.
SpO reading 2above
95% indicates normal, 90% indicate minor desaturation and less than 85%
indicate hypoxemia. The 85% saturation in pulse oximetry is equivalent to 55
mmHg in PaO in blood gas analysis. The desaturation is not evident clinically as cyanosis until
enough deoxygenated blood is present to produce blue discolouration. Once
cyanosis is observed, such patients are already in a severe state of hypoxemia,
the consequences of which may be difficult to reverse. In anaesthesia, pulse
oximetry provides warning signals of hypoxemia, anaesthetic equipment failure,
disconnection of patient from oxygen source, endotracheal tube etc. In a normal
healthy patient subjected to elective surgical procedures, pulse oximetry is
not required obviously but it is certainly essential in emergency and critical
care of compromised animal patients along with arterial blood gas analysis or
alone. Pulse oximeter is also a useful tool in assessing the intestinal
perfusion and vitality.
PRINCIPLES:
The
principle of pulse oximetry is quite simple. As the blood deoxygenates, it
becomes increasingly less permeable to red light. The tissues then loose its
pinkish appearance, taking on a blue tint. Pulse oximetry is based on two
physical principles (1) the absorbance spectra of oxygenated haemoglobin is different
from that of deoxygenated haemoglobin and (2) the pulsatile component
of arterial blood can be distinguished as volume fluctuations between the
source of light and detector. By design, pulse oximeter utilizes two light
emitting diodes (LEDs) with wavelength of red light at approximately 660 mm and
infrared light at approximately 920 mm. to determine oxy haemoglobin
saturation. When a pulse oximeter probe is placed over a bed
of tissues, two light emitting diodes (LEDs) on one side of the probe emit red
and infrared light. The light is detected on the other side of the tissue bed
by a photo detector
that produce a current proportional to the intensity of light transmitted
through the tissues. The values displayed digitally are based on the
empirically derived tables, and a pulsating arterial supply is essential for
getting the readings in pulse oximeter. A photo detector, placed opposite to
these LEDs across the arterial vascular bed, measures the intensity of transmitted
light across the vascular bed. The difference in the intensity of transmitted
light between two LEDs is caused by the difference in the absorption of light
by oxygenated and deoxygenated haemoglobin contained within the vascular bed.
The determination of arterial haemoglobin oxygen saturation is computed by the
pulse oximeter from the relative amount of light transmitted to the photo
detector and are displayed digitally.
TECHNIQUE:
Two
types of probes are available transmission (lingual) and reflectance (rectal
and oesophageal).The probe of the pulse oximeter can be attached on the tongue,
ear, oesophagus, rectum, vulval lips or on the nipple of the teat. The size,
location, application and positioning of the probe of pulse oximeter present
unique challenges in veterinary patients. The difference in
tissue thickness and pigmentation can affect signal detection. The pressure on
the probe site can result in vasoconstriction and inadequate signal detection,
which can be rectified by relocating the probe. In anaesthetized / unconscious
animal, the tongue appears to provide the most
reliable signal detection site but this is rarely tolerated by an awake animal.
Apart from the site mentioned earlier, the probe can apply on digits, Achilles
tendon, lip, skin fold of the flank or axila, etc. In general, use a site with
thinnest skin area is selected. Once the site has been selected, the hair is
clipped and the area is cleaned before attaching the probe. While attaching the
probe, make sure that the probe is in full contact without creating too much
compression and make sure that the light emitting diodes are aligned.
Factors adversely affect the readings
1.
Presence of intense light sources like O.T. light can give false information
and this be prevented byplacing a towel over the sensor.
2.
Circumstances of low perfusion, only very small amount of arterial blood may
flow into the
arteriolar
bed results in weak pulsatile activity. Clinically this may be recognized with
hypothermic
or hypotensive patients.
3.
Increased motion may result in increased activity at the sensor and the
photographic detector may be unable to differentiate between pulsation that are
due to motion and that which are truly arterial.
4.
Long hairs, thick and pigmented skin etc. can inhibit light transmission.
5.
Dye injection / icterus, oedematous tissues etc gives false results.
CONCLUSION
Pulse
oximetry is a minimum standard method for monitoring patients during
anaesthesia and
critical care situations. Pulse oximetry can provide early warning signs of
dangerous desaturation
events, which enables clinicians to respond with preventive or corrective
measures before
severe problems occur. But it is up to the clinician, the ultimate"Signal
Processor"in interpreting
the data.
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