Page 411 - Clinical Small Animal Internal Medicine

38 Respiratory Monitoring in Critical Care 379

a more holistic approach to interpreting oxygenation above) would be on room air then subtracts 20 mmHg
VetBooks.ir parameters is to consider gas exchange efficiency. from that value, one obtains the lowest value for PaO 2
that could be present with that level of ventilation and no
Gas exchange efficiency measures are a means of tak-
ing a given PaO 2 and SpO 2 value and asking, “What alve-
olar partial pressure of oxygen was required to achieve venous admixture. One finds that the PACO 2 and PaO 2
obtained this way always add up to greater than
that value?” These tools are primarily a means of assess- 120 mmHg. Thus, for an arterial blood gas result
ing how efficient the lung is at bringing pulmonary capil- obtained on room air, the finding that the PaCO 2 and
lary and alveolar partial pressures of oxygen into PaO 2 add up to more than 120 mmHg suggests that
equilibrium. When patients are breathing room air, the venous admixture is unlikely to be the cause of hypox-
most widely used tool for this purpose is the alveolar‐ emia (if present). This is essentially a rapid, nonquantita-
arterial PO 2 gradient (A‐a gradient). This approach is tive means of determining whether the A‐a gradient is
based on the ideal alveolar gas equation (equation 1), greater or less than 20–mmHg using an arterial blood
which provides a means by which alveolar PO 2 may be gas obtained on room air. It is the author’s understanding
approximated. that this clever shortcut was developed by Dr Steve
Haskins at UC Davis. It has proven to be an effective and
/ RQ ] (Eq. 1)

[PAO 2 FiO 2 *(P B P H O2 ) PaCO 2 efficient screening tool for venous admixture in critically
In order to perform the calculation, one must know ill dogs and cats.
the following factors which are assumed or obtained The A‐a gradient performs more poorly when
from an arterial blood gas analysis: (1) FiO 2 , the fraction patients are receiving supplemental oxygen because of
of inspired oxygen (0.21 on room air), (2) P B , barometric the wide range of gradients that can be obtained from
pressure (760 mmHg at sea level), (3) P H2O , water vapor normal, healthy animals under these conditions. In this
pressure (varies with body temperature, but usually setting, the ratio of PaO 2 to FiO 2 (or P:F ratio) is more
taken as 50 mmHg), (4) PACO 2 , alveolar carbon dioxide typically employed. Healthy animals have P:F ratios
tension (arterial value is usually substituted), and (5) RQ, approximating 500 (unitless). For example, a healthy
respiratory quotient (which varies with metabolic sub- dog with a PaO 2 of 105 mmHg on room air (21% or 0.21
strate used as a fuel source, 0.8–0.9 typically used). A oxygen) would have a P:F ratio of 500 (i.e., 105/0.21 =
simplified version (equation 2) is often used clinically. 500). Humans and animals with acute lung injury (ALI)
PAO 2 FiOP B2 ( 50) 12.( PaCO 2 ) (Eq. 2) typically have P:F ratios of less than 300. This equates
to a P a O 2 of less than 63 mmHg on room air. Humans
Once one has calculated PAO 2 , the measured value of and animals with P/F ratios of less than 200 are consid-
PaO 2 is subtracted from it to obtain the difference ered to have lung function comparable to patients with
between the two (A‐a gradient). Lower values indicate acute respiratory distress syndrome (ARDS). The P:F
more efficient pulmonary gas exchange, with values less ratio is of no use on room air. All that would represent
than 15 considered normal and greater than 20 is a transformation (x/0.21) of the PaO 2 , which would
abnormal. provide no additional information. Moreover, since the
The normal range for the A‐a gradient is affected by P:F ratio does not take ventilation into account, abnor-
the inspired oxygen concentration. One rule of thumb is mal values obtained from patients breathing room air
that a normal animal may have a gradient of 1 mmHg for could suggest that lung injury/venous admixture is pre-
each percent oxygen they are inspiring (e.g., up to a sent when in fact hypoventilation is the sole cause of
20 mmHg gradient is normal on room air and up to a hypoxemia.
100 mmHg gradient is normal on 100% oxygen). These The P:F ratio is best used to compare gas exchange
values are rough approximations only, however, and efficiency using values obtained from the same patient
serial comparisons are best made on room air whenever while receiving supplemental oxygen or on mechanical
possible. A‐a gradients greater than 20 mmHg indicate ventilation (or both). However, the P:F ratio is highly
that venous admixture is present and contributing to labile and prognostication and lung recovery assessment
hypoxemia. This does not rule out hypoventilation as a are best performed with many serial measurements
secondary contributor (both may be present). Conversely, rather than just two.
hypoxemia with concurrent hypoventilation and a nor- Recently, it has been suggested that the ratio of SpO 2 to
mal (<20 mmHg on room air) A‐a gradient suggests that FiO 2 might serve as a noninvasive alternative to the P:F
hypoventilation is the sole cause of hypoxemia. ratio. This approach has been validated in human
The “Rule of 120” is a nonquantitative shortcut for patients with ARDS, but as of this writing no such valida-
assessing whether venous admixture is present. If one tion has been published in the veterinary literature. One
takes any physiologically plausible value for PACO 2 and small pilot study tends to support the utility of this
calculates what the resultant PAO 2 (using equation 1 approach.