Respiratory Support in Critical Care

Indications of non-invasive ventilation in intensive care: a practical approach

Matthias Leist, MD
Department of Anaesthesiology, Emergency and Intensive Care Medicine
University Hospital Göttingen
D-37075 Göttingen

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See the author giving the Clinical Window presentation at the 23rd ISICEM congressin in Brussels (March 2003).

Introduction

Mechanical ventilation with an endotracheal tube or via tracheostomy has become a major component of intensive care. However, unavoidable drawbacks related to the invasive character of this treatment have been observed, including ventilator-associated pneumonia (VAP).

This has led to wide acceptance of ‘non-invasive ventilation’ (NIV) as a long-term treatment for patients with hypercapnic respiratory failure due to neuromuscular diseases or chest wall deformities. In the NIV method the non-invasive airway access is provided by tightly fitting masks. Interest in the use of this method appears to be growing for other types of respiratory failure as well. This article discusses indications and contraindications of NIV as well as practical aspects of the method necessary for successful treatment.

Chronic obstructive pulmonary disease (COPD)

NIV leads to improved gas exchange and may lower the frequency of endotracheal intubations. Using NIV in acute respiratory failure (ARF) related to acute exacerbations of COPD may reduce the duration of hospitalization compared to standard care, in which no ventilatory support is given [3]. Based on a meta-analysis of different prospective trials NIV has also been shown to reduce mortality. Hence, acute exacerbation of COPD is now considered to be an unequivocal indication for NIV [7].

Acute hypoxemic respiratory failure (ARF)

No reduction of intubation rate, length of ICU stay or mortality was observed in patients with hypoxemic ARF without pre-existing COPD [12], but possible indications for NIV may exist in certain subsets of patients.

Compared to standard treatment without ventilatory support, NIV improves oxygenation and leads to fewer intubations in immuno-suppressed patients and patients with community-acquired pneumonia. However, mortality was reduced only in immuno-suppressed patients most probably because serious infectious complications [4, 6] were avoided.

In early hypoxemic ARF, NIV improves oxygenation as efficiently as invasive ventilation. Hence, a lower frequency of infections may also contribute to a shorter duration of ICU stay [1]. Based on these findings, NIV may with some limitations be recommended as an additional component for treatment of early stage ARF not related to COPD.

Acute cardiogenic pulmonary edema

NIV has been suggested as a suitable approach in the treatment of acute cardiogenic pulmonary edema. This is based on pathophysiological findings such as a reduction of left ventricular preload or end-diastolic volume, with secondary improvement in the left ventricular ejection fraction.

Comparing CPAP treatment with medical treatment alone demonstrated a decreased frequency of intubation, and there was a suggestive finding of decreased hospital mortality. Thus CPAP treatment is now a well-established component in the treatment of acute cardiogenic pulmonary edema [11].

However, trials with NIV applied in a pressure support mode have shown conflicting results. Compared to standard treatment, NIV does not appear to reduce mortality despite clinical improvement, better oxygenation and a lower intubation rate [8]. In fact NIV may even be associated with a higher incidence of myocardial ischemia when compared to CPAP treatment [9]. Hence, the method may be disadvantageous if used without caution, particularly in the case of acute cardiogenic edema caused by myocardial infarction.

NIV as a weaning strategy

Although most authors have focused on the use of NIV in an early stage of ARF to avoid intubation, NIV may also be used during weaning from mechanical ventilation. Even patients who do not meet typical criteria (like a 2-hour T-piece trial) for successful weaning from mechanical ventilation, could first be extubated and thereafter weaned using NIV. In this group of patients, the duration of endotracheal mechanical ventilation was significantly shorter than in the control group undergoing conventional weaning procedures – and the overall success rates were comparable.

These results with patients suffering acute respiratory failure and COPD, and usually requiring cumbersome conventional weaning strategies, suggest that complications related to prolonged invasive ventilation may be avoided. However, the total duration of ventilatory support was longer in the NIV group [5]. Based on these data, NIV may be recommended as a means of preventing re-intubation in patients with COPD after early extubation.

Contraindications

Despite the broad spectrum of indications for NIV, contraindications of this method must also be observed. They include facial trauma, recent surgery of the upper airway or upper gastrointestinal tract, and difficulties in protecting the airway (i.e. lowered consciousness, bowel obstruction or copious tracheal secretions). Endotracheal intubation should always be preferred in the case of marked hemodynamic or respiratory instability with life-threatening hypoxemia [2].

Monitoring

Monitoring should not only be limited to technical measurements because clinical observation of the patient is also important. Successful treatment will result in higher tidal volumes, reduced respiratory rate, improved chest wall movement and adequate synchronization of the patient’s own breaths with the respirator.

With reversal of hypoxemia and hypercapnia, the patient’s hemodynamic stability and mental status are likely to improve. Precise monitoring of oxygenation and frequent measurement of arterial blood gases are crucial, especially in the early stage of treatment.

Treatment failure

Regular clinical evaluation of the patient’s status is important for successful treatment. Deterioration in the patient’s status can occur even after 48 hours or more, and such cases must be identified immediately. In fact such ‘late failures’ are often associated with a poor prognosis and high mortality. Typical criteria for a ‘late failure’ include a rapid drop in arterial pH to below 7.34, a possible 15-20% rise in PaCO2, and dyspnoe or deterioration of the patient’s mental status [10].

Technical considerations and choice of ventilators

Most ventilators commonly used in an ICU setting allow continuous measurement of FiO2, tidal volumes and respiratory frequency. If not, it may be helpful to measure these parameters and observe the associated alarms.

On the other hand, frequent leakage alarms may bother patients and staff. In fact many modern ICU respirators are equipped with full leakage compensation and the most commonly used non-invasive ventilation modes such as pressure support ventilation or bi-level pressure ventilation can be used with them. It must be emphasized, though, that leakage control and correct ventilator function often depend on practical issues like careful fitting of nasal or full-face masks.

Ventilator settings

Theoretical considerations suggest that application of a controlled ventilation mode leads to complete unload of respiratory muscles and reduction of work of breathing. In ARF this may be favorable because it may result in faster recovery than with assisted ventilation.

Although more research on the matter needs to be done, the following guidelines may at this point be suggested:

• With COPD, a controlled mode of ventilation may be applied if tolerated by the patient. Alternatively, a pressure-supported mode should be preferred in cases of poor synchronization with the ventilator.
• Other types of respiratory failure may benefit from pressure support ventilation. These include pneumonia or cardiogenic pulmonary edema, where fatigue of the respiratory muscles may have a less important role.
• In patients with acute exacerbation of COPD, relatively high pressure support may initially be necessary since too low pressure support would be counteracted by high airway resistance. In such a case, alveolar ventilation would then not be improved, but increased breathing frequency with a decrease in tidal volumes could occur. Initial adjustment of ICU ventilator’s respiration rate is also important, as the patient’s own effort should be synchronized with the respirator to achieve a sufficient unloading of the respiratory muscles.
• If patients ventilated with an adequate minute volume still experience dyspnoea, the cause may be an inadequate pressure rise time in the early phase of inspiration. It may be helpful in such cases to use ventilators capable of delivering high flow rates and correctly adjusting the pressure rise time.

Conclusions

NIV has been used increasingly for several indications of acute and chronic respiratory failure. In certain indications, such as the acute exacerbation of COPD, NIV is about to replace invasive ventilation as a standard regime.

Potential advantages like reduced rates of infection, and growing experience with other indications of mechanical ventilation, may increase the use of NIV as an alternative to conventional, invasive ventilatory techniques.

Referenses

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Last updated: 1 March 2003Created
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