Obstructive Pulmonary Disease and Ventilatory Management

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Obstructive Pulmonary Disease and Ventilatory Management

I Obstructive Pulmonary Diseases Are Characterized by Airflow Limitation

II Specific Diseases in This Category Include:

B Chronic bronchitis

E Cystic fibrosis

III In Obstructive Pulmonary Disease Airflow Limitation Results from:

B Airway hyperactivity

D Loss of structural integrity of the lung

IV All of the Above Lead to:

B Increased work of breathing

C Ventilatory muscle dysfunction

D These are the major concerns affecting the approach and ease of providing ventilatory support to patients with obstructive pulmonary disease.

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

FIG. 21-1 Relationship between alveolar, central airway, and ventilator circuit pressure under (A) normal conditions and in the presence of severe dynamic airway obstruction, (B) with expiratory port open, and (C) with expiratory port occluded. Auto-positive end-expiratory pressure (PEEP) level is identified by creating an end-expiratory hold, allowing alveolar, central airway, and ventilator circuit pressure to equilibrate. Note that during equilibration auto-PEEP level can be read on the system manometer.

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

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

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

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

< ?xml:namespace prefix = "mml" />auto–PEEP=VT[(C)×(eKE/TE–1)] (1)

(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.

E In obstructive lung disease auto-PEEP develops because of:

1. Increased compliance: All diseases except asthma

2. Increased airway resistance: All diseases but especially asthma

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

G This is referred to as dynamic airway obstruction.

H 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.

1. Auto-PEEP caused by this mechanism is usually not increased by the addition of applied PEEP until the applied PEEP level is ≥80% of the auto-PEEP level.

2. To trigger an assisted breath the patient must first decompress the auto-PEEP.

3. Appling PEEP in the presence of dynamic airway obstruction (see Figure 21-1) decreases the pressure threshold needed to trigger the ventilator.

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

1. In this setting auto-PEEP level is not beneficially affected by the application of PEEP.

2. For patients with asthma the application of PEEP usually does not offset the auto-PEEP as in dynamic airway obstruction. For those with asthma applied PEEP is normally additive to auto-PEEP.

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

1. The greater the minute ventilation, the greater the auto-PEEP.

2. The lower the minute ventilation, the lower the auto-PEEP.

K 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.

FIG. 21-2 Esophageal pressure (P_es), airway opening pressure (P_ao), and flow at the tracheostomy in a patient with dysynchrony on volume-cycled (assist/control) mode. The units are cm H₂O for the pressure tracing and L/min for flow. The patient’s inspiratory efforts are identified by the negative P_es swings. The positive end-expiratory pressure is set at 0. P_ao appropriately decreases to 0 during expiration, demonstrating little circuit or valve resistance. Dysynchrony is evident with one triggered breath every three to four efforts. Prolonged expiratory flow is caused by airflow limitation. P_esswings have little effect in retarding the expiratory flow and even less effect on P_ao, depending on the phase of expiration.

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

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

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

• Airway pressure =	0.0 cm H₂O
• Auto-PEEP level =	10.0 cm H₂O
• Pressure gradient needed to cause flow into the airway	10.0 cm H₂O
• Normal transpulmonary pressure during breathing	3.0 cm H₂O
• Total pressure gradient to breath in the presence of 10.0 cm H₂O auto-PEEP	13.0 cm H₂O

C 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:

1. Anterior abdominal wall moves inward.

2. Lateral rib cage moves inward.

3. Expansion of the upper chest wall.

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

A Acute or chronic ventilatory failure

B Unloading work of breathing

C Resting ventilatory muscles

D Improving bronchial hygiene

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).

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

1. A decreased need for intubation

2. Decreased length of mechanical ventilation

3. Decreased length of intensive care unit (ICU) stay

4. Decreased rate of nosocomial pneumonia and other infections

5. Decreased cost of care

6. Improved mortality

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

BOX 21-1 Initial Setting of Ventilator During NPPV in Acute Respiratory Failure

Ventilator: Provides waveforms of airway pressure and flow

Mode: Pressure support or pressure assist/control (B_IPAP)

Peak airway pressure: 20 cm H₂O maximum

PEEP: 3 to 10 cm H₂O

Ventilating pressure: 8 to 12 cm H₂O

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