Emergency physicians (EPs) are experts in emergent airway management and thus must be confident managing mechanical ventilation. Hospital-wide bed shortages mean that EPs will be managing admitted patients for longer periods of time, and if you work in a hospital without intensivist coverage you must be the ventilator expert. A recent study suggests implementing a ventilator protocol in the emergency department can improve mortality in patients started on mechanical ventilation.(1) This supports the idea that early and appropriate ventilator management can save lives. In the first of this two part series we will discuss management of the ventilator and the intubated patient. Part two will discuss the management of patients who are difficult to oxygenate or ventilate, including patients with obstructive lung disease and acute respiratory distress syndrome (ARDS).
Regardless of the reason for intubation, the primary functions of mechanical ventilation are to oxygenate and ventilate the patient.
Oxygenation can be improved with mechanical ventilation by both higher levels of oxygen (increasing the FiO2, or inspired partial pressure of oxygen) and through increases in airway pressure via increased positive end expiratory pressure (PEEP). (2) PEEP has various mechanisms, but generally improves oxygenation through recruiting additional lung and increasing surface area for gas exchange and redistribution of lung water. We recommend a strategy of setting initial PEEP at 5 cm H2O in most patients unless they have severe hypoxemia or are obese, in which case initial PEEP can be set at 8 cm H2O, but will likely need to increase from there. When you encounter patients who are difficult to oxygenate despite initial PEEP and FiO2 settings, we recommend titration of PEEP by a PEEP table, (see Table 1), note that those requiring 90-100% FiO2 may benefit from PEEP of 16-20 cm H20 or higher. Use either the high PEEP or low PEEP table, both are acceptable, to incrementally increase your PEEP to the point of desired oxygenation. Improvements through increased PEEP are not immediate, so use incremental changes in PEEP by 2 cm H2O every few minutes rather than making rapid adjustments because there is potential for unanticipated hemodynamic, intrathoracic or intrapulmonary changes.
Choosing a safe oxygen level
Many studies have shown increased mortality in mechanically ventilated patients with hyperoxia.(3-5) A recent observational cohort study showed exposure to hyperoxia (defined as PaO2> 120 mm Hg) in the ED was associated with an increase in mortality.(4) The relationship of hyperoxia and increased mortality risk is dose-dependent.(5) For these reasons, it is important to quickly decrease the FiO2 to achieve a PaO2 60-100 mm Hg or an oxygen saturation of 94-96%. Even lower goals can be tolerated in severe ARDS such as a PaO2 of 55-80 mm Hg or an oxygen saturation of 88-95%. Weaning by oxygen saturation can start as soon as the tube is secured and your patient has recovered from the apneic induction period. Simply stated, mechanically ventilated patient should have oxygen saturation no greater than 94-96% — not 100%.
Removal of carbon dioxide is affected by changing the minute ventilation, the product of the tidal volume and respiratory rate over a minute. In prescribing ventilator settings, it is important to match the respiratory rate and tidal volume to the underlying physiology. For reference, normal minute ventilation is 5-8 L/minute.(6) If intubating an intoxicated young patient with normal lungs for airway protection, a minute ventilation of 5-8 L/min is most likely adequate. However patients with a metabolic acidosis may partially compensate with significantly elevated minute ventilations. If your patient has a severe metabolic acidosis you will want to try to match their pre-intubation minute ventilation, generally through increasing their RR because of the increased risk of volutrauma with increasing Vt > 8 ml/kg.(7) Respiratory rates up to 30-35/min can be safe but pay careful attention to evidence of air trapping (shown in Figure 2).(7) In most patients we target a pH rather than a strict pCO2. A pH of > 7.20-7.25 is generally considered safe.(8) One caveat are those with pulmonary hypertension and right ventricular failure who may not tolerate hypercapnia.
All ventilators will offer the same basic functions, but nomenclature can differ between brands. The ventilator will either target a pressure or a volume based on the ventilator mode. Volume control (VC), also known as volume assist-control, sets a targeted tidal volume at a targeted respiratory rate. The tidal volume is delivered as a flow of gas over a set inspiratory time. An example of this is seen in Figure 1, where a 600 mL Vt is delivered over one second, at a constant flow rate of 36 L/min. The same respiratory goals can be achieved in volume or pressure targeted mode, but in our opinion, volume targeted ventilation (i.e. VC) should be the go-to mode for hypoxemic patients.
The prescribed Vt is given at a set interval regardless of patient effort (also known as “controlled” breaths), but the patient can trigger additional “assisted” breaths. This mode is sometimes referred to as volume control-assist control (VC-AC). Whether triggered or automatic, the patient receives the same preset volume. Where volume has been determined by the operator the pressure generated will vary with patient physiology. This creates a risk of delivering excessive pressures while delivering the desired volume, but this risk can be mitigated by checking the plateau pressure as described below. This is in contrast to pressure control ventilation (PC), where the pressure and inspiratory time are set, while the Vt delivered will vary.(9) The risk of the pressure control mode is delivering constant pressure, but varying tidal volumes, too high or too low depending on a patient’s dynamic lung mechanics.
We prefer volume targeted ventilation because ARDSnet and subsequent trials have shown that attention to tidal volume and minimizing the risk of volutrauma is important for our patients.(10,11) Recent evidence suggests lower Vt can reduce mortality particularly in patients with ARDS. In all patients keeping Vt ≤ 8 ml/kg is likely to improve patient outcomes as compared to larger volumes.(1,7,11) For this reason, we recommend starting at 8 ml/kg ideal body weight (IBW) for patients not being treated for ARDS and 6 ml/kg IBW for those with or at high risk for ARDS. After initiation of mechanical ventilation, check a plateau pressure. If the plateau pressure is > 30 cm H20 decrease the tidal volume by 1 ml/kg, down to as low at 4 ml/kg, until plateau pressures are <30 cm H20 (1,7). Note it is important to measure your patients’ height as tidal volume is based on ideal body weight not actual body weight. This is important in our obese patient population, who can be wildly over ventilated if tidal volumes are selected based on their actual body weight. One way to easily incorporate this measurement into the post intubation work flow is to attach a small tape measure to the ventilator.
In a patient with an intact respiratory drive and without significant hypoxia (such as those intubated for airway protection) pressure support ventilation (PS) should be considered once the paralytics have worn off. In PS the clinician sets an inspiratory pressure to augment the patient’s spontaneous respiratory effort above a set PEEP. You may set an inspiratory pressure of 10 cm H2O and a PEEP of 5 cm H20, sometimes known as “10/5.” Every time the patient triggers a breath the ventilator will raise the pressure 10 cm H2O over a PEEP of 5 cm H20 giving the patient a peak pressure of 15 cm H20. You can think of it like bilevel positive airway pressure (i.e. Bi-PAP ™) through an endotracheal tube.
The patient decides the rate, the volume and the duration of the breath. The ventilator delivers a set inspiratory pressure when the patient triggers a breath and stops delivering pressure when the patient’s inspiratory flow slows.(12) Generally you would increase the inspiratory pressure until your patient has a comfortable work of breathing with a normal Vt and RR. This mode is more comfortable for the spontaneously breathing patient and can help minimize sedation needs in patients without significant lung pathology.
There are additional modes of ventilation such as synchronized intermittent mandatory ventilation (SIMV) and pressure regulated volume control (PRVC) that may be used in your ED. These modes of ventilation are some combination of a set control (i.e. a targeted pressure or volume) and independent, patient-determined variables (such as RR, flow, pressure, or volume). While these modes each have their pros and cons, it is important to know what the prescribed controlled variables are and which are patient controlled to avoid injurious pressures (plateau > 30 cm H2O) or volumes (> 8 ml/kg IBW).
Recently, more attention has been placed on airway pressure release ventilation (APRV) in patients with ARDS, but we do not currently see a role for this as a primary ventilator mode in the ED.
Emergency physicians must be ready to manage mechanical ventilation in various disease processes. We think about this in terms of oxygenation, ventilation and avoiding harm. In this article (part 1 of 3) we have discussed the basics of ventilator management in terms of oxygenation and ventilation. While the nomenclature varies, the EP will most commonly use volume targeted modes (i.e. volume control) and pressure support. It is important to measure your patient’s height and keep the Vt < 8 ml/kg IBW. For patients with hypoxia, higher PEEP, lower tidal volumes, plateau pressure <30 cm H2O and avoidance of hyperoxia will improve mortality. Titrate the oxygen down in all your patients as soon as possible, targeting a SpO2 of no greater than 96%. In those who are intubated for airway protection, with grossly normal lung parenchyma, adjust the ventilator for comfort to modes such as pressure support once the RSI medications have worn off.
Next month, Part 2: Do No Harm: waveform interpretation, sedation and vent bundles.
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