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814 AClffE CARE HANDBOOK FOR PHYSICAL THERAPISTS

Cuff

Approximately 0.5 in. from the end of an endotracheal or tracheal

tube is a cuff (balloon). The cuff is inflated to (1) ensure rhar all of rhe

supplemenral oxygen being delivered by rhe venrilator via the artificial airway enters the lungs and (2) help hold the artificial airway in place. Cuff inflation pressure should be adequate to ensure that no air

is leaking around the tube; however, cuff pressures should not exceed

20 mm Hg. High cuff pressures have been linked to tracheal damage

and scarring, which can cause tracheal stenosis.

Clinical Tip

• A cuff leak should be suspected if the patienr is able to

phonate or audible sounds come from his or her mouth.

• Cuff leaks can occur if the endotracheal tube is shifted

(positional leak) or if the pressure changes in the cuff.

• If a cuff leak is suspected, then the respiratory therapist

or nurse should be notified. (Physical therapists who specialize in critical or cardiopulmonary care may be able to add air to the cuff according to the facility's guidelines.)

Positive Pressure Ventilators

Positive pressure ventilators are classified based on the method used

to stop the inspiratory phase and allow expiration (cycling method)

to occur.' There are three basic cycling methods: (1) pressure cycled,

(2) volume cycled, and (3) time cycled.

Pressure-cycled ventilators Stop inspiration at a preset pressure,

volume-cycled ventilators stOp inspiration at a preset volume, and

time-cycled ventilators stop inspiration at a preset time interval.

Although these methods allow for increased conrrol of certain variables during inspiration, holding only one variable constant for the termination of positive pressure inhalation allows other factors to

affect inspiration and potentially cause barotrauma or reduced

inspiratory volumes. These factors include position changes and manual techniques.

For example, with a volume-cycled ventilator, a preset volume will

be delivered regardless of the patienr's position, and reductions in

chest wall expansion owing to a patienr's position (e.g., side lying)

APPENDIX 111-8: MECHANICAL VENTILAT10N

815

may increase the pressure placed on the dependent lung tissue and

result in barotrauma. Conversely, in the same scenario, if a patient is

on a pressure-cycled ventilator, then pressure will be delivered to the

predetermined level, but because the patient's position may hinder

chest expansion, a resultant lower volume of inspired air may be

delivered, because the preset pressure limit was reached. Many newer

ventilators provide the clinician with more than one cycling option,

and certain modes of ventilation allow for more than one parameter

ro derermine the inspirarory phase (as discussed throughout this

appendix).

Clinical Tip

• Being aware of the cycling method, the therapist can

pay attention to changes in the pressure or tidal volume

(VT) associated with his or her interventions.

• Wide bore plastic tubing is used to create the mechanical ventilator's circuit. The terminal end of this circuit directly connects ro an endotracheal or tracheal tube or,

less commonly, to a face mask.4 Some ventilator circuits

have an extra parr at their terminal end for an "in-line"

suction catheter, which allows for suctioning without the

removal of the ventilator circuit from the patient.4

Modes of Ventilation

Modes of ventilation can range from providing rotal support (no

work performed by the patient) to minimal support (near-total

work performed by the patient). Modes of ventilation are geared

toward allowing the patient to do as much of the work of breathing as is physiologically possible, while meeting the intended objectives of ventilatory support. Even short periods (11 days) of

complete dependence on positive pressure ventilation can lead to

respiratory muscle atrophy and concomitant reductions in diaphragm strength (25%) and endurance (36%).5 Figure III-B.2 provides a schematic of the conventional modes of ventilation based on the amount of support they provide. Characteristics of conventional and alternative modes of ventilation are presented in Tables III-B. I and III-B.2, respectively.

00

�

Sp«trum ofPatienl Participation

'"

,.

No

Good Spontaneous

lndependent with

Spontaneous

Some Spontnneous

Spontaneous Breathing:

Brearbing: RR> 10

�

ventilation and

Breathing

(RR < 10)

-

Breathing: RR < 10

Total

But inspiratory effort

able to

Ventilator

Weak. inspiratory

Good EfTon:

9

still weak:

physiologically

Dependence

effons:

VT> 10 m.Vkg

i:;

Vt < 10 mllkg

support self

VT<6mL1kg

NIF> 20 em H:O

NlF < 10 em H10

s;

�

�

o

"

Continuous

Synchronous

lntenni

Positive

�

ttem

Support

Airway

Mandatory

Ventilation

p..,....

OJ!

Ventilation

�

§

,..

Figure ill-B.2. Schematic of mechanical ventilator modes: the ability of patients to participate in the process of ventilation and i

oxygenation in part determines the mode of ventilation. Parameters used to determine the patient's ventilatory effort include the iii

presence of spontaneous breaths, the number of breaths per minute he or she initiates (respiratory rate [RR)). the volume of

>-

'"

those breaths per breath (tidal volume /VT}). and the negative inspiratory force (NIF) generated with those breaths. The values B

indicated above are examples 0/ patient characteristics along the spectrum (rom complete ventilatory dependence to independence. The effectiveness of the patient's efforts needs to be assessed based not only 011 the above parameters. but on "outcon-te"

variables, such as oxygenation (Pao'}! oxygen saturation as measured by pulse oximetry). ventilation (Paco2), and overall status (hemodynamic stability. symptonts).

APPENDIX III-B: MECHANICAL VENTILATION

817

Table HI-B.l. Conventional Modes of Ventilation

Modes

Characteristics

CV

Toral control of the parient's ventilation: preset rate; Fiol; VT;

flow rate; I:E ratio.

Patients may be sedated or pharmacologically paralyzed.

No active respirarory muscle activity is necessary.

AV

Patient controls cespirarory parrern and rare; breath initiated

by patient creares negative airway pressure in circuit; once

initiated, the volume is delivered with eirher a preset

volume or pressure and flow ratc; respiratory muscles are

still working.

Patient can trigger RRs that are roo high, leading ro

respirarory alkalosis or auto PEEP (see text).

A ssist!

Combination of CV and AV; delivers breath of predetermined

control

tidal volume with the patient'S inspiratory effort.

ventilation

If the patient does not initiate a breath within a specified time

period, the ventilator will deliver a breath ro maintain

preset RR.

IMV

Delivers breaths intermittently at preset time intervals with a

preset RR, VT,and flow rare.