Sample of typical initial ventilator settings for an adult:

• Mode (eg, AC, SIMV)
• FIO2 30–100%
• Rate 10–18 breaths/min
• Tidal volume 4–6 mL/kg
• PS (5–20 cm water)
• PEEP (5 cm water or higher, if needed)

Ventilator Setting Changes

The choice of ventilator settings should be guided by clearly defined therapeutic end points.

In most instances, the primary goal of mechanical ventilation is to correct abnormalities in arterial blood-gas tensions. In most patients, this is accomplished easily by adjusting the minute volume to correct hypercapnia and by treating hypoxemia with oxygen (O2) supplementation. Because the volume, frequency, and timing of gas delivered to the lungs have important disease-specific effects on cardiovascular and respiratory systems functions, the physician must avoid simply managing the blood-gas tensions of the ventilator-dependent patient. After a brief review of the capabilities of modern ventilators, this chapter discusses the mechanical determinants of patient–ventilator interactions and defines therapeutic end points in common respiratory failure syndromes. These sections provide background for the major thrust of the chapter, which is to detail the physiologic consequences of positive-pressure ventilation and to develop recommendations for ventilator settings in various disease states based on this knowledge.

The incorporation of microprocessors into ventilator technology has made it possible to program ventilators to deliver gas with virtually any pressure or flow profile. Significant advances have been made in producing machines that are more responsive to changes in patient ventilatory demands, and most full-service mechanical ventilators display diagnostic information contained in airway pressure (Paw), volume (V), and flow () waveforms. Because of these added capabilities, the practitioner is being challenged with a staggering array of descriptive acronyms for so-called new modes of ventilation. To avoid unnecessary confusion, it is useful not to focus on specific modes for the moment but rather to consider three general aspects of ventilator management: (a) the choice of inspired-gas composition, (b) the means to ensure the machine’s sensing of the patient’s demand, and (c) the definition of the machine’s mechanical output.

Five basic respiratory parameters (FIO2, minute ventilation, PS, PEEP, I/E ratio) can be changed to improve ventilation, oxygenation, and compliance.

• 1. FIO2: Choose an initial FIO2 that ensures adequate arterial O2 saturation (SaO2> 90%). Increasing the level of PEEP is often a helpful means of decreasing the FIO2 requirement while maintaining adequate oxygenation. Once adequate oxygenation is established, decrease the FIO2 to avoid O2 toxicity (avoid FIO2 > 60%).

• O2 toxicity: Damage to lungs occurs if the intraalveolar O2 concentration is > 60% (injury actually occurs after a few hours if FIO2 = 1.0). The mechanism probably involves generation of reactive O2 species that oxidize the cell membranes. O2toxicity has not been documented if FIO2 is maintained < 60%.

• 2. Minute ventilation: Adjust to maintain PCO2 within a normal range (35–45 mm Hg). Because minute ventilation is the product of rate and tidal volume, make this adjustment by varying either of these values. Once a tidal volume is chosen, set the respiratory rate (≈ 8–16 breaths/min) for adequate minute ventilation. For a spontaneously breathing patient, PS can be increased to achieve a target minute ventilation.
• 3. Pressure support: After the patient’s respiratory pattern is established on SIMV, add PS at an initial level of 5–10 cm water. Increase PS to the level at which the patient can achieve reasonable tidal volume and breathe at a comfortable rate (< 30 breaths/min). Depending on the overall stability and mental status of the patient, turn down the number of SIMV backup breaths so that the patient assumes more control of ventilation. PS rarely has to exceed 20 cm water.
• 4. PEEP: With the addition of PEEP, the ventilator maintains positive airway pressure at the end of expiration even though net airflow is zero. PEEP increases alveolar ventilation by preventing small-airway collapse, thereby improving lung compliance and FRC. Increasing levels of PEEP are typically used in the care of hypoxemic patients who need FIO2 > 50%. With PEEP, FIO2 can be reduced and O2 toxicity limited. PEEP ≈ 5 cm water is considered physiologic. If oxygenation remains marginal, PEEP is added in 2- to 3-cm increments until oxygenation is improved. In acute lung injury, the PEEP at which lung compliance is optimized can be determined by observation of pressure-volume loops.
• High-Dose PEEP: The elevated intrathoracic pressure associated with high PEEP can compromise venous return and thus decrease stroke volume and CO, particularly in hypovolemic patients. The result is decreased oxygen delivery. Consider placement of a PA catheter. Because ICP can become elevated, titrate PEEP upward with caution in patients with intracranial hypertension.
• 5. Inspiratory to expiratory (I/E) ratio: The normal I/E ratio is 1/2 or 1/3. Inverse-ratio ventilation (eg, 2/1) results in progressive recruitment of alveoli and elevation of mean airway pressure, which improves oxygenation. This beneficial effect on oxygenation is lost if “breath stacking” or auto-PEEP occurs. Note: This technique may be inappropriate in the care of patients with obstructive lung disease, in which longer expiratory times are required. Inverse-ratio ventilation is poorly tolerated by awake patients and typically requires heavy sedation. Shorter expiratory times can result in hypercapnia; this “permissive hypercapnia” is sometimes accepted to improve oxygenation. Balance FIO2 and peak pressure to keep peak pressure < 35 cm water. If unable to do so, move to pressure support ventilation.

Ventilator Weaning

Before weaning the patient from the ventilator, assess pulmonary mechanics and oxygenation (Table 20–7).

Pulmonary Mechanics:

Information about the patient’s ability to perform the work of respiration. Routine pulmonary mechanics consist of:

• Vital capacity
• Tidal volume
• Spontaneous respiratory rate
• Lung compliance

Inspiratory Force:

The maximum negative pressure that can be exerted against a completely closed airway (a function of respiratory muscle strength). An inspiratory force between 0 and –25 cm water suggests that the patient may be incapable of generating adequate inspiratory effort for successful extubation.

Weaning Modes:

Ventilators are designed to facilitate weaning. Once the preceding criteria have been met, select a ventilator mode appropriate to the clinical situation. SIMV and PSV are considered weaning modes because the patient assumes more of the workload of breathing as mechanical support is reduced.

Order of Weaning:

Take the following steps to wean the patient from the ventilator:

• 1. Reduce FIO2 to 40% while monitoring SpO2.
• 2. Sequentially reduce the IMV rate to a level of 4–8 breaths/min. Add PS to maintain adequate minute ventilation. Closely monitor minute ventilation (on ventilator display).
• 3. Sequentially reduce PEEP in increments of 2- to 3-cm water while maintaining SpO2 > 90% until a level of 5 cm water is achieved.
• 4. Sequentially reduce PS by increments of 2–3 cm water while maintaining minute ventilation (goal: 5–10 cm water); monitor respiratory rate, work of breathing, and minute ventilation.

Checklist for Extubation

• Correction of primary problem that prompted intubation and mechanical ventilation (eg, successfully treated pneumonia, returned hemodynamic stability)
• Level of consciousness stable or improved
• Stable vital signs
• Pulmonary mechanics and oxygenation meet acceptable criteria (see Table 20–7)

Extubation Trials:

Once weaning has been achieved, attempt trials with minimal mechanical support while the patient is still intubated. CPAP trials (with 5 cm water positive pressure) is the most commonly used method. A CPAP trial with an FIO2 of 40% should result in a PaO2 of > 70 mm Hg, and a respiratory rate < 25 breaths/min. One of the best predictors of successful extubation is the ratio of respiratory rate to tidal volume (f/Vt, or Tobin index). Extubation frequently is unsuccessful in patients with a rapid–shallow breathing pattern. A ratio > 100 has been shown in some studies to be predictive of extubation failure (N Engl J Med1991;324:1445–1450). These trials may vary in duration from 30 min to several hours and are used primarily as the last test before extubation.

Extubation:

A patient who is able to maintain a PO2 > 70 mm Hg, a PCO2 < 45 mm Hg, and a respiratory rate < 25 breaths/min for 1–2 h on a CPAP trial is ready for extubation.

• 1. Disconnect the ET tube from the ventilator or T-piece.
• 2. Suction the ET tube and oropharynx.
• 3. Have the patient take a deep breath.
• 4. As the patient expires forcefully, deflate the cuff and remove the tube.
• 5. Suction any secretions and administer O2 through a nasal cannula at 2–4 L/min.
• 6. Check postextubation ABG if adequate ventilation and oxygenation are in doubt.