Page 444 - Clinical Small Animal Internal Medicine

412 Section 5 Critical Care Medicine

that is delivered to the capillaries via the arteries and the The most significant advantage of venous hemo­
VetBooks.ir amount found in the venous blood draining the capillar­ globin saturation monitoring is that increases in OER
(i.e., lower than expected venous hemoglobin satura­
ies. This ratio indicates the proportion of oxygen that
the mitochondria are using versus the amount supplied.
metabolism, giving the clinician an essential head start
If venous blood has high hemoglobin saturation, then the tion) may be evident before the onset of anaerobic
mitochondria did not consume as much oxygen as in the treatment of shock. An ScvO 2 of <70% indicates
expected. This may indicate low metabolic rate or mito­ that a patient is experiencing tissue oxygen debt. When
chondrial dysfunction, as seen in sepsis. However, if performing resuscitation, therefore, the goal is to main­
venous oxygen saturation is low, it suggests that the tain an ScvO 2 of >70%. If this is not possible with the
mitochondria are extracting more oxygen than expected administration of vascular expansion and vasopressor
and that an oxygen debt may be present. support alone then treatment with red blood cells may
Importantly, venous hemoglobin saturation only reflects be indicated. Venous hemoglobin saturation may be
the oxygen consumption that is occurring in the tissue bed low for several reasons, including slow capillary transit
that the blood is draining. As an example, if you were to time (poor microperfusion), increased tissue demand
draw blood from a cephalic vein, you will only be gaining for oxygen, low arterial hemoglobin saturation or ane­
knowledge about oxygen usage in the distal limb. Blood mia. Once again, venous hemoglobin saturation should
drawn from the jugular vein will provide information be interpreted with the patient’s overall condition in
about oxygen consumption in the head, including the mind.
brain. Under most circumstances, samples drawn from
the jugular vein are approximately reflective of oxygen
consumption throughout the body, but a better represen­ Conclusion
tation of the global oxygen usage would be venous blood
that has been mixed from all parts of the body. This ideal A simplified and systematic approach to the patient
sample is retrieved from the pulmonary artery and is in shock is necessary for successful management. It is
termed the mixed venous oxygen saturation (SvO 2 ). The easy to become overwhelmed with the totality of clini­
pulmonary artery is not routinely catheterized in people cal implications and lose sight of the ultimate goal.
and even less frequently in veterinary patients, but there is Approaching shock from the standpoint of oxygen
a strong correlation between mixed venous samples and delivery makes the interventions that need to be
central venous samples (ScvO 2 ) obtained from the cranial considered finite and easier to implement. Frequent
vena cava or right atrium. Obtaining an ScvO 2 sample is reevaluation of the patient through the endpoints of
much more feasible in clinical practice and can prove very resuscitation will allow determination of the efficacy
useful for guiding resuscitation efforts. of therapy.

Further Reading

Allen SE, Holm JL. Lactate: physiology and clinical utility. Silverstein DC. Pruett‐Saratan II A, Drobatz KJ.
J Vet Emerg Crit Care 2008; 18: 123–32. Measurements of microvascular perfusion in healthy
Hall JE. Guyton and Hall Textbook of Medical anesthetized dogs using orthogonal polarization spectral
Physiology, 12th edn. Philadelphia, PA: Saunders imaging. J Vet Emerg Crit Care 2009; 19: 579–87.
Elsevier, 2011. Zacher LA, Berg J, Shaw SP, et al. Association between
Lisciandro GR. Abdominal and thoracic focused outcome and changes in plasma lactate concentration
assessment with sonography for trauma, triage, and during presurgical treatment in dogs with gastric
monitoring in small animals. J Vet Emerg Crit Care dilatation‐volvulus: 64 cases (2002–2008). J Am Vet
2011; 21: 104–22. Med Assoc 2010; 236: 892–7.