Page 66 - Clinical Application of Mechanical Ventilation

32 Chapter 2

Positive
Pressure
Ventilation

Increase
in Intrathoracic
Pressure

Compression
of Pulmonary
Vessels

Reduction
in Stroke
Volume

Reduction of Cardiac
Output and
Pulmonary Blood Flow

(High) V/Q Mismatch

Hypoxemia

Decrease
in O
2
Content

Decrease © Cengage Learning 2014
in O 2
Delivery
Figure 2-2 Positive pressure ventilation leads to a decrease in O 2 delivery.

Pulmonary Blood Flow and Thoracic

Pump Mechanism

During positive pressure ventilation, intrathoracic pressure changes according to the
pressure transmitted across the lung parenchyma. In turn, changes in intrathoracic
pressure can affect the pulmonary blood flow entering and leaving the ventricles.

Left ventricle. In the left ventricle, the effect of an increase in lung volume on pulmo-
thoracic pump mechanism:
Alternations in pulmonary nary venous blood flow is dependent on the relative state of filling of the pulmonary
blood flow caused by changes circulation. In patients who are hypotensive, an increase in tidal volume causes a de-
in intrathoracic pressure during
positive pressure ventilation. In crease in pulmonary venous return to the left ventricle (Figure 2-3) (Pinsky, 1990).
hypotensive conditions, positive
pressure ventilation decreases In hypertensive patients, use of large tidal volumes increases venous return to the
the blood flow to the left heart. left ventricle (Figure 2-4) (Pinsky, 1990). This is because compression of pulmo-
In hypertensive conditions, this
mechanism enhances the outflow nary blood vessels is minimal in hypertensive conditions. It is also due in part to the
of blood from the right ventricle thoracic pump mechanism where the blood flow from right to left ventricle is enhanced
and into the left heart.
during the expiratory phase of positive pressure ventilation (DiCarlo et al., 1994).

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