Respiratory Care

As the first paper in this Journal Conference on intensive care unit controversies, the editors wished us to set the tone for the debate by discussing the ethics of medical “adventurism” in the intensive care unit. More life-or-death decisions are made in the intensive care unit than elsewhere in the hospital, and the critical care specialist often sees himself or herself as a warrior in a battle with death. This adrenaline-charged calling attracts highly intelligent, hard-working, and compassionate caregivers, as well as fiercely independent clinicians. The result of this is that critical care specialists passionately debate about the meaning and application of published “evidence” and this leads to thoughtful debate, as exemplified by the papers in this and the next issue of Respiratory Care, as well as thoughtless and often dangerous disregard for evidence-based medicine. Physicians are morally obligated to provide the best and most appropriate care possible for their patients, but when accepted approaches are failing and a critically ill patient is getting worse, the critical care physician must make a decision regarding innovative therapy, based on the patient's prognosis, the available evidence, the resources on hand, the expertise of the physicians, and the values of the patient and the physician. This decision may lead, at times, to trying unproven and innovative strategies to achieve a clinical goal. In such cases, it is to be hoped that this can be done in such a way that data are formally and prospectively collected to increase our knowledge.

Ventilator-associated pneumonia (VAP) significantly increases intensive care unit morbidity, mortality, and costs. VAP is thought to be caused by bacterial entry into injured airways, which produces tracheobronchitis that evolves into diffuse pneumonia. The use of aerosolized antibiotics is conceptually attractive, especially when the infection is early and limited to the airway epithelium. Data show that aerosolized antibiotics kill airway bacteria and improve outcomes in cystic fibrosis. The clinical evidence for aerosolized antibiotics to prevent VAP is weak but suggestive. Concerns about the high cost, possible development of antibiotic resistance, and other potential risks of aerosolized antibiotics led several evidence-based consensus groups to recommend against routine use of aerosolized antibiotics for VAP prevention until better data are available. Importantly, the clinical evidence that aerosolized antibiotics can treat established VAP is negative, and multiple consensus groups recommend against treating established VAP with aerosolized antibiotics.

One of the most important aspects of caring for a critically ill patient is monitoring. Few would disagree that the most essential aspect of monitoring is frequent physical assessments. Complementing the physical examination is continuous monitoring of heart rate, respiratory rate, and blood oxygen saturation measured via pulse-oximetry, which have become the standard of care in intensive care units. Over the past decade one of the most controversial aspects of monitoring critically ill patients has been capnography. Although most clinicians use capnography to confirm endotracheal intubation, few clinicians use continuous capnography in the intensive care unit. This article reviews the medical literature on whether every mechanically ventilated patient should be monitored with capnography from intubation to extubation. There are numerous articles on capnography, but no definitive, randomized study has even attempted to address this specific question. Based on the available literature, it seems reasonable to use continuous capnography, for at least a subset of critically ill patients, to ensure integrity of the endotracheal tube and other ventilatory apparatus. However, at this point definitive data are not yet available to clearly support continuous capnography for optimizing mechanical ventilatory support. We hope that as new data become available, the answer to this capnography question will become clear.

Cardiac arrest is a common and lethal medical problem; each year more than half a million people in the United States and Canada suffer cardiac arrest treated by emergency medical personnel or in-hospital providers. Of those who survive to hospital admission or suffer in-hospital arrest, 40–60% die prior to discharge. Neurologic injury is the major source of morbidity and mortality after recovery of spontaneous circulation. Therapeutic options to prevent neurologic injury are limited, but recent randomized trials showed that moderate therapeutic hypothermia improves neurologic outcome in selected patients following cardiac arrest. Clear consensus statements recommend that unconscious adult patients with spontaneous circulation after out-of-hospital cardiac arrest should be cooled if the initial rhythm was ventricular fibrillation, and that therapeutic hypothermia should be considered for other patients (other rhythms or in-hospital arrest). However, the position that all patients should be cooled following cardiac arrest is probably too broad, given the lack of studies on patients with non-ventricular-fibrillation rhythms, in-hospital arrest, or non-cardiac causes of arrest. Further research is needed to determine the broadest application of moderate therapeutic hypothermia.

Airway pressure-release ventilation (APRV) is a mechanical ventilation strategy that is usually time-triggered but can be patient-triggered, pressure-limited, and time-cycled. APRV provides 2 levels of airway pressure (P_high and P_low) during 2 time periods (T_high and T_low), both set by the clinician. APRV usually involves a long T_high and a short T_low. APRV uses an active exhalation valve that allows spontaneous breathing during both T_high and T_low. APRV typically generates a higher mean airway pressure with a lower tidal volume (V_T) and lower positive end-expiratory pressure than comparable levels of other ventilation strategies, so APRV may provide better alveolar recruitment at a lower end-inflation pressure and therefore (1) decrease the risk of barotrauma and alveolar damage in patients with acute lung injury or acute respiratory distress syndrome (ALI/ARDS), and (2) provide better ventilation-perfusion matching, cardiac filling, and patient comfort than modes that do not allow spontaneous breaths. However, if the patient makes a spontaneous breath during T_high, the V_T generated could be much larger than the clinician-set target V_T, which could cause the end-inflation transpulmonary pressure and alveolar stretch to be much larger than intended or produced in other ventilation strategies. It is unknown whether a patient's inspiratory effort (and consequent larger V_T) can damage alveoli in the way that mechanically delivered, positive-pressure breaths can damage alveoli in ALI/ARDS. Other ventilation modes also promote spontaneous breaths, but at overall lower end-inflation transpulmonary pressure. There is a dearth of data on what would be the optimal APRV inspiratory-expiratory ratio, positive end-expiratory pressure, or weaning strategy. The few clinical trials to date indicate that APRV provides adequate gas exchange, but none of the data indicate that APRV confers better clinical outcomes than other ventilation strategies.

Positive end-expiratory pressure (PEEP) and inspired oxygen fraction (F_IO2 ) are the primary means of improving P_aO2 during mechanical ventilation. Patients with acute respiratory distress syndrome (ARDS) typically present with a large intrapulmonary shunt, which makes even high F_IO2 ineffective in improving P_aO2 . PEEP decreases intrapulmonary shunt by recruiting collapsed alveoli, but PEEP is associated with important adverse effects, whereas prolonged exposure to high F_IO2 may cause oxidative lung injury. The improved survival found in the National Institutes of Health's ARDS Network low-tidal-volume study may suggest that their PEEP/F_IO2 titration tables represent the best method for adjusting these variables. Based upon an extensive literature review of PEEP and respiratory system mechanics in ARDS, we conclude that: (1) for most patients the therapeutic range of PEEP is relatively narrow, so the ARDS Network PEEP/F_IO2 strategy is reasonable and supported by high-level evidence, (2) how best to adjust PEEP to prevent or ameliorate ventilator-associated lung injury is unknown and still under investigation, and (3) in a small subset of patients with severe lung injury and/or abnormal chest-wall compliance, highly individualized titration of PEEP, based upon the respiratory-system pressure-volume curve, PEEP/tidal-volume titration grids, or a recruitment maneuver and a PEEP decrement trial is a reasonable alternative.

Traditional mechanical ventilation is provided with either a constant volume or constant pressure breath. In recent years, dual-control (adaptive pressure control) has been introduced in an attempt to combine the attributes of volume ventilation (constant tidal volume and minute ventilation) with the attributes of pressure ventilation (rapid, variable flow and reduced work of breathing). Adaptive pressure control is a pressure-controlled breath that utilizes closed-loop control of the pressure setting to maintain a minimum delivered tidal volume. Prior to the introduction of adaptive pressure control, no clinical studies were accomplished. Studies have shown that adaptive pressure control reduces peak inspiratory pressure, compared to volume control. When compared to traditional pressure-control ventilation, no differences have been identified. While adaptive pressure control can guarantee a minimum tidal volume, it cannot guarantee a constant tidal volume. One concern is that the ventilator cannot distinguish between improved pulmonary compliance and increased patient effort. Clinicians should be aware of the limitations of adaptive pressure control and understand when other breath delivery techniques are more suitable.