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Patient is allowed to breathe spontaneously through a

separate circuit between machine-delivered breaths.

Different from AV in that attempts ro breathe by the patient

are not assisted by the ventilaror.

Patient may "fight" the ventilator if patient tries [0 exhale

during the mechanical inspiratory phase, leading to

dyssynchrony; this mode has largely been abandonedreplaced by SIMY.

SIMV

Similar to IMV: Mandarory breaths are delivered with a

preset RR, Vpand flow; however, like AV, it will assist a

patient-initiated breath.

Mandarory breaths are only delivered when the patient is nor

initiating enough breaths ro allow a preset minute

venrilation.

Pressure

Patient-initiated breaths are augmented wirh a preset flow of

supponed

gas from the ventilator to maintain consrant inspirarory

ventilation

pressurej when the inspirarory flow drops to a preset value,

the flow of gas terminates.

Patient controls RR, inspiratory time, and flow; patient and

ventilator determine VT and minute ventilation.

The VT received from the machine is relared nor only to the

patient's effort but also to the amounr of pressure provided

by the ventilator.

818 ACUTE CARE HANDBOOK FOR PHYSICAL THERAI'ISTS

Table m-B.1. Continued

Modes

Characteristics

Continuous

Intended to decrease work of breaching by reducing the

positive

airway pressure necessary to generate inspiration

airway

throughout the respiratory cycle while the patient is

pressure

spontaneously breathing.

Positive pressure is constantly maintained above atmospheric pressure.

Most commonly used during weaning from the mechanical ventilator or in an artempt to postpone intubation

(can be delivered via an endotracheal tube or a specially

designed face mask, respectively).

Commonly used at night for the treatment of sleep apnea.

AV = assisted venrilation; CV = conrrolled venrilation; FlO: = fraction of inspired oxygen; l:E ratio = inspiratory rime to expiratory rime ratio; IMV = Intermittent mandatory ventilation; PEEP = positive end-expiratory pressure; RR = respiratory rate; SIMV =

synchronous IMV; VT = tidal volume.

Sources: Dara from P Marino. The ICU Book (2nd cd), Philadelphia: Lea & Febiger,

1998; AS Slutsky. Mechanical ventilation. American College of Chest Physicians' Consensus Conference Isee comments). Chest 1993; 104: 1833; and SF !-lowman. Mechanical ve'nrilation: a review and update for clinicians. Hospital Physician 1999;December: 26-36.

Ventilatory Settings

Ventilatory settings are parametets established ro provide the necessary support to meet the patient'S individual ventilarory and oxygenation needs."· Establishment of the ventilatory serrings is dependent on the patient'S (1) atrerial blood gas levels, (2) vital signs, (3) airway

ptessures, (4) lung volumes, and (5) pathophysiologic condition,

including the patient'S ability to spontaneously breathe."· Ventilator

serrings, subdivided into those that influence oxygenation and those

that influence ventilation, are presented in Table III-B.3.

Complications of Mechanical Ventilation

Auto Positive End-Expiratory Pressure

Auto positive end-expirarory pressure (PEEP) occurs when lung volumes fail to return ro functional residual capacity before the onset of the next inspiration. The process leading ro auro PEEP is referred ro

API'ENDIX [II-B: MECHANICAL VfNIlLATION

819

Table III-B.2. Alternative Modes of Ventilation

Mode

Characteristics

Pressure control

Delivers a preset airway pressure for a predetermined

ventilation

inspiratory time interval.

Inspiratory time is usually prolonged, and patients are

generally sedated because of discomfort due CO the

prolonged mechanical inspiration.

VT is determined by lung compliance; useful in cases in

which barotrauma is thought to exacerbate the

acute lung injury (ARDS); now also available in

ACV or SIMV modes.

High-frequency

A relatively rare technique of ventilation that is

oscilla.rion

administered with frequencies of 100-3,000 breaths

ventilation

per minute and consequently small tidal volumes of

1-3 ml/kg.

Primary advantage is dramatic reduction of airway

pressure and has shown beneficial results in

neonates and adults with ARDS.

Inverse rario

A rarely used technique involving the use of an

ventilation

inspiratory to expirarory ratio of morc than 1 ro 1;

can be delivered as a pressure-controlled or volumecycled mode.

The proposed benefit is the recruitment of a greatcr

number of lung units during the respiratory pause

and longer inspiration.

Potential risk of generating autO PEEP and dynamic

hyperinnation (see text).

Mandatory minute

Only set parameter is the minute ventilation; if

ventilation

spontaneous, unassisted breaths do not meet that

minute ventilation, the venrilator makes up the

difference by supplying mechanical breaths.

Noninvasive

Delivery of ACV, SIMV, and PSV modes of ventilation

positive pressure

via a nose or face mask to reduce the need for

ventilators

intubation.

(NIPPV)

Shown to be useful in reducing in-hospital mortality in

CO PO patients and long-term use in patients with

neuromuscular diseases.

Negative pressure

Exposes the chest wall to subatmospheric pressure

ventilators

during inspiration ro reduce intrapleural pressure,

thereby allowing air ro emer the lungs.

Efficacy has not been demonstrated in patients with

COPD and is unproven for acute respiratory failure.