• Non-invasive positive-pressure ventilation (NIPPV) via face mask or nasal mask can be a safe and effective means of improving the pathophysiology of respiratory failure and correcting gas exchange abnormalities.
• Avoiding complications of endotracheal intubation and decreasing the incidence of nosocomial infections are factors involved in improving outcome of patients supported with NIPPV.
• Patient selection, correct implementation, and monitoring are critical to its success.
• Pressure support ventilation is preferred because it minimizes peak inspiratory mask pressure and air leakage and is better tolerated.
• With pressure support ventilation, tidal volume, gas exchange, respiratory rate, and diaphragmatic activity are improved in proportion to the amount of pressure supplied.
In many patients with acute respiratory failure, non-invasive positive-pressure ventilation (NIPPV) via a mask can be as effective in improving the pathophysiology of respiratory failure and correcting gas exchange abnormalities as conventional ventilation via an endotracheal tube ( Meduri 1996). When effective, NIPPV avoids the complications associated with endotracheal intubation, improves patient comfort, and preserves airway defense mechanisms, speech, and swallowing. Furthermore, NIPPV provides greater flexibility in instituting and removing mechanical ventilation.
NIPPV via a face mask or nasal mask (Fig 1) can be a safe and effective means of recruiting alveoli and augmenting ventilation in patients with acute respiratory failure resulting from chronic obstructive pulmonary disease, cardiogenic pulmonary edema, postextubation, cystic fibrosis, endstage lung disease, postoperative complications, post-traumatic injury, community-acquired pneumonia, or AIDS. Randomized studies have shown it to be superior to conventional management in patients with chronic obstructive pulmonary disease; the procedure significantly decreased the need for intubation, rate of complications, incidence of nosocomial infections, length of stay, mortality, and cost.
The duration of mechanical ventilation with continuous NIPPV is shorter than with conventional mechanical ventilation. In our group experience of 210 patients, the mean duration of mechanical ventilation was 25 h, similar to findings from other studies. The short duration of continuous successful NIPPV is consistently seen across all types of acute respiratory failure, indicating an intrinsic advantage of this procedure ( Meduri.efa/ 19.9.6). Factors that may be involved in shortening the duration of mechanical ventilation include earlier intervention, avoiding sedation and paralysis, reduced incidence of mechanical-ventilation-induced respiratory muscle atrophy, eliminating the imposed work by the endotracheal tube, a lower rate of complications (particularly infections), and earlier removal.
Avoiding complications of endotracheal intubation and decreasing the incidence of nosocomial infections are factors involved in improving outcome of patients supported with NIPPV. With NIPPV, the trachea is not invaded by the endotracheal tube or by suction catheters (patients can remove the face mask and expectorate) and the vocal cords are not kept open, thereby lowering the risk of aspiration, which is the leading mechanism of nosocomial pneumonia. Furthermore, patients rarely require invasion by nasogastric, urinary, or central intravenous catheters, which are significant contributors to nosocomial sepsis. Monitoring for clinical and laboratory signs of infection, our group has identified only two cases of pneumonia (and no other infection) among 210 NIPPV-treated patients. A randomized study of chronic obstructive pulmonary disease patients showed a significantly lower rate of intubation (26 per cent) among 43 NIPPV-treated patients and a lower incidence of nosocomial infections (Brochard. etaj 1995). The standard treatment group had seven pneumonia patients and three episodes of sepsis among intubated patients, while the NIPPV-treated group had two pneumonia patients and two episodes of sepsis (all but one occurring after intubation in patients failing NIPPV). Death related to a nosocomial infection occurred in 10 per cent of the standard treatment group and in only 2 per cent of NIPPV-treated patients. Among 16 studies involving 462 patients treated with NIPPV, the overall complication rate was 15 per cent, including 42 with facial skin necrosis, six with gastric distention, four with pneumonia, nine with conjunctivitis, and five with other complications.
Because correct implementation and monitoring are critical to the success of NIPPV, physicians, hospital respiratory technicians, and nurses should develop familiarity with this methodology. Patient selection is also essential in avoiding complications ( Table 1). Patients with morbid obesity (over 200 per cent of ideal body weight) or with unstable angina or acute myocardial infarction should be closely managed, but only by experienced personnel. The response to treatment and duration of mechanical ventilation cannot be predicted by the severity of the underlying lung disease or by the arterial blood gas values obtained before initiating mechanical ventilation.
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Table 1 Criteria for selecting patients for NIPPV
The methodology for NIPPV in patients with acute respiratory failure used at the University of Tennessee is shown in Table2. Although no study has directly compared efficacy, one group has reported a higher success rate with face-mask NIPPV versus their institutional historical controls using a nasal mask. Improvement in arterial blood gases appears to be slower in some studies using a nasal mask. Because patient tolerance is essential to the success of NIPPV, a tight uncomfortable fit should be avoided when possible. Even in patients with hypoxemic respiratory failure who are receiving continuous positive airway pressure (CPAP) by mask alone, a small leak will not cause the airway pressure to drop. Placing a nasogastric tube is indicated only for patients developing gastric distension or to provide access for enteral feeding. Gastric distention rarely occurs (less than 2 per cent) in patients treated with NIPPV or mask CPAP. Furthermore, in our experience, none of the patients developing pneumonia on NIPPV had prior gastric distention. In normal patients, the resting upper esophageal sphincter pressure is 33 ± 12 mmHg. When air enters the stomach during NIPPV, a gurgling noise can be heard over the epigastrium with the stethoscope, and a vibration can be felt by palpation. Nurses and therapists should be instructed to observe patients for signs of abdominal distension.
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Table 2 Methodology for NIPPV in patients with acute respiratory failure (University of Tennessee, Memphis)
In either hypercapnic or hypoxemic acute respiratory failure, we utilize both CPAP and intermittent positive-pressure ventilation. Pressure support ventilation is the preferred form of intermittent positive-pressure ventilation because it minimizes mask peak inspiratory pressure and air leakage and is better tolerated. The effects of CPAP and pressure support ventilation delivered by a mask, either alone or in combination, on gas exchange and transdiaphragmatic pressure in patients with obstructive and restrictive lung disease are shown in T.a.b!e..,3. With pressure support ventilation, tidal volume, gas exchange, respiratory rate and diaphragmatic activity are improved in proportion to the amount of pressure applied (Carreyefai 1990; App§DdiQ! §t§L 1994). CO2 rebreathing can occur during bi-level positive airway pressure (BiPAP) ventilatory assistance using the standard exhalation device (Whisper Swivel) and can be eliminated with a new plateau exhalation device or a non-rebreather valve. Applying expiratory positive airway pressure (3 4 cmH 2O) decreases inhaled CO2 which is eliminated at a level of 8 cmH2O. If air leakage is not improved, despite manipulations of the mask and application of a skin patch, first CPAP and then applied pressure (or tidal volume) are decreased to reduce the peak mask pressure. Portable units, such as the Respironics Bi-PAP device and the Puritan Bennett PB-335, automatically compensate for mask leaks and mouth opening.
Table 3 Effects of CPAP and pressure support ventilation delivered by a mask, alone and in combination, on gas exchange and trandiaphragmatic pressure in patients with obstructive and restrictive lung disease
In our experience, after the first hour of uncomplicated NIPPV, most patients do not require bedside observation; ventilator and oximetry alarms provide warnings for early intervention if necessary. We have found that time involvement with NIPPV is proportional to the level of experience. In two randomized studies, bedside time commitment by nurses and therapists was similar for patients receiving NIPPV or conventional treatment. After the initial stabilization period on NIPPV (4-6 h), patients with hypercapnic respiratory failure or with hypoxemia on low-level CPAP (£ 5 cmH 2O) can safely remove the mask for 5 to 15 min, during which time they can talk, drink small amounts of liquid, expectorate, or receive nebulized bronchodilator therapy. Because mask ventilation provides a large degree of flexibility, it can be adjusted to meet a patient's individual needs. Continuous oximetry with alarms should be provided, and in patients with severe hypercapnia it is advisable to have an arterial catheter in place. Ventilator settings should be adjusted based on results of arterial blood gases obtained within 1 h and, as necessary, at intervals of 2 to 6 h. Providing reassurance and adequate explanation to the patient about what to expect is of the utmost importance. Patients are instructed to call the nurse if they have needs or develop complications.
Criteria to discontinue NIPPV are shown in Jable..4. Following improvement in acute respiratory failure, patients are weaned from mechanical ventilation either by lowering the amount of delivered pressure or by titrating periods off mechanical ventilation to patient tolerance and objective findings, similar to a T-piece weaning trial. At a low level of pressure support (5-8 cmH2O), the patient is disconnected from the ventilator while receiving supplemental oxygen by nasal cannula or face mask. During weaning, NIPPV eliminates the reintubation factor associated with prematurely removing conventional ventilation, and this may contribute to the shorter duration of ventilation.
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In patients with hypercapnic acute respiratory failure receiving NIPPV, most studies have found that the arterial blood gas response and need for intubation cannot be predicted by the severity of the underlying lung disease (forced expiratory volume in 1 s and arterial blood gases) or by the arterial blood gas values ( Paco2, pH) obtained before implementing NIPPV. Two recent studies involving 125 patients with chronic obstructive pulmonary disease and acute respiratory failure found that non-responders had a higher Paco2 at initiation of NIPPV (MedyLleLal 1996). In other studies, the underlying cause of acute respiratory failure did not predict the outcome of NIPPV. Patients with chronic obstructive pulmonary disease and pneumonia or congestive heart failure as the cause of acute respiratory failure have a higher intubation rate. We have consistently found that a reduction in Paco2 or an increase in pH within 1 to 2 h of NIPPV predicted a sustained improvement in gas exchange and requirement for shorter duration of ventilatory support (mean 26 h of NIPPV versus 323 h of mechanical ventilation with endotracheal intubation). This observation has been confirmed by others.
An understanding of this method of mechanical ventilation by physicians and hospital respiratory therapy teams is an extremely important adjunct to ensuring the safety of this technique and its proper use on patients with acute respiratory failure. A dedicated team (therapist and intensivist) should provide in-service training to respiratory therapists, critical care nurses, and house staff prior to implementing this method of ventilation. Continuous supervision is required until providers are fully comfortable in all aspects of non-invasive ventilation. With proper training, NIPPV can be transferred to clinical practice with results similar to those of small studies conducted by a research team. We have produced an instructional videotape, which is available upon request, and developed a comprehensive course to instruct therapists and physicians on correct NIPPV methodology.
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