Blood gas results

Published on 03/06/2015 by admin

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Last modified 03/06/2015

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CHAPTER 5 Blood gas results

David Cartwright, Mark Davies

Blood gas report

Each intensive care unit should have an on-site blood gas machine. All give similar results.

The blood gas machine discussed here as an example is the Radiometer ABL 730. For blood gas analysis it measures three blood gas parameters — pH, PaO2 and PaCO2. A printed report from this machine is shown on page 54 as an example. Since, clearly, different machines will give different reports, it is important to be aware of exactly what your unit’s machine measures and reports. The report in the example is divided into eight parts:

1. patient identification
2. blood gas values — the other values derived from these, in subsequent sections of the report, are either calculated or estimated. They have the subscript ‘c’ for calculated, or ‘e’ for estimated (for those involving the alveolar gas equation, which makes some assumptions)
3. blood oximetry values — displaying measured values for total haemoglobin (ctHb), oxygen saturation (sO2), and proportions of some unusual haemoglobin compounds, such as fetal haemoglobin (HbF)
4. electrolyte values for Na+, K+, Cl−, ionised Ca2+
5. values for glucose, lactate, bilirubin
6. oxygen status
7. acid–base status calculations — actual base excess and actual bicarbonate (see below)
8. other calculated values — anion gap, pO2(A–a)e, pO2(a/A)e (see below).

Figure 5.1 shows an example from the Intensive Care Nursery (ICN) gas machine at the Royal Brisbane and Women’s Hospital.

image

Figure 5.1 Example blood gas result

Notes on calculated values

Standard bicarbonate, cHCO3−(P,st)c

The standard bicarbonate is a notional value for the bicarbonate concentration that would be present in the sample of blood if the PaCO2 were 40 mmHg. Its variance from a normal value of 24 indicates metabolic acidosis (<24) or metabolic alkalosis (>24).

We suggest using base excess (see below) rather than bicarbonate for these types of interpretations.

Actual bicarbonate

The actual bicarbonate is the bicarbonate content of the sample, calculated from the Henderson-Hasselbalch equation:

image

This value varies in proportion with changes in PaCO2, in contrast with the standard bicarbonate which will not change with changes in PaCO2, only with changes in acid or base content of the blood sample. Note in the examples below, all calculated on the same sample of blood, how the actual bicarbonate reflects the PaCO2 changes, while the standard bicarbonate and base excess remain constant.

Care must be taken to use the standard

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