3 Pulse Oximetry for the Measurement of Oxygen Saturation …
69
sensitivity. Equation (3.32) is simpler than Eq. (3.27) as computation of slopes are
not required.
A comparison of Eq. (3.9) employed in traditional pulse oximeters and Eq. (3.27)
or Eq. (3.32) employed in the present method for computation of SpO 2 reveals the
following:
(1) Here SpO 2 is computed without using the ratio of ratios R as well as an empirical
equation obtained through calibration.
(2) Since the methods described above do not require calibration constants K 1 and
K 2 , the methods are ‘calibration-free’.
(3) Since calibration is not required, the SpO 2 values obtained are not dependent
on the composition of volunteers engaged in obtaining constants K 1 and K 2 .
(4) To compute SpO 2 , only the slope and peak-to-peak values of the AC part of the
red and IR PPGs after applying natural logarithm alone are used. The DC part is
not at all used and hence the SpO 2 values obtained are not influenced by patient
and sensor dependent parameters.
The details of experiments conducted to validate Eqs. (3.27) and (3.32) and the
practicality of the method are given in [41, 42].
3.10 Problems Associated with Pulse Oximetry
Using the red and IR PPGs, not only oxygen saturation in arterial blood but also
heart rate and heart rate variability in a wide variety of clinical situations can be
determined [43]. Pulse oximeters enable quick, continuous and reliable measurement
of oxygen saturation at the same time avoid discomfort and risks of an arterial
puncture. However, present-day commercial pulse oximeters have shortcomings and
research is being carried out all over the globe in removing these shortcomings [44,
45]. The factors that adversely affect the accuracy of pulse oximeter output are:
(1) Probe placement [38],
(2) Peripheral vasoconstriction (poor perfusion) [39],
(3) Dyshaemoglobins [43],
(4) intravascular dyes [44],
(5) ambient light [46]
(6) nail polish [47, 48],
(7) skin pigmentation [49], and
(8) reliability problems due to movement of patient [50, 51].
(9) Design of pulse oximeter circuitry and the principle of measurement employed.
As the pulse oximeters use two wavelengths only, they can measure only
two haemoglobins and thus, estimate functional saturation, not fractional saturation. Other compounds that absorb light at the same wavelengths will thus introduce errors. Important limitation of pulse oximetry arises due to the presence
of abnormal haemoglobins. The most common and potentially most serious is
69
sensitivity. Equation (3.32) is simpler than Eq. (3.27) as computation of slopes are
not required.
A comparison of Eq. (3.9) employed in traditional pulse oximeters and Eq. (3.27)
or Eq. (3.32) employed in the present method for computation of SpO 2 reveals the
following:
(1) Here SpO 2 is computed without using the ratio of ratios R as well as an empirical
equation obtained through calibration.
(2) Since the methods described above do not require calibration constants K 1 and
K 2 , the methods are ‘calibration-free’.
(3) Since calibration is not required, the SpO 2 values obtained are not dependent
on the composition of volunteers engaged in obtaining constants K 1 and K 2 .
(4) To compute SpO 2 , only the slope and peak-to-peak values of the AC part of the
red and IR PPGs after applying natural logarithm alone are used. The DC part is
not at all used and hence the SpO 2 values obtained are not influenced by patient
and sensor dependent parameters.
The details of experiments conducted to validate Eqs. (3.27) and (3.32) and the
practicality of the method are given in [41, 42].
3.10 Problems Associated with Pulse Oximetry
Using the red and IR PPGs, not only oxygen saturation in arterial blood but also
heart rate and heart rate variability in a wide variety of clinical situations can be
determined [43]. Pulse oximeters enable quick, continuous and reliable measurement
of oxygen saturation at the same time avoid discomfort and risks of an arterial
puncture. However, present-day commercial pulse oximeters have shortcomings and
research is being carried out all over the globe in removing these shortcomings [44,
45]. The factors that adversely affect the accuracy of pulse oximeter output are:
(1) Probe placement [38],
(2) Peripheral vasoconstriction (poor perfusion) [39],
(3) Dyshaemoglobins [43],
(4) intravascular dyes [44],
(5) ambient light [46]
(6) nail polish [47, 48],
(7) skin pigmentation [49], and
(8) reliability problems due to movement of patient [50, 51].
(9) Design of pulse oximeter circuitry and the principle of measurement employed.
As the pulse oximeters use two wavelengths only, they can measure only
two haemoglobins and thus, estimate functional saturation, not fractional saturation. Other compounds that absorb light at the same wavelengths will thus introduce errors. Important limitation of pulse oximetry arises due to the presence
of abnormal haemoglobins. The most common and potentially most serious is