Page 68 - Clinical Application of Mechanical Ventilation

34 Chapter 2

In children with right ventricular dysfunction, high positive pressure (up to 40 cm
H O) and large tidal volumes (20 to 30 mL/kg) may reduce the workload of the
2
right heart by the action of the thoracic pump mechanism (DiCarlo et al., 1994).

HEMODYNAMIC CONSIDERATIONS

One of the major adverse effects of mechanical ventilation is the changes in a pa-
central venous pressure (CVP):
Pressure measured in the vena tient’s hemodynamic status. The major hemodynamic measurements affected by
cava or right atrium. It reflects positive pressure ventilation include central venous pressure (CVP) and pulmo-
the status of blood volume in
the systemic circulation. Right nary artery pressure (PAP). The pulmonary capillary wedge pressure (PCWP)
ventricular preload. is not affected to a great extent because of the capability of the systemic venous
circulation to compensate or regulate changing blood pressure and volume.
pulmonary artery pressure
(PAP): Pressure measured in the Positive Pressure Ventilation
pulmonary artery. It reflects the
volume status of the pulmonary
artery and the functions of the Positive pressure ventilation causes an increase in intrathoracic pressure and compres-
ventricles. Right ventricular
afterload. sion of the pulmonary blood vessels. Partial recovery is observed during the expira-
tory phase. It is estimated that 15% to 20% of pulmonary blood volume is shifted to
the systemic circulation at a tidal volume of 1 L. An increase in intrathoracic pressure
pulmonary capillary wedge and compression of the pulmonary blood vessels causes an overall decrease in ven-
pressure (PCWP): Pressure
measured in the pulmonary artery tricular output, stroke volume, and pressure readings (Versprille, 1990).
with a balloon inflated to stop
pulmonary blood flow. It reflects Table 2-4 shows the general effects of positive pressure ventilation on hemody-
the volume status and functions namic measurements. It is essential to remember that the severity of these hemo-
of the left heart. Left ventricular
preload. dynamic changes is dependent on the level of airway pressures, lung volume, and
compliance characteristics of the patient.

Positive End-Expiratory Pressure
Positive pressure ventila-
tion causes an increase in
intrathoracic pressure and Positive end-expiratory pressure (PEEP) is a modality used in conjunction with
compression of the pulmonary
blood vessels leading to an positive pressure ventilation. PEEP has a profound effect on the PAP and mPaw.
overall decrease in ventricular In one study, when PEEP was initiated and increased to 15 cm H O over 90 sec,
output, stroke volume, and 2
pressure readings. the CVP and PAP showed a drastic increase while the aortic pressure and cardiac
output showed a significant decrease (Versprille, 1990). PEEP must be used with
extreme care in a clinical setting because PEEP, in addition to positive pressure

PEEP increases CVP and ventilation, can potentiate the reduction in cardiac output.
PAP but decreases aortic pres- Table 2-5 outlines the general effects of PEEP on hemodynamic measurements. It
sure and cardiac output.
is important to remember that PEEP is used in conjunction with positive pressure
ventilation. For this reason, the hemodynamic changes may be different from those
caused by positive pressure ventilation alone. The severity of these hemodynamic
changes is also dependent on the lung volume and compliance.
The decrease in cardiac output due to positive pressure ventilation and PEEP can
be managed by using appropriate intravascular volume expansion and positive ino-
tropic support. A patient with adequate intravascular volume or one who receives a
positive inotrope may have a smaller decline in cardiac output during positive pres-
sure ventilation and PEEP (Perkins et al., 1989).

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