Obstructive Pulmonary Disease and Ventilatory Management

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Obstructive Pulmonary Disease and Ventilatory Management

Obstructive Pulmonary Diseases Are Characterized by Airflow Limitation

II Specific Diseases in This Category Include:

III In Obstructive Pulmonary Disease Airflow Limitation Results from:

IV All of the Above Lead to:

Air Trapping (see Chapter 40) (Figure 21-1)

The trapping of gas at end-exhalation in the lung parenchyma.

This results in an increase in the functional residual capacity and an alteration in the end-expiratory position of the diaphragm.

Air trapping results in the development of auto-positive end-expiratory pressure (PEEP).

The level of auto-PEEP is determined by the tidal volume, respiratory system compliance, airway resistance, and expiratory time:

< ?xml:namespace prefix = "mml" />autoPEEP=VT[(C)×(eKE/TE1)] (1)

image (1)

    where Ke = 1/Re × C, e is the base of the natural logarithm, Te is expiratory time, Re is expiratory airways resistance, and C is respiratory system compliance.

In obstructive lung disease auto-PEEP develops because of:

With most obstructive diseases there is instability of the small airways. As a result these airways dilate during inspiration but narrow during expiration.

This is referred to as dynamic airway obstruction.

In most obstructive lung disease it is the combination of dynamic airway obstruction and increased respiratory system compliance that accounts for the development of auto-PEEP.

In asthma the limitation of airway flow is a result of increased fixed airway resistance caused by bronchospasm and edema.

Beyond altered lung mechanics, minute ventilation has the greatest overall effect on the level of air trapping and auto-PEEP.

Refer to Chapter 40 for a full discussion on identification of auto-PEEP level.

1. As noted in Figure 21-2 for spontaneously breathing patients triggering the ventilator, auto-PEEP is most commonly observed by a difference in the patients’ ventilatory rate and the ventilator response rate.

2. If the patient’s rate is higher than the ventilator response rate it is almost ensured that the cause is auto-PEEP.

3. To offset the effect of auto-PEEP on patient triggering (in dynamic airway obstruction) PEEP should be slowly applied in 1- to 2-cm H2O steps until every patient effort triggers the ventilator.

4. In some patients this may require applied PEEP as high as 15 cm H2O.

VI Increased Work of Breathing

The biggest adverse impact of auto-PEEP for patients with obstructive lung disease is an increase in work of breathing.

As discussed previously if a patient must decompress the auto-PEEP level to inspire, work of breathing increases. For example:

In patients with severe obstructive lung disease with air trapping and auto-PEEP, the diaphragm is flattened. This prevents its contraction from increasing the anterior-posterior diameter of the thorax and laterally expanding the lower rib cage. This frequently results in paradoxical breathing:

VII The Overall Indications for Ventilation in Patients With Chronic Pulmonary Disease Are:

VIII Noninvasive Positive Pressure Ventilation

    For all patients with obstructive pulmonary disease requiring ventilatory support, except those with asthma, noninvasive positive pressure ventilation (NPPV) should be the first ventilatory option (see Chapter 43).

The data clearly indicate that the use of NPPV in these patients results in:

NPPV should be applied with a ventilator that allows assessment of patient-ventilator synchrony; waveforms should be available (Box 21-1).