Plus-Hex CLINICAL
Table 3 . The patient ' s blood gas values .
Parameter
It was crucial to identify the underlying cause of the low
PaO 2 in this patient so appropriate action could be taken to resolve the issue . The possible causes included :
• Low FiO 2
• Hypoventilation
Sample taken at 10:30
• Diffusion impairment
• Right-to-left intracardiac shunt
Sample taken at 11:10
Normal value on 100 % O 2
pH 7.420 7.440 7.350 – 7.450
PaCO 2
( mmHg ) 58 52 35 – 45
PaO 2
( mmHg ) 289 376 > 500
HCO 3
–
( mmol / l ) 31.2 30.08 24 – 30 BE ( mmol / l ) 4 3 – 4 to 4
SaO 2
(%) 100 100 100 Lactate 2.45 2.04 < 2
PaCO 2
, partial pressure of carbon dioxide in arterial blood ; PaO 2 , partial pressure of oxygen in arterial blood ; HCO 3
–
, bicarbonate ;
BE , base excess ; SaO 2 , arterial oxygen saturation .
occur in patients that are lying down for long periods of time . However , even with adequate ventilation , perfusion issues can arise , leading to a mismatch in which ventilation does not align with perfusion , such as in cases of pulmonary embolism or anaemia , in which inspired O 2 is met with a diminished blood supply . When the PaO 2 is too low , treatment is required . As discussed , when a patient is disconnected from 100 %
O 2 and starts to breathe room air , the PaO 2 would further reduce dramatically . The partially collapsed alveoli can be inflated using the alveolar recruitment manoeuvre ( ARM ) [ 14 ] , and kept open by a positive end-expiratory pressure ( PEEP ) valve , which is placed on the expiratory end of the breathing circuit before administering ARM . In this case , an inspiratory breath was given to 20 cmH 2
O for 10 – 15 seconds ; this was repeated with a pressure increase to 25 cmH 2
O , then
30 cmH 2 O , 5 min apart .
A 10 cmH 2
O PEEP valve ( Vital signs , BD , UK ) ( Figure 3 ) was attached to the expiratory section of the breathing circuit before ARM was administered . The PEEP valve prevents the alveoli re-collapsing by creating resistance in the circuit and positive pressure within the lung , which keeps the alveoli open . Using the adjustable pressurelimiting ( APL ) valve or ‘ pop-off ’ can also create this resistance – and therefore PEEP – but great care must be taken in using it for this purpose [ 15 ] . Subsequent blood gas analysis showed that the action was a success .
• Ventilation versus perfusion ( V / Q ) mismatch . As the patient was being delivered 100 % O 2
, low FiO 2 was deemed not to be the cause of the low PaO 2
. Diffusion impairment and a right-to-left shunt were also considered unlikely , as the patient had been assessed as healthy prior to the operation .
Hypoventilation has a detrimental effect on oxygenation . As CO 2 rises in the blood and alveoli , it affects the haemoglobin ' s ability to bind and hold on to O 2
[ 13 ] and can therefore cause hypoxaemia ( lower than normal levels of O 2 in the blood ). However , with oxygen at 100 %, it is not possible to see hypercapnia ( higher than normal levels of CO 2 in the blood ) to the level that could reduce the PaO 2 down to hypoxic levels ( below 60 mmHg ) before the death of the patient . So , while the level of CO 2 could have had some effect on the PaO 2 it would not have been enough to reduce the
PaO 2 to 289 mmHg , so it was determined that a second factor was contributing to this result .
A V / Q mismatch was deemed to be a cause . The donkey was in lateral recumbency , so it was likely that atelectasis ( collapsed alveoli ) had developed in the dependent lung ( the lower part of the lung , which is most effected by gravity ), which limited the amount of O 2 available to diffuse into the circulating pulmonary blood . In a V / Q mismatch , ventilation can be inadequate although perfusion is adequate . This can
Figure 3 . A PEEP valve .
The patient ' s pH values remained within the normal range , but an elevated PaCO 2 and hypoventilation indicated respiratory acidosis . An increased level of
CO 2 due to hypoventilation will cause a decrease in the blood pH , which in turn activates the buffer bicarbonate ( HCO 3
–
), as shown in the first blood sample results ( Table 3 ). When CO 2 levels drop , as seen in the second sample , HCO 3
– also decreases to maintain pH stability . If this buffering system is overwhelmed , acid – base imbalances can develop [ 16 ] .
Volume 40 ( 2 ) • April 2025
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