Page 410 - Clinical Small Animal Internal Medicine

378 Section 5 Critical Care Medicine

partial pressure of oxygen can be inferred from the satu- blood. Mixed and central venous samples are preferable
VetBooks.ir ration due to the known relationship between the two as they represent a larger pooled sample than those
obtained from a peripheral vein. Values for PvO 2 below
(the oxyhemoglobin equilibrium curve). If pulse oxime-
try were to be employed for this purpose then the cut‐
or regional perfusion and oxygen delivery. Values below
offs for hypoxemia and severe hypoxemia become 95% 30 mmHg should prompt the clinician to assess global
and 92%, respectively. 20 mmHg should prompt immediate patient assess-
Hypoxemia may be due to hypoventilation, low ment and intervention. Recently published work has
inspired partial pressure of oxygen (PiO 2 ), or venous suggested that central venous hemoglobin saturations
admixture. Hypoventilation typically only results in greater than 68% are associated with improved out-
hypoxemia when the patient is breathing room air. comes in critically ill small animal patients. This value
Supplemental oxygen is sufficient to overcome hypox- was obtained on an instrument not validated for canine
emia due to increased alveolar carbon dioxide tension in blood and may only be directly relevant when that
all cases (except apnea). Low inspired partial pressure of instrument is used; however, the value reported is
oxygen (PiO 2 ; often incorrectly interpreted as FiO 2 ) can intriguingly similar to that reported for critically ill
commonly be due to decreased total inspired gas humans and titrating therapy to maintain ScvO 2 in the
pressure (low barometric pressure at elevation) or on high 60s is likely to be associated with improved out-
very rare occasion to a decreased fraction of inspired comes in populations, if not necessarily in each indi-
oxygen (e.g., rebreathing circuit with inadequate oxygen vidual patient.
inflow rates; asphyxiation). Analysis of venous blood gases reinforces an impor-
Venous admixture is a term to describe all the means tant point: adequate partial pressure and adequate total
by which venous blood may fail to properly equilibrate oxygen content are both required for optimal oxygen
with alveolar oxygen tensions during pulmonary perfu- delivery. Total content is essential to ensure that suffi-
sion. The principal means by which venous admixture cient oxygen is available to be extracted due to oxygen’s
occurs are as follows: (1) right‐to‐left anatomic shunting poor solubility in plasma. Partial pressure must also be
(congenital defects predominantly, although acquired high enough to drive diffusion from capillary beds to
shunting may occur if pulmonary hypertension devel- mitochondria. Neither inadequate carrying capacity
ops), (2) ventilation–perfusion mismatch (most common (hemoglobin concentration) nor inadequate partial
cause), and (3) diffusion impairment (rarely a sole cause, pressure (PaO 2 ) will allow for optimal cellular utilization
but contributes to a degree any time alveolar surface area of oxygen. Adequate partial pressure maintenance is par-
is lost due to collapse or flooding). Ventilation–perfusion ticularly important when the diffusion distance is
mismatching may be severe enough to result in physio- increased by tissue edema. The combination of
logic shunting when perfusion is maintained to alveoli hypoxemia and anemia represents a major challenge to
that are receiving no fresh gas via ventilation. The other homeostasis and oxygen supplementation is advisable
extreme (ventilation of nonperfused alveoli) results in while blood products are being obtained.
increased dead space ventilation and does not directly
lead to hypoxemia.
Both PaO 2 and SpO 2 tell one very little by themselves Evaluation of Gas Exchange
about the adequacy of oxygen delivery. In an anemic Efficiency
patient, the PaO 2 and SpO 2 may be normal while total
arterial oxygen content is completely inadequate. It As mentioned earlier, PaO 2 and SpO 2 evaluation taken in
serves little purpose to assess PaO 2 or SpO 2 in isolation isolation may tell one very little about the adequacy of
from other factors. If one is seeking to determine the oxygen delivery. They also poorly define lung function
adequacy of oxygen delivery to the tissues then venous when the clinical context is not considered. When one
partial pressures (PvO 2 ) and saturations (SvO 2 ) are more evaluates either parameter, the following factors must be
informative in this setting. PvO 2 cannot be used to eval- considered at the time the reading was obtained: (1) the
uate pulmonary oxygenating performance as the blood nature of the gas the patient is currently breathing and
has already been modified by cellular oxygen extraction. (2) the patient’s current ventilatory status. A low PaO 2 on
Properly handled samples can inform the clinician of the room air may be a result of poor lung function and
lowest value that PaO 2 could be (i.e., if PvO 2 is 64 mmHg venous admixture or it may be simply due to hypoventi-
then the patient cannot be severely hypoxemic as PaO 2 lation. An SpO 2 reading of 96% on room air may be
must be at least 64 mmHg). acceptable, whereas it is cause for significant concern on
PvO 2 serves as a reasonable marker of tissue hypoxia. 100% oxygen. These other factors cannot be ignored if
The partial pressure in the venous blood will be a reflec- any PaO 2 and SpO 2 readings are to be correctly inter-
tion of the partial pressure present in the end‐capillary preted. One approach to incorporating these factors into