Rate of Respiratory Normal Range: 12–24 breaths per minute
117
Measurement is normally done to analyze the oxygenation and saturation of hemoglobin in the blood. There are a few variables
used such as partial pressure in mmHg of oxygen in arterial blood
ࡼࢇࡻ
, which refers to a technique to measure the arte-
rial percentage of blood. ࡿࢇࡻ
And ࡿࢇࡻ
refers to direct and indirect measurement of the percentage in blood oxygen satura-
tion level. The former is determined using pulse oximetry and the latter is measured using arterial blood gas sampling. Even
though, ࡿࢇࡻ
and ࡿࡻ
appear to be similar actually these two variables are different from one another. The reading that is ob-
tained by arterial blood gas sampling can be affected by condi- tions such as anticoagulant medications and thrombolytic.
These variables are linked to respiration as inhalation brings oxygen into the lungs and exhalation releases carbon dioxide
out from the lungs.
ࡼࢇࡻ
Refer to a measurement of gas which can be determined using polarographic oxygen electrode as
shown in Figure 4.8. Polarographic oxygen electrode contains a platinum cathode and a silver chloride anode in which generates
an electrical current; the electrical current is proportional to the oxygen. The blood sample must be isolated from the electrode
by a membrane to avoid protein deposition. The equipment must be kept in an oven that has a temperature analogous to the
human body temperature which is approximately 37
◦C. The membrane is should not contain any protein deposit that might
accumulate on the surface over time. Pulse oximetry is a non- invasive method of continuous arterial oxygen saturation moni-
toring. The pulse OX meter is a tiny portable device which par- amedics can carry to the site of an accident. It can measure the
arterial oxygen saturation
ࡿࢇࡻ
of a patient. According to the formulae, the maximum amount of oxygen that can be carried
by the blood is determined as:-
ࡿࢇࡻ
ൌ
ை
మ
ை
మೌೌ
ݔͳͲͲΨ
4.1
118
The formulae above will give an idea of an oxyhaemoglobin dissociation curve. In order to get precise measurement, it re-
quires an oxygen meter that uses light source like red and infra- red LED that able to shine through certain body parts where a
considerably translucent area of blood flow can be exposed to light. Figure 4.9 show that oxygenated hemoglobin and deoxy-
genated hemoglobin absorbs infrared light and red light respec- tively. The measurement is usually taken from the fingers or ear
lobe. The process involves light passing through the blood ves- sel where certain portion of red and infrared light beam is ab-
sorbed. The photocell will then receive whatever is left and then deduce the red-to-infrared ratio of absorbed light through blood.
Figure 4.9 Infrared energy absorption by hemoglobin versus wavelength FONG et al 2011
Figure 4.10 shows the arrangement, where a 100 ࡿࡻ
pro- duces a received light ratio of about 50. It is important to note
that calibration is a must as the extent of light absorbed by skin and tissue varies.
119
Figure 4.10 Pulse oximeter oxygen saturation
ࡿࡻ
FONG et al 2011 The accuracy of the measurement can also be affected as the
amount of arterial blood flow is affected by the sequence of heart beat. Therefore, it is a must to measure for a sufficient period of
time that covers two successive heart beats to get the average value. In the case of an accident, the measurement of oxygen sat-
uration is a crucial step to detect hypoxia in order to facilitate emergency treatment when the victim arrives at the hospital.
Hereby, problems such as tricuspid regurgitation, hypovolaemia and vasoconstriction affecting the blood flow may influence the
measurement given by an oximeter. An oximeter is also unable to differentiate carboxy hemoglobin from normal hemoglobin in the
case of carbon monoxide poisoning and it will also cause the reading to be of higher value than the actual value.