Page 49 - Clinical Application of Mechanical Ventilation

Principles of Mechanical Ventilation 15

TABLE 1-6 Interpretation of Shunt Percent

Physiologic Shunt Interpretation

,10% Normal

10% to 20% Mild shunt
20% to 30% Significant shunt

.30% Critical and severe shunt
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one for noncritical patients (e.g., spontaneous breathing, moderate level of F O ,
I
2
moderate level of continuous positive airway pressure) and one for critically ill pa-
tients (e.g., mechanical ventilation, high F O , high level of positive end-expiratory
2
I
pressure).
For noncritical patient:
See Appendix 1 for
example. Q (CcO - CaO )
Estimated SP = 2 2
Q T [5 + (CcO - CaO )]
2
2
For critical patient:
Q (CcO - CaO )
Estimated SP = 2 2
Q T [3.5 + (CcO - CaO )]
2
2
Classic Physiologic Shunt Equation. The classic physiologic shunt equation requires an arte-
The classic physiologic rial blood sample and a mixed venous blood sample from the pulmonary artery . It is
shunt equation requires arte-
rial and mixed venous blood the most accurate among all shunt equations.
samples.
Q (CcO - CaO )
Classic SP = 2 # 2
Q T (CcO - CvO )
2
2
When the shunt percent is too high, oxygenation becomes an extremely difficult
See Appendix 1 for
example. task for the cardiopulmonary system to support. Over time, the respiratory muscles
fatigue, resulting in ventilatory failure. This is usually followed by oxygenation fail-
ure if ventilatory interventions are unsuccessful (Rochester, 1993).
Since intrapulmonary shunting is a result of lack of adequate ventilation due to col-
lapsed or fluid-filled lung units, positive end-expiratory pressure (PEEP) or continuous
positive airway pressure (CPAP) may be used to open up and ventilate these lung units.
Chapter 4 describes the use of PEEP or CPAP in the management of intrapulmonary
shunting during mechanical ventilation (PEEP) or spontaneous breathing (CPAP).

Diffusion Defect

Diffusion of gases (including oxygen and carbon dioxide) across the alveolar-
capillary (A-C) membrane is mainly dependent on the gas pressure gradients. Oxy-
gen diffuses from the alveoli (P O 5 109 mm Hg) to the pulmonary arterial
2
A
#
capillaries (PvO = 40 mm Hg) with a pressure gradient of 69 mm Hg. Carbon
2

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