How is peak expiratory flow rate measured in the Intensive Care Unit (ICU)?

Measuring Peak Expiratory Flow Rate in the ICU

In mechanically ventilated ICU patients, peak expiratory flow rate (PEF) can be measured directly using the ventilator's built-in flow meter without disconnecting the patient, or alternatively with a portable electronic peak flow meter connected to the endotracheal tube. 1

Methods for PEF Measurement in Ventilated ICU Patients

Using the Ventilator's Built-In Flow Meter (Preferred Method)

• Modern ICU ventilators incorporate flow velocity measurement capabilities that allow direct assessment of cough peak expiratory flow without patient disconnection. 1
• This method is feasible in routine practice with cooperative mechanically ventilated patients who are in the weaning process. 1
• The technique works best in patients ventilated with pressure support <15 cm H₂O and PEEP <9 cm H₂O. 1
• Ventilator-measured CPF values correlate moderately well (r=0.63) with portable peak flow meter measurements, though ventilator readings are typically higher (mean 72.6 L/min vs 31.1 L/min). 1

Using a Portable Electronic Peak Flow Meter

• An electronic portable handheld peak flow meter can be connected directly to the endotracheal tube for measurement. 1
• This requires temporary disconnection from the ventilator circuit, which may not be ideal for unstable patients. 1
• The portable device provides lower absolute values but maintains correlation with ventilator measurements. 1

Measurement Technique for Ventilated Patients

Patient Preparation

• The patient must be cooperative and able to follow commands to generate a maximal cough effort. 1
• Ensure the patient is in a semi-recumbent or upright position if tolerated. 2
• The measurement is typically performed on the day of planned extubation when the patient is most alert and cooperative. 1

Execution of Measurement

• Instruct the patient to take a deep breath to total lung capacity, then cough as hard and fast as possible. 2, 1
• The exhalation should be forceful and explosive rather than prolonged. 2
• Record the highest value from three attempts. 2
• Results are expressed in liters per minute (L/min). 2, 3

Clinical Interpretation in the ICU Context

Threshold Values

• A ventilator CPF with area under the ROC curve of 0.84 can predict a portable meter CPF <35 L/min, which is a commonly used threshold for extubation readiness. 1
• However, neither ventilator CPF nor portable meter CPF reliably predicted re-intubation within 72 hours in one study. 1

Important Caveats

• The ventilator-measured values are systematically higher than portable meter values due to differences in measurement technique and resistance characteristics. 1
• PEF is highly effort-dependent and requires proper patient cooperation for accurate results. 2
• The measurement may be less reliable in patients with altered mental status, heavy sedation, or neuromuscular weakness. 4

Equipment Considerations

• Ensure the ventilator flow meter is properly calibrated according to manufacturer specifications. 4
• If using a portable device, use disposable mouthpieces (or in this case, ensure sterile connection to the endotracheal tube) to prevent cross-contamination. 2
• The resistance of the measurement device should not exceed American Thoracic Society recommendations, particularly at flow rates >600 L/min. 5

Alternative Approach for Non-Ventilated ICU Patients

For ICU patients who are not mechanically ventilated:

• Use standard peak flow meter technique with the patient in sitting or standing position if possible. 2
• The patient should take a deep breath to maximum inspiration, seal lips tightly around the mouthpiece, and blow out as hard and fast as possible. 2
• Ensure the peak flow meter indicator is at zero before starting. 2
• Record the highest of three attempts. 2

Limitations in the ICU Setting

• Maximum inspiratory pressure measurements have poor reproducibility in critically ill patients and are of limited use for decision-making. 4
• Traditional predictors like vital capacity and minute ventilation are frequently falsely positive or negative for weaning outcomes. 4
• The ratio of respiratory frequency to tidal volume remains the most reliable simple predictor of weaning outcome, more so than isolated PEF measurements. 4