Page 83 - Basic Monitoring in Canine and Feline Emergency Patients

and is insensitive for detection of earlier, milder While very small shifts in the curve may happen
losses of lung function. even within different tissue beds, larger systemic
VetBooks.ir also be shifted either left (increased Hb affinity for abnormalities in pH or temperature can cause more
The oxygen–hemoglobin dissociation curve can
global effects on oxygen delivery in the body. The
O ) or right (decreased Hb affinity for O ; see Fig.
2
2
curve makes the ‘normal’ expected relationship
4.4). As seen by the dashed lines in Fig. 4.4, when reason that this is important is that shifting of the
the curve is shifted to the right, the same PaO cor- between PaO and SaO invalid. For example,
2
2
2
relates with a decreased SaO versus the normal practitioners should be aware that in a patient who
2
curve. For a left shift, it is the opposite; the same has an extreme fever, the curve will be shifted to the
PaO correlates with an increased SaO versus right and the SaO will be lower than expected
2
2
2
normal. Factors that will right shift the curve given the measured PaO (Fig. 4.4). In this sort of
2
include increased concentration of hydrogen ions setting, the best option for a clinician is to measure
+
(H ; i.e. decreased pH), CO , and temperature. A the PaO directly rather than trying to predict it
2
2
practical example of this occurs in skeletal muscles from the SaO reading.
2
during exercise. Exercise increases the amount of
+
CO , H , and temperature, right shifting the curve
2
and reducing the affinity of Hb for oxygen. The Abnormal hemoglobin forms
goal is to improve offloading of oxygen in the
metabolically active tissues. Conversely, decreases Methemoglobin
in H ions, increased pH, decreased CO , and Oxidative damage to red blood cells can cause the
+
2
decreased temperature cause the curve to be formation of methemoglobin (MetHb). The dam-
shifted to the left. These circumstances improve age results in the iron molecule at the core of the
2+
uptake of oxygen onto Hb and are similar to what heme being converted from the ferrous (Fe ) to the
3+
occurs in the pulmonary capillaries where oxygen ferric state (Fe ). In this state, MetHb is unable to
diffuses first into the plasma and is then loaded bind to oxygen. Because some oxidative damage is
onto the Hb. always occurring in nature, red blood cells contain
Left shift
Decreased temperature
100 Decreased CO 2
Decreased [H+]
90
80
Oxyhemoglobim (% saturation) 60 Right shift
70
(reduced affinity)
50
Increased temperature
Increased CO 2
40
Increased [H+]
30
dissociation curve. The normal curve is
depicted in the solid line, with the dotted
20 Fig. 4.4. Shifts in the oxyhemoglobin
lines indicating either right or left shifting
caused by the listed factors. PO , partial
10 2
+
pressure of oxygen; [H ], hydrogen ion
concentration; CO , carbon dioxide. Open
2
0 10 20 30 40 50 60 70 80 90 100 source illustration from https://litfl.com/
oxygen-haemoglobin-dissociation-curve/.
PO (mmHg)
2 (accessed 3 August 2019).
Pulse Oximetry 75