Editorials Mucus On the Move: Embed It or Expel It—The Patient, the Clinician, and Now the Ventilator Intubation and mechanical ventilation may impair muco- A “saw tooth” pattern on the expiratory flow waveform ciliary clearance and lead to sputum retention, airway occlu- and coarse expiratory sounds over the trachea are good sion, atelectasis, and ventilator-associated pneumonia.1,2 Early indicators of retained secretions in the major airways.10 work demonstrated that airway clearance can be augmented Endotracheal tube and bronchial obstruction can be de- by an expiratory flow bias generated through the 2-phase tected by a characteristic increase in the early expiratory gas-liquid transport mechanism.3,4 Higher expiratory flow than time constant9 and by acoustic reflectometry.10-12 We have inspiratory flow combined with dynamic airway compression yet to accurately diagnose secretion retention, but flow contributes to the clearance of airway secretions in intubated waveform analysis and good clinical examination may be patients,3,4 and can be achieved by physical methods, includ- useful. ing manual lung hyperinflation.5 Patent Airway SEE THE ORIGINAL STUDY ON PAGE 1287 Adequate humidification and airway suctioning are re- 6,7 Clinicians may find it challenging when repositioning quired to maintain airway patency. Closed suctioning a heavily sedated, intubated, ventilated patient who has can minimize potential adverse physiologic effects of air- poor or absent cough and secretion retention, because way suctioning but may not be as effective for secretion 13-16 repositioning may result in substantial arterial hypox- clearance. Ventilator breaths triggered by closed suc- emia, hypercarbia, and increased patient work of breath- tioning may migrate secretions away from the suction cath- 13,17 ing. The prevalence, sequelae, and optimal methods to eter tip (due to inspiratory flow bias). Open suctioning, manage or prevent these episodes are yet to be ade- higher suction pressure, a wider-bore suction catheter, or quately described in the literature. The patient’s respi- the removal of positive end-expiratory pressure (PEEP)/ ratory-muscle strength, position, volume, viscosity, and pressure support can improve secretion clearance and may location of airway secretions, depth of sedation, and be required under certain conditions, such as major airway “cough strength” (expiratory flow generated spontane- occlusion, but may compromise gas exchange in the short 13,16 ously or in response to suction) are some of the vari- term. The “pipe cleaner” effect of the suction catheter ables that may determine the impact of secretion reten- may additionally assist to ensure airway patency and needs 16 tion on ventilation, gas exchange, and patient outcome. to be further investigated. The minimum standard of airway care currently advo- Using a shorter suction catheter (to prevent contact with cates adequate humidification and suctioning.6,7 the carina) can minimize adverse physiologic effects and Below I will discuss various definitions and means to may be as effective as suctioning with a standard-length monitor secretion-retention in an intubated and ventilated catheter, in terms of duration of intubation, intensive-care- patient, and methods to maintain a patent artificial airway, unit stay, intensive-care-unit mortality, and incidence of 18 enhance secretion clearance, and prevent secretion reten- pulmonary infections. The act of disconnecting the ven- tion. tilator circuit for open suctioning may explain the equiv- alent outcomes between shorter and conventional-length Diagnosis and Monitoring catheters (simulated cough due to elastic recoil), and re- quires further investigation.16,18 Lung auscultation and chest palpation may be useful to identify the presence of airway secretions, but may be Enhanced Secretion Clearance unreliable.8 Airway secretions may also be suspected with changes in pulmonary mechanics at the bedside. However, The expiratory flow bias required to generate annular measurements of airway pressure and flow are often taken 2-phase gas-liquid flow can be created with manual lung at the proximal end of the endotracheal tube, and hence are hyperinflation.5,19 Ventilator hyperinflation can also en- mainly affected by the mechanical properties of the tube.9 hance secretion clearance,20 and hyperinflation may be 1276 RESPIRATORY CARE • OCTOBER 2008 VOL 53 NO 10 MUCUS ON THE MOVE effective up to the 10th generation of airways.4 Manual ant lung to the more compliant lung. These laboratory and ventilator hyperinflation can transiently improve air- findings cannot be directly extrapolated to clinical care but way resistance and dynamic lung/thorax compliance.20,21 should stimulate in vivo research. Gravity-assisted drainage (head-down tilt) combined with manual lung hyperinflation may further enhance secretion Overview and Recommendations clearance.22 Chest-wall vibration, with or without manual lung hy- The work by Volpe et al27 supports earlier findings that perinflation and suctioning, can improve expiratory flow, expiratory flow bias can augment secretion clearance3,4 airway resistance, and dynamic lung/thorax compliance, and demonstrates how secretions can move between lungs but has not been demonstrated to improve secretion clear- at the carina; it is unclear if this principle extends to the ance.23,24 Manual lung hyperinflation with PEEP more peripheral airway branchings. Unfortunately, airway Ͼ 10 cm H2O may retard the expiratory flow to below that suctioning, cough, and gravity were not investigated, but required for 2-phase gas-liquid flow.25 they can play an important role in mucus movement.28 In a rabbit model with pressure-controlled ventilation, Ventilator settings that have an expiratory flow bias may no PEEP, and artificial mucus instilled to create atelecta- cause “pooling” of secretions near the central or major sis, chest-wall vibration was associated with significant airways, which could lead to adverse changes in ventila- deterioration in dynamic lung/thorax compliance and gas tion and gas exchange, for example in a sedated or para- exchange. Unfortunately, those authors did not measure lyzed patient.26 Hence, adjuncts such as manual lung hy- airway resistance.26 The resultant deterioration in lung me- perinflation, chest-wall vibration, postural drainage, and chanics and gas exchange in that animal model may sup- airway suctioning could be important tools to prevent se- port the hypothesis that chest-wall vibration can mobilize cretion retention and ensure major airway patency. Newer peripheral secretions to the proximal airways.26 These ad- artificial airways, such as the “mucus slurper” and “mucus verse changes in ventilation probably occur during routine shaver,”6 combined with ventilator settings/procedures that patient care (eg, patient repositioning) but have yet to be cause an expiratory flow bias may reduce the need for studied. Adding manual lung hyperinflation, head-down conventional suctioning.5,16,27 positioning, and airway suctioning to chest-wall vibration Even though impaired mucociliary clearance may lead may be a more effective strategy to manage or prevent to pulmonary complications such as secretion retention these secretion-related adverse events. and ventilator-associated pneumonia,1,2 studies of chest physiotherapy have had mixed results,29-32 but chest phys- Mechanical Ventilator Settings iotherapy warrants further investigation.31 The current practice of manual lung hyperinflation,5 with In this issue of RESPIRATORY CARE, Volpe et al27 report a inspiratory flow of 90 L/min and expiratory flow of 196 L/ study in which they used a test-lung model connected to a min, and an inspiratory-flow-to-expiratory-flow ratio of mechanical ventilator to investigate the effects of tidal- 0.6, far exceeded the flow-magnitude threshold, expiratory ventilation airflow, airflow bias, and lung mechanics on flow bias, and minimum difference between inspiratory the movement of simulated mucus. Ventilator settings that and expiratory flow required for mucus movement during produced an expiratory flow bias moved mucus toward the tidal ventilation in test lung or animal models.3,4,27 It is ventilator, whereas settings that produced an inspiratory also conceivable that chest-wall vibration during mechan- flow bias moved mucus deeper into the lung model, away ical ventilation may move airway secretions “to and fro” from the ventilator. Thresholds for flow magnitude (min- through the alternating expiratory and inspiratory flow bias imum of 40 L/min, with a strong association between peak (negative pleural pressure on the removal of the chest flow rate and mucus movement) and phase differential compression may trigger ventilator breaths and create an would shift simulated mucus either closer to or away from inspiratory flow bias), similar to what may occur during the ventilator.27 The absolute difference between the in- closed suctioning.13 The work of Volpe et al and others spiratory flow and expiratory flow better explained the may also explain the “inexplicable” major-airway occlu- direction of mucus movement than did the ratio of expi- sion and deterioration in gas exchange and ventilation when ratory flow to inspiratory flow. Intrinsic PEEP (from in- mechanical ventilator settings favor an expiratory flow creased minute ventilation) moved mucus toward the ven- bias and the patient has highly viscous secretions, deep tilator, whereas retarding expiratory flow with an airway sedation, and/or paralysis and absent