Page 67 - Clinical Application of Mechanical Ventilation
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Effects of Positive Pressure Ventilation 33
A B
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Figure 2-3 In hypotensive conditions, positive pressure ventilation decreases the blood flow
to left heart. (A) Spontaneous breathing; (B) Positive pressure ventilation causes compression of
the pulmonary blood vessels in hypotensive conditions.
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Figure 2-4 In hypertensive conditions, positive pressure ventilation increases the blood flow
to left heart in part due to the thoracic pump mechanism (see text). (A) Spontaneous breathing;
(B) Positive pressure ventilation does not cause significant compression of the pulmonary blood
vessels in hypertensive conditions.
Right ventricle. In the right ventricle, high airway pressures and large tidal volumes
used in positive pressure ventilation stretch and compress the pulmonary blood
vessels and limit their capacity to hold blood volume. During expiration, the pul-
monary vessels, no longer under high pressure and large tidal volumes, are free
to fill to their holding capacity with the blood leaving the right ventricle. This
thoracic pump mechanism facilitates the outflow of blood from the right ventricle
(Figure 2-5) (DiCarlo et al., 1994).
A B
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Figure 2-5 Thoracic pump mechanism. (A) During the inspiratory phase of positive pressure
ventilation, the pulmonary blood vessels are compressed and the blood flow from right to left
ventricle is decreased. (B) During the expiratory phase, the pulmonary blood vessels are no longer
under alveolar compression and the blood flow from right to left ventricle is increased.
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