PRACTICAL GUIDELINES FOR MECHANICAL VENTILATION

PRACTICAL GUIDELINES FOR MECHANICAL VENTILATION
IN DIFFERENT CLINICAL SETTINGS
Regardless of the specific issues associated with each of the following settings, the following general principles should be followed when mechanically ventilating acutely-ill patients:
•Maintain peak alveolar pressure (end-inspiratory plateau pressure) ≤ 30 cm H2O
•Look for and avoid the development of dynamic hyperinflation and auto-PEEP
•Use the lowest FIO2 necessary to maintain acceptable arterial PO2
Short-Term Ventilatory Support in Pts with Normal Underlying Lung Function
In this category are post-operative patients, patients requiring heavy sedation for procedures or to control muscular activity, and patients with acute ventilatory failure following accidental or intentional drug overdose. Almost any approach to mechanical ventilation is acceptable in this setting since no acute or chronic lung disease is present.
•AMV or SIMV (PCV requires more monitoring and ventilator adjustment)
•Tidal volume 10-12 mL/kg (note: the "lung protective" strategy does not apply
here; the use of lower tidal volumes may lead to atelectasis)
•Initial rate 10-15 breaths/min, adjusted to maintain arterial PCO2 35-45 mm Hg
•Peak inspiratory flow 60-80 L/min
•PEEP 5 cm H2O (optional)
•FIO2 = 0.30-0.50 (as required to maintain arterial PO2 > 70 mm Hg)
Intracranial Hypertension in Closed Head Injury
These patients generally have normal lungs, so that the above guidelines apply but with the following exceptions:
•No PEEP (may raise intracranial pressure) unless needed for hypoxemia
•If PEEP necessary, head of bed raised to counteract increased hydrostatic
pressure, and use as little PEEP as possible
•Insure appropriate sedation during suctioning to prevent patients from coughing
violently or fighting the ventilator
•Avoid "routine" chest physical therapy, especially in head-down position
•Optional: Hyperventilate to reduce acutely increased intracranial pressure: rate
>15 breaths/min in order to keep arterial PCO2 30-35 mm Hg [controversial;
usually used only short-term until other measures can be employed]
Acute Respiratory Failure in Obstructive Lung Disease (especially COPD)
This is the ideal setting for noninvasive positive-pressure ventilation (NPPV), and whenever possible this should be tried prior to intubation unless the patient has a complicating process like pneumonia or is too obtunded or agitated to cooperate.
Dynamic hyperinflation and auto-PEEP are common and may produce life-threatening cardiac compromise, especially on initial intubation or when patients become agitated. Rapid cycling rates and large tidal volumes (>10 mL/kg) should be avoided.
Work of breathing is already acutely increased in these patients, and is generally the reason mechanical ventilation has become necessary. Thus, the ventilator should be adjusted to maximize their comfort and minimize imposed (external) work of breathing.
A major goal should be to avoid alkalemia: arterial PCO2 should be lowered only enough to correct acutely life-threatening acidosis, and generally not to "normal" in these patients. Shoot for an arterial pH in the 7.30-7.35 range.
•Either assist-control or SIMV; pressure support also effective but harder to avoid
over-ventilation and keep tidal volume down
•Tidal volume 5-8 mL/kg
•Initial rate 6-8 breaths/min
•Pressure support 5 cm H2O (to minimize work of breathing during
spontaneous breaths if SIMV used)
•Peak inspiratory flow 70-90 L/min (note: this is higher than traditionally used)
•PEEP adjusted to about 80 percent of measured auto-PEEP level
•FIO2 sufficient to maintain arterial PO2 ≥ 60 mm Hg (avoiding hyperoxic ventilatory
depression is not an issue once the patient is intubated)
•Sedation as necessary to insure that patient rests during first 24 hours
Most COPD patients can be weaned and extubated within 48-72 hours, which should be the goal unless they have pneumonia or other serious complicating problem. Alkalemia, over-sedation, and iatrogenic auto-PEEP are the commonest reasons for failure.
Acute Neuromuscular Disease
Patients with spinal cord injury and other neurologic problems typically require larger tidal volumes than normal to provide the sensation of full inspiration. As such patients usually have normal lungs, tidal volumes larger than in other settings do not pose a significant threat of barotrauma. Neuromuscular patients also usually prefer a rapid inspiratory flow. Repeated adjustments of tidal volume, inspiratory flow, mode, and other settings may be necessary to achieve optimal comfort in these patients, who are typically fully awake.
•Assist-control or SIMV (with sufficient mandatory rate to prevent hypercapnia)
•Tidal volume 10-12 mL/kg (sometimes 15-18 mL/kg or more for optimum comfort)
•Initial rate 8-12 breaths/min; avoid acidosis
•Peak inspiratory flow adjusted to patient comfort; may require > 80 L/min
•PEEP 5 cm H2O (optional)
•FIO2 sufficient to keep arterial PO2 ≥ 80 mm Hg. Even mild hypoxemia is often
distressing to neuromuscular patients.
•Sedation as needed to avoid severe alkalosis
Acute Lung Injury
Current definitions and criteria for diagnosis:
•Acute clinical illness
•Bilateral diffuse infiltrates on chest X-ray
•Absence of left atrial hypertension (e.g. wedge pressure <18 mm Hg if
available; otherwise no clinical evidence of heart failure as explanation)
•Appropriate risk factor or clinical setting; no other apparent explanation
When the above conditions are met, the patient has either acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS), depending on the severity of oxygenation impairment:
ALI: PaO2/FIO2 < 300 mm Hg ARDS: PaO2/FIO2 < 200 mm Hg
Of all the clinical scenarios requiring ventilatory support, this one is most challenging to the clinician. In severe acute lung injury it can be expected that normal gas exchange cannot be achieved, and new targets for both PO2 and PCO2 must be set. In addition, because of the extent of the injury, small tidal volumes are necessary in order to prevent local pulmonary overdistension and to maintain peak alveolar pressures below the "damage threshold" of 30 cm H2O.
•Assist-control (AMV) or SIMV (or PCV)*
•Tidal volume 6 mL/kg or less per ARDS Net protocol
•Rate 20-35 breaths/min, limited by development of auto-PEEP
•Peak inspiratory flow 60-80 L/min, higher if auto-PEEP develops
•PEEP: adjusted according to ARDS Net PEEP-FIO2 ladder; at least 8-10 cm H2O
initially; most patients need 15 cm ± 5 cm H2O
•Consider other means of augmenting tissue oxygen delivery (e.g. RBC
transfusion; dobutamine) if the above measures do not produce PO2
> 50-55 mm Hg
•Sedate patient to prevent fighting the ventilator and/or increased respiratory
rates over those set (to minimize auto-PEEP); the great majority of patients
do not need to be paralyzed, however
•FIO2: as low as possible to maintain arterial PO2 ≥ 50 mm Hg. The PaO2 target
should be 55-70 (SpO2 85-92%), not higher, to minimize FIO2 and PEEP;
use of ARDS Net PEEP-FIO2 ladder is strongly recommended.
•Permissive hypercapnia: allow arterial PCO2 to increase into the 60-90 mm Hg
range if necessary to achieve the target tidal volume and pressures.
Bicarbonate infusion is optional when pH falls below 7.25-7.30 but is
unnecessary unless patient has acute intracranial pathology or severe
cardiac dysfunction.
Care must be taken when decreasing PEEP once patients improve, as premature reduction worsen atelectasis and require a return to even higher levels of PEEP. Titrating PEEP according to the ARDS Net PEEP-FIO2 ladder is strongly recommended.
*Pressure control (PCV) is an acceptable alternative here (although not proven to be better), but tidal volume is harder to keep in the “lung-protective” range, and its use requires closer monitoring, more frequent ventilator adjustments, and a trained team familiar with its problems and idiosyncrasies.