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Horses are known to be highly susceptible to hypoxemia,
especially when a recumbent horse is placed in the dorsal
position for surgery. Anesthetic drugs can induce hypoventilation
or apnea, which will lead to ventilation-perfusion mismatch (V/Q
defects) in the lungs.
The gold standard for measuring PaO
2
is through an analysation
of an arterial sample, by checking for oxygen in the arterial
blood. However, another (non-invasive) way to measure oxygen
concentrations is to place a small SpO
2
probe clip on the tip of the
tongue (SpO
2
), where the tissue is soft and nonpigmented.
Due to the pressure of the clip on the tongue, in some cases
blood doesn't pass well, which can disturb readings. To prevent
this from happening, ensure you regularly reposition the clip on
the tongue. However, this generally only happens with small
foals and isn't a common issue for adult horses.
TIP It is important to check that the tongue is moisturized
throughout surgery, and if it does become dry, wet it with a
moistening gel and reattach the clip.
When recording SpO
2
, a reading between 95-98% haemoglobin
saturation is normal (see graph shown below
2
). If this falls below
90% (hypoxemia), apply positive end-expiratory pressure (PEEP)
during mechanical ventilation to ensure the patient is respirating
at a proper level to get enough oxygen circulating through the
blood. Hypoxemia can also be treated with medication and the
recruitment manoeuvre.
0
10
20
30
40
P0
2
(mm Hg)
pH
DPG
Temp
pH
DPG
Temp
Oxyhaemoglobin
(%
saturation)
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
SpO
2
clip on tongue
Pulse oximetry – SpO
2
Equine anesthesia tips
