Guidelines for Ventilator Settings in Different Clinical Circumstances

Table 8 [43k PDF*] summarizes the recommended approach to setting the ventilator depending on the patient’s acute respiratory problem and the clinical setting.

Routine ventilatory support

Most patients who require a period of invasive mechanical ventilation have relatively normal underlying lung function. What may be referred to as “routine ventilatory support” is encountered most frequently in the postoperative period or in the setting of short-term loss of spontaneous ventilation, such as with a drug overdose. In such settings, a volume-targeted mode such as A/C ventilation is simplest and most reliable, and usually requires fewer adjustments than does pressure-targeted ventilation. Some clinicians prefer to add 5 cm H₂O of PEEP routinely in order to counteract the modest drop in functional residual capacity (FRC) that has been shown to occur with endotracheal intubation, although this is probably unnecessary in most patients.

Healthy individuals normally breathe with a Vt of 5-7mL/kg. However, diffuse microatelectasis and an increased difference between alveolar and arterial oxygen tensions [P(A-a)O₂] soon develops because of decreased surfactant function if these individuals do not more fully expand their lungs several times per hour by sighing. The same problem exists with intubated patients who have normal lungs and who are ventilated with ‘normal’ Vts in the absence of sigh breaths. The need for sighs, which in the presence of pulmonary disease might overdistend and rupture alveoli, can be obviated if a larger Vt (10-12 mL/kg) is used.

The cycling rate and hence minute ventilation are adjusted to provide normal arterial pH and PaCO₂ values (e.g., 7.40 ± 0.05 units and 40 ± 5 mmHg, respectively).  Enough supplemental oxygen is used to prevent hypoxemia, although maintaining PaCO₂> 100 mm Hg is unnecessary.

Obstructive lung disease

Compared to individuals who do not have chronic lung disease, patients who have severe COPD or asthma are at increased risk for circulatory impairment and barotrauma when subjected to invasive mechanical ventilation, and a number of modifications of the routine approach are required to avoid these.

The most common problem is pulmonary hyperinflation, which patients who suffer obstructive lung disease typically have at baseline, and which worsens during times of acute exacerbation. The three main goals of invasive mechanical ventilation in patients who have acutely exacerbated COPD or acute severe asthma are to:

• rest the ventilatory muscles;
• avoid further dynamic hyperinflation;
• avoid overventilation and acute alkalemia.

Resting the ventilatory muscles may be achieved either by providing full ventilatory support using volume-targeted ventilation (with either A/C ventilation or SIMV), so that the patient makes no respiratory effort, or by providing partial ventilatory support using PSV, such that tachypnea and respiratory distress are relieved.  If the former strategy is used, Vts of 5-7 mL/kg and a rapid inspiratory flow (e.g., 80–100 L/min) to maximize expiratory time and avoid air trapping (see subsequent section on auto-PEEP) are recommended.

This is one of the two clinical settings in which permissive hypercapnia is appropriate, the other being acute lung injury as discussed below. When the degree of airflow limitation is severe, it may not be possible to provide a sufficient minute ventilation (i.e., respiratory rate x Vt) to reduce the PaCO₂ enough to produce a normal pH without a resultant worsening in hyperinflation. In obstructive lung disease, PEEP serves a different function from that in acute lung injury. Its purpose here is not to increase lung volume (which is already excessive), but to decrease the muscular effort required to trigger the ventilator or breathe spontaneously in the presence of dynamic hyperinflation and auto-PEEP.

Acute Lung Injury and the Acute Respiratory Distress Syndrome

The goals of mechanical ventilation in acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) are to:

• support oxygenation;
• avoid circulatory compromise; and
• avoid ventilator-induced lung injury.

The first of these goals is accomplished through manipulations of FIO₂ and PEEP, the aim of which is to balance the risks of pulmonary oxygen toxicity with those of raised intrathoracic pressures and lung volumes. Attempts to avoid ventilator-induced lung injury include a ‘lung-protective’ ventilation strategy that involves low Vt, limited alveolar pressure, and permissive hypercapnia.

Either volume- or pressure-targeted ventilation may be used to manage ALI-ARDS. The use of PCV has several potential advantages, although whether these lead to improved outcomes or fewer complications remains to be determined. Whatever mode is used, it has been demonstrated both in animal models and in patients that the larger-than-physiologic Vts used for routine ventilatory support contribute to lung injury.  Current best evidence indicates that a "lung-protective" ventilation strategy that keeps VT to a maximum of 6 mL/kg predicted body weight and avoids end-inspiratory plateau (static) pressures above 30 cm H₂O minimizes ventilator-induced lung injury and reduces mortality. 

Because of the increasing prevalence of obesity in the developed world, and the risk of unintended lung overdistention in overweight patients if admission weight rather than predicted body weight is used to determine VT, the following formulas are used to determine initial settings:

Males

Predicted body weight (kg)  = 50 + 2.3 [(height in cm - 152) divided by 2.54]
                                        = 50 + 2.3 (height in inches - 60)

Females

Predicted body weight (kg)  = 45.5 + 2.3 [(height in cm - 152) divided by 2.54]
                                        = 45.5 + 2.3 (height in inches - 60)

The rate must be adjusted so that auto-PEEP does not develop, a limitation that results in hypercapnia in many patients.  Most patients tolerate arterial pH values in the range 7.20-7.30 without difficulty, and bicarbonate infusion is not used by most authorities unless the value falls well below this range.

In managing patients who have severe ARDS, the clinician may need to accept ‘permissive hypoxemia’ as well as permissive hypercapnia. Although the goal is to maintain PaCO₂ in the normal range, this may not be attainable in some patients without the use of potentially injurious levels of PEEP. A PaCO₂ of 50-60 mm Hg [oxygen saturation as measured by pulse oximetry (Spo2) 80–90%] is usually well tolerated if hemoglobin concentration and cardiac function are adequate. As both hypercapnia and hypoxemia distress patients, appropriate sedation is required when pursuing the ‘lung protective’ ventilatory strategy in ARDS.  The "PEEP-FIO₂ Ladder" (Table 9 [20k PDF*]) titrates these two ventilator settings according to the original ARDS Network protocol.   A second study comparing this "ladder" for setting PEEP and FIO₂ to a similar titration employing higher PEEP levels and correspondingly lower FIO₂s found no differences in patient outcomes.

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