RESPIRATORY RATE
There is considerable variation of this parameter e.g. the "normal" respiratory rate for a neonate is significantly different to that of a 60-year old adult.
HYPNOSIS
This is an unreliable sign of asphyxia and varies with:
- skin pigmentation
- level of haemoglobin
- ambient light
- skin perfusion
OXYGEN SATURATION
This is assessed by pulse optometry and has become an integral part of the assessment of respiratory status and indeed is considered an additional vital sign. Like the other vital signs it must be assessed in context and its limitations understood.
The pulse taximeter will display:
- oxygen saturation
- pulse rate
- pulse volume
The last parameter is displayed as a ethnographically waveform and is subject to more error than the oxygen saturation reading. Alarms are usually present for low saturation and for pulse rates at upper and lower limits. The accuracy for oxygen saturation is +/- 2% only between 70-90% range.
It must be remembered that saturation is not the same as the partial pressure of oxygen and there is not a linear relationship between the two. The relationship is described by the hemoglobin-oxygen dissociation curve and this in turn is not a constant as it is affected by temperature, pH and PC O 2.
Rough correlations would be :
75% sat = Pa O 2 40 mm.Hg
90% sat = Pa O 2 60 mm.Hg
Factors influencing pulse optometry readings :
- Signal interference - shivering, movement, high intensity light from another source
- Decreased light transmission - dirty skin, dark nail polish
- Decreased signal volume - poor peripheral perfusion
- Abnormal hemoglobin - car boxy hemoglobin (causes an overestimation of saturation)
Anemia (especially at levels < 5 g % also affect the signal)
- Others e.g. intravenous dyes
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