Page 254 - Clinical Application of Mechanical Ventilation

220 Chapter 8

INITIAL VENTILATOR SETTINGS

When it becomes necessary to provide mechanical ventilatory support for a patient,
the following basic ventilator settings must be determined: mode, frequency, tidal
volume, F O , inspiratory:expiratory ratio, inspiratory flow pattern, and various
I
2
alarm limits.
These initial ventilator settings are mainly based on a patient’s body size, diagno-
sis, pathophysiology, and laboratory results. These settings only serve as a starting
point and they should be adjusted according to changes in the patient’s condition
and requirements.

Mode

The first step in selecting the ventilator mode is to decide whether the patient
should receive full ventilatory support (FVS) or partial ventilatory support (PVS).
Full ventilatory support is achieved by any mode that assumes essentially all of the
Full ventilatory support work of breathing. The majority of ventilator patients initially require full support,
may be necesssary if the
patient is not breathing with the control mode or the assist/control mode. The synchronized intermittent
spontaneously between mandatory ventilation (SIMV) mode also provides full ventilatory support if
mechanical breaths.
the patient is not breathing spontaneously between mechanical breaths, and the
mandatory frequency is set at 12/min or higher.
Partial ventilatory support is achieved by any mode that provides less than the total
Partial ventilatory sup- amount of the work of breathing. Partial support would be inappropriate initially for
port is achieved by any mode
that provides less than the patients with ventilatory failure, and it is more commonly used during the weaning
total amount of the work of process. Some examples are bilevel positive airway pressure (BiPAP), and pressure
breathing.
support ventilation (PSV). These topics are discussed elsewhere in this text.

Dual Control Mode

In traditional ventilation, a single control variable is selected to achieve a desired
goal. Examples of the single-variable control include volume-controlled ventilation
(VCV) and pressure-controlled ventilation (PCV). In VCV, a desired tidal volume
(a control variable) is set and the pressure changes from breath to breath depending
on the characteristics of the patient-ventilator system. In PCV, the desired pressure
(a control variable) is set and the delivered volume changes according to the charac-
teristics of the patient-ventilator system (Campbell et al., 2002).
A dual control mode combines two control variables (e.g., pressure and vol-
dual control mode: A combined
mode between two control ume) that are regulated by independent feedback loops so that the delivered breath
variables (e.g., pressure and switches between pressure-controlled and volume-controlled. In short, a dual con-
volume) that are regulated by
independent feedback loops so trol mode is a combined mode between two control variables. When VCV and PCV
that the delivered breath switches are combined, the patient receives mandatory breaths that are volume-targeted,
between pressure-controlled and
volume-controlled. pressure-limited, and time-cycled (Campbell et al., 2002). Since there are many
dual control modes on the market and more are forthcoming in the future, selection

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