VENTILATORY STRATEGIES IN CHRONIC OBSTRUCTIVE PULMONARY DISEASE

Chronic obstructive pulmonary disease (COPD) is a disease characterized by air flow obstruction (FEV1 <80% of predicted or FEV1/ FVC <70%). It is usually progressive, not completely reversible and does not change markedly over several months. The disease is predominantly caused by smoking. A small percentage can be due to alpha1 antitrypsin deficiency. Pathophysiologically there is airway and lung parenchymal damage leading to airway obstruction. These changes give rise to various pulmonary symptoms (cough, shortness of breath, wheezing) resulting in disability and impaired quality of life. As anaesthetists and critical care doctors, we are involved in ventilatory management of COPD patients in following situations,

Chronic obstructive pulmonary disease (COPD) is a disease characterized by air flow obstruction (FEV 1 <80% of predicted or FEV 1 / FVC <70%).It is usually progressive, not completely reversible and does not change markedly over several months.The disease is predominantly caused by smoking.A small percentage can be due to alpha-1 antitrypsin deficiency 1 .
Pathophysiologically there is airway and lung parenchymal damage leading to airway obstruction.These changes give rise to various pulmonary symptoms (cough, shortness of breath, wheezing) resulting in disability and impaired quality of life.
As anaesthetists and critical care doctors, we are involved in ventilatory management of COPD patients in following situations, • Ventilatory support/ long term oxygen therapy in long term management of COPD patients.• Ventilatory support in acute exacerbations in COPD patients.• Intra operative and postoperative ventilation in patient with COPD who undergo major surgery.
Oxygen therapy (2L per minute via face mask or nasal cannula) need to be given at least 15 hours per day to get the above mentioned survival benefit.If oxygen therapy is given for more than 20 hours per day, it has been shown to reduce pulmonary vascular resistance.
In the long term management certain patients need non invasive ventilatory support other than LTOT.

Clinical indications for non invasive ventilator (NIV) support in COPD
• Symptomatic despite optimum medical treatment for COPD.• PaO 2 > 55 mmHg.
What is the rationale of NIV in the management of COPD?It is important to know the changes happening in the COPD lung as opposed to normal lung to find the answer for the above question.All these changes make the energy supply demand balance unfavouarble and the end result would be chronic respiratory muscle fatigue.NIV provide resting of the chronically fatigued muscles and this was the basis for muscle resting hypothesis.Later Fleetham J & colleague and Catterall JR & colleague came up with another hypothesis which was based on sleep studies done among COPD patients.They found that obstructive sleep apnea is highly prevalent among COPD patients and obstructive sleep apnea got improved when they were given nocturnal NIV.More importantly day time gas exchange was improved in these patients.
Firstly, negative pressure ventilation was introduced to test the muscle resting hypothesis.Negative pressure ventilation was given via a wrap ventilator which was a rigid plastic or metal cage.Most  >50mmHg and randomized to two groups.One group received conventional treatment and other group received conventional treatment and NPPV.There was an improvement in survival among conventional treatment and NPPV group though it was not statistically significant.Similar findings were obtained in European multi centre trial.In conclusion, although evidence is conflicting and far from definitive, patient with severe carbon dioxide retention, particularly those with nocturnal oxygen desaturation appear most appropriate to respond favourably to NPPV.

Ventilatory support in acute exacerbation of COPD
An exacerbation is a sustained worsening of the patient's symptoms from their usual stable state which is beyond normal day to day variations, and is acute in onset.Enhancing ventilation by unloading fatigued ventilatory muscles is an important goal in the treatment of acute exacerbations of COPD which is complicated by respiratory failure.Multicentre randomized controlled trials and meta analysis have shown NIV reduces numbers, reduced hospital mortality, hospital stay and accelerate symptomatic and physiological improvement 6 .

One of the suggested treatment plans in acute COPD is as follows:
Start optimum medical treatment.
• Provide optimum oxygen therapy -using 24% or 28% venti mask or nasal cannulae flow rate adjusted to achieve oxygen saturation 88-92%.Oxygen therapy starts from low FiO 2 as these patients have hypoxic respiratory drive.Most of the studies on NIV in acute management of COPD were compared with medical treatment.Conti G & colleague did a randomized controlled trial comparing NIV and invasive ventilation in patients with acute respiratory failure due to COPD.They have concluded that NIV was no worse than invasive ventilation and if successful there are advantages both in the short term and in the year after hospital discharge.In both spontaneous and positive pressure ventilation exhalation continues till functional residual capacity.If inspiration commences before passive expiration is complete, the lung become progressively distended and gives rise to dynamic pulmonary hyperinflation.

Indications for invasive ventilation in acute COPD
DPH occurring during positive pressure ventilation is called intrinsic positive end expiratory pressure (PEEPi).Amount of PEEPi depends on lung compliance, airway tierce and respiratory variables.Some COPD patients need 20-30 seconds for full expiration and some retain up to 4L above functional residual capacity in their lungs.
Intrinsic PEEP has both dynamic and static components.Experimental studies have shown dynamic PEEPi is much less than static PEEPi in patients with air flow limitation who typically have significant inhomogenity. (DPH)

Figure 1 :
Figure 1: Changes in COPD lung as apposed to normal lung.• Hyperinflation in patients with emphysema has been found to place the respiratory muscles at a mechanical disadvantage.• Flattened diaphragm shortens the sarcomere length and reduces maximal muscle force.• Reduced bucket handle movement of the ribs impair thoracic wall compliance.• Reduced zone of opposition between diaphragm and chest wall.• Alveolar wall tension increase in emphysema.This (according to Law of Laplace) leads to reduced blood supply.• Recruitment of accessory muscles for tidal breathing.
of the early work had been done by Bruno and Marino which was published in Chest in 1985.They studied 16 COPD patients and found improvement in vital capacity, maximum inspiratory pressure, maximum expiratory pressure and day time PaCO 2 .Later studies have shown both favourable and unfavourable results with wrap ventilators.A cross over study done by the Zibrak & colleagues did not show any benefit and they found wrap ventilator was poorly tolerated by most patients.