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The management of Non-invasive ventilation in chronic airway diseases
 
发布时间: 2010/09/23    阅读:1338
 
The management of Non-invasive ventilation in chronic airway diseases
Enhai Cui, Department of Respiratory Medicine, Huzhou Central Hospital,
Medical College of Zhejiang University, 313000, China
Without mechanical support for respiration, many patients would die within hours to days due to respiratory failure. Non-invasive ventilation (NIV) is generally defined as the delivery of ventilatory support to the lungs through the patients upper airway using a mask or similar device. The use of NIV is increasing for patients with acute and chronic respiratory failure, because of its convenience, lower cost and minimal complications. The greatest advantage of NIV is that endotracheal intubation is not required. Observational, physiological and case/control studies form a large body of evidence demonstrating that NIV can be used in many situations to decrease a patientsdyspnoea and work of breathing, improve gas exchange and ultimately avoid the need for endotracheal intubation. Although it is an extremely useful technique, it cannot be used in all patients and correct patient selection is important. It has been extensively used in patients with chronic airway diseases with acute or acute-on-chronic respiratory failure.
NIV includes two common modalities: continuous positive airway pressure (CPAP) and bi-level positive airway pressure(BiPAP). The effects of CPAP include increasing mean airway pressure, recruitment of under-ventilated alveoli, increasing minute ventilation and stabilizing the upper airway. BiPAP delivers a higher pressure during inspiration and then returns to a preset expiratory positive airway pressure. By providing an increased level of inspiratory support, BiPAP can decrease the work of breathing and rest fatigued respiratory muscles in addition to having the other effects of CPAP.
In recent years, with the rapid progress in staff expertise and equipment for NIV, including ventilator technology, ventilatory modes and close fitting interface, NIV has become an effective treatment in the management of patients with acute and chronic ventilatory failure. An increasing number of evidences are available to support the use of NIV in chronic obstructive pulmonary disease (COPD), cardiogenic pulmonary edema, chronic alveolar hypoventilation, and hypoxemic respiratory failure due to some certain clinical conditions. This review focuses on the indications, benefits and supportive evidences for the use of NIV in COPD and asthma in adult patients.
NIV is distinguished from invasive ventilation that bypasses the upper airway by tracheal tube, laryngeal mask or tracheotomy, therefore preserving normal swallowing, speech, cough and expectoration as well as airway defense mechanisms. Moreover, specific complications associated with endotracheal intubation, such as ventilator-associated pneumonia (VAP), sinusitis, intubation related trauma, hemorrhage and tracheal stenosis are avoided. Accordingly, NIV is associated with reduced respiratory intensive care unit (RICU) demands, a reduction in intubation rates, a shorter length of stay, enhanced patient comfort and, ultimately, reduced health-care expenditure. It has been regarded as a cost-effective intervention.
1.      COPD
NIV reduces mortality and hospital stay in patients with acute hypercapnic ventilatory failure due to COPD; it is also an effective weaning strategy for those who require intubation. As for severe stable COPD patients, NIV may be beneficial in certain highly selected ones and further studies are needed.
a. Hypercapnic exacerbations of COPD
There has been an accumulating amount of clinical evidences to support the effectiveness of NIV in acute hypercapnic ventilatory failure in COPD.
A number of early small uncontrolled studies showed that the application of NIV in patients with acute exacerbations of respiratory failure due to COPD was associated with variable improvements in arterial blood gas tensions. Twenty-two cases of acute hypercapnic respiratory failure with pH value less than 7.30 were delivered with NIV via nasal or face mask in the study. NIV was well-tolerated and the cure rate was 86.4%. Only 3 of the total 22 patients needed intubation. For those suffering from severe hypercapnic respiratory failure, NIV could be ineffective and, frequent monitoring is necessary to ensure timely intubation.The experience of staff is crucial factors for the success of NIV.
There is now sufficient evidence that the use of NIV in patients with acute respiratory failure due to COPD is advantageous, resulting in reductions in rates of intubation and mortality. NIV can be used in the intensive care unit, in the ward, or in the accident and emergency department. The criteria for patient inclusion in most of the clinical studies are a useful guide, and NIV should be considered when the pH is less than 7.35, respiratory rate above 30 breaths/min, and PaO2 on air below 6kPa. This means that NIV will be used at a much earlier stage in the course of respiratory failure than that at which intubation is normally considered, with the aim of preventing the development of muscle fatigue and more severe acidosis.
The largest prospective multicentric randomised controlled study was performed by Plant et al. 236 mildly and moderately acidosis patients with COPD in the general ward setting were recruited from 14 UK hospitals over 22 months. The entry criteria were hypercapnic exacerbation of COPD with pH 7.25-7.34 following optimization of medical treatment (including controlled oxygen therapy). Patients were randomized to receive standard medical care or standard medical care plus NIV. The use of NIV significantly reduced the need for intubation (27% vs l5%, p=0.02). In-hospital mortality was also reduced by NIV (20% vs 10%, p=0.05). However, NIV led to a more rapid improvement in pH in the first hour (p=0.02) and a greater fall in respiratory rate at 4 h (p=0.035). The duration of breathlessness was also reduced by NIV (p=0.025).
A recent systematic review, involving 14 randomised controlled trials (RCTs) comparing NIV plus usual medical care (UMC) versus UMC alone in adult patients with respiratory failure due to an exacerbation of COPD with respiratory rate >28/min, PaCO2 > 45 mmHg and pH < 7.35, found that NIV resulted in decreased need for endotracheal intubation (RR=0.41), a relative risk reduction in mortality (RR=0.52), reduction in treatment failure (RR=0.48), and more rapid improvement in arterial blood gases. In addition, complications associated with treatment and length of hospital stay was also reduced in the NIV group. Therefore, NIV should be used as first line intervention as an adjunct therapy to UMC in all suitable patients in the early course of respiratory failure secondary to an acute exacerbation of COPD.
Despite above encouraging results, the use of NIV in the management of acute respiratory failure in COPD is not always successful. Severe conditions, low value of pH, poor Glasgow Coma Score and Apache II Score, as well as metabolic alterations were reported as the best predictors of the failure. Patients with risk factors of NIV failure should be monitored more frequently. Although nearly all COPD patients with acute hypercapnic respiratory failure could benefit from NIV, facial and skull trauma, inability to cough or clear secretions, pneumothorax and epistaxis have been listed as contraindication to the use of NIV. In case of NIV failure and contraindication, mechanical ventilation via artificial airway should be delivered without hesitation.
Success has also been reported in using NIV for weaning patients from intubation and mechanical ventilation.When intubation is required, weaning from ventilation is facilitated by NIV. Early extubation and sequential NIV can decrease the invasive and total durations of ventilatory support, the risk of VAP, and the duration of ICU stay. A systematic review of five RCTs found that the NIV-weaning strategy was associated with significantly lower mortality and reduced length of hospital stay by a mean of 7.3 days.
b. respiratory failure in chronic stable COPD
Despite the success of short-term NIV in treating acute hypercapnic COPD, this group of patients had a high rate of exacerbation and mortality rate in the subsequent year. This phenomenon leads to a number of studies on whether the application of NIV is beneficial for chronic stable COPD.
The early studies had small number of patients followed for only a short period of time and most of them failed to show any advantage in using NIV in stable COPD. In 2002, the result of a 2-year multicentric RCT carried out by Clini was published, favoring the use of nocturnal, home NIV in patients with chronic ventilatory failure due to advanced COPD. Ninety eligible patients were randomly assigned to NIV plus long-term oxygen therapy (LTOT) (n=43) or to LTOT alone (n=47). There were small improvements over time in the NIV group in PaCO2, resting dyspnoea and health-related quality of life, but no improvement in survival or hospital admissions. Nevertheless, overall hospital admissions showed a different trend in the NIV plus LTOT(decreasing by 45%) as compared with the LTOT group (increasing by 27%), when compared with the period before the study. ICU stay was reduced in both groups, but more so in the NIV than in the LTOT group (75% vs 25%). A retrospective study showed that in severe hypercapnic stable COPD with RV/TLC>75%, long-term nocturnal NIV was positively correlated with reductions in PaC02 (r=0.56; P<0.05) and RV/TLC (r=0.50;P<0.05). Therefore, NIV should be considered in patients who are clearly hypercapnic, who can tolerate an effective level of ventilatory support, and who get enough time to adjust to the ventilator.
Besides, NIV enables a higher intensity of training in patients with severe COPD, leading to greater improvements in maximum exercise capacity and may prove to be a useful adjunct in pulmonary rehabilitation, but will not take effect in the pulmonary rehabilitation in mild COPD patients.
In summary, additional NIV has been shown to improve daytime blood gas parameters and quality of life with reduction of hyperinflation in selected hypercapnic patients with stable COPD. However, its effect on reducing the frequency and severity of COPD exacerbation, and economic evaluation should be clarified before widespread use.
2. Severe asthma
The pathophysiological condition of acute respiratory failure in asthma is in many ways similar to that of acute respiratory failure in COPD, leading to the question on whether NIV could also be successful in patients with severe acute asthma.
To our knowledge, there is only one prospective, randomized, placebo-controlled study on the effect of NIV in patients with a severe asthmatic attack. Thirty patients with FEV1 on presentation < 40% predicted and without hypercapnia were randomized into BiPAP ventilation plus conventional treatment and conventional treatment alone. The use of NIV (mean IPAP 13 cmH20, mean EPAP 4 cmH2O for 3 hours) can significantly improve lung function (FEV1, FVC, PEFR and respiratory rate), alleviate the attack faster, and reduce the hospital admission rate.
The data from a seven-year period retrospective observational study presented that 15 of 17 patients with acute hypercapnic asthma (mean pH 7.25 and PaCO2 65 mmHg) who were treated with NIV avoided invasive ventilation. Compared with baseline, NIV showed significant improvements in pH and PaCO2. In another three-year period retrospective observational study , 12 acute asthma patients (mean pH 7.335±0.047 and PaCO2 53±14 mmHg) were treated with NIV (mean IPAP 13.3±3.6 cmH2O, mean EPAP 4.9±1.1 cmH2O), and 1 patient failed to respond and required intubation. Compared with baseline, significant improvements were achieved in pH, PaCO2, heart rate and respiratory rate after 2 hours of NIV without life-threatening complications.
A recent systematic review by Ram pointed out that despite some promising preliminary results and physiological benefits, large, prospective, randomised controlled trials are needed to establish the role of NIV in the management of status asthmaticus.
Conclusions
Strong evidences are available for the use of NIV in the following clinical conditions: (1) acute hypercapnic respiratory failure due to COPD; (2) selected stable COPD with hypercapnic respiratory failure. Despite the existing evidences, further study is required to elucidate the optimal modalities, target population, best timing to be initiated, long-term benefits and economical effectiveness.
NIV is advantageous in many aspects. For important, it is effective in avoiding intubation, reducing mortality, improving quality of life and decreasing health-care expenditure in properly selected patients. Also, the convenience of NIV makes it possible to be delivered anywhere, including areas outside the ICU, such as ambulance, emergency, operating recovery rooms, and even domicile settings. NIV can be useful in many conditions that lead to respiratory failure. NIV is a relatively new technique and has not found a place in all intensive care units. It is expected that this modality will be used more frequently in the near future.

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