Page 81 - Basic Monitoring in Canine and Feline Emergency Patients

fully saturated as OxyHb (four O molecules) or dissociation curve is due to the cooperation of
2
fully desaturated (DeOxyHb, no O ). the four oxygen binding sites on the Hb mole-
2
VetBooks.ir Hb-binding sites bound to oxygen. Due to the con- cule. As the PaO increases, SaO rises rapidly as
The term ‘SaO ’ refers to the percentage of total
2
2
2
depicted by the steep slope of the curve. On the
formational changes described above, Hb tends to
small changes in SaO (e.g. 95% to 90%) actu-
either be fully bound (oxygen on all four sites – upper ‘flatter’ part of the curve, what seem to be
2
OxyHb – or deoxygenated). In other words, if 100 ally represents larger significant changes in oxy-
Hb proteins had a SaO of 75%, it would typically gen concentration in the blood. As the curve
2
indicate 25 were fully desaturated as DeOxyHb becomes steeper, relatively small losses of lung
and 75 were fully saturated as OxyHb, as opposed function affecting PaO can result in relatively
2
to all 100 proteins each having three out of four large changes in SaO . The top of the curve is
2
binding sites filled with oxygen. flattened because once all Hb molecules are fully
saturated (SaO = 100%), no matter how much
2
the PaO is increased, no further gain in SaO is
Oxyhemoglobin dissociation curve 2 2
possible.
How many Hb are fully saturated as OxyHb is For patients who are breathing room air and
determined by the partial pressure of oxygen in the therefore are still within the sigmoidal portion of
plasma (PaO ). Oxygen diffuses from the alveolus the curve, the relationship between SpO and
2
2
into the plasma first and then loads onto Hb. PaO allows SpO to be used as a surrogate marker
2
2
Conversely, when oxygen diffuses from the plasma of PaO . For example, in a patient breathing room
2
into the tissues, oxygen is released from the Hb air with a pulse oximetry reading of 95%, it can be
to replenish the plasma and allow for continued assumed that the PaO is approximately 70 mmHg.
2
oxygen diffusion into tissues. The oxygen–hemo- The relationship between the dissolved oxygen and
globin dissociation curve (Fig. 4.3) describes the Hb-bound oxygen has significant clinical implica-
relationship between the SaO and the PaO . tions for the total oxygen content able to be carried
2
2
The dissolved oxygen (PaO ) is the independ- in the blood as depicted in Box 4.1.
2
ent variable and is determined by lung function The oxygen–hemoglobin dissociation curve is a
(see Chapter 5). The sigmoid shape of the OxyHb way to use the SpO to approximate the PaO
2
2
100
90
Hemoglobin oxygen saturation (%) 60
80
70
50
40
30
20
10
0
0 50 100 150
Oxygen partial pressure (mmHg)
Fig. 4.3. The oxygen–hemoglobin dissociation curve describes the relationship between the percentage of Hb binding
sites that are saturated with oxygen (SaO ) and the partial pressure of oxygen in arterial blood (PaO ) measured in
2
2
millimeters of mercury (mmHg). Open source illustration from https://www.researchgate.net/figure/Hemoglobin-Binding-
Curve-Hemoglobin-oxygen-saturation-as-a-function-of-partial-pressure_fig10_309876232 (accessed 3 August 2019).

Pulse Oximetry 73