**2. Modes of noninvasive ventilation**

## **2.1 Positive pressure ventilation**

Positive pressure ventilation delivers either a tidal volume which is either at a supra-atmospheric pressure or at a preset volume which leads to the inflation of the lungs. Exhalation is itself a passive event; it relies on the elastic recoil of the lungs for the deflation of the lung until equilibrium is attained with the pressure of atmosphere or PEEP.

It is the most commonly utilized mode of NIV in the present time, where the interface with the patient can be in the form of a full face mask, a nasal mask or a nasal pillow.

Benefits of positive pressure ventilation are as follows includes avoidance of intubation and other risks as well as complications which are associated with it. There is also preservation of swallowing along with speech and cough reflex which is beneficial for the patient. There is improvement in the exchange of gas and reduction in the word of breathing through resting of the muscles of respiration.

Candidates for positive pressure ventilation are all the patients with respiratory failure irrespective of the type of respiratory failure and it's type. Patients with acute exacerbation of chronic obstructive pulmonary disease, acute pulmonary edema, pneumonia and exacerbation of bronchial asthma, cystic fibrosis or intrinsic lung disease can be managed with positive pressure ventilation.

Portable ventilators are utilized in order to provide continuous positive airway pressure (CPAP) or BIPAP. CPAP is used to deliver a pressure which is constantly set during inspiration as well as expiration which leads to an increase in the functional residual capacity resulting in improvement of oxygenation, however, it is not strictly a form of ventilatory assistance. Contraindication for the use of positive pressure ventilation is uncooperative patient, patient having a copious amount of secretions where airway protection is not possible, patient with unstable hemodynamics and patients with decreased mental state.

BIPAP is used to provide positive airway pressure in a manner which is biphasic. There is an inspiratory positive airway pressure (IPAP) which is set for inspiration and a lower expiratory positive airway pressure (EPAP) which is set for expiration. Difference obtained from subtracting EPAP from IPAP yields the degree of ventilatory assistance.

EPAP provides a dual benefit by ensuring proper flow in order to flush carbon dioxide from the single tube of ventilator and avoiding rebreathing along with increasing functional residual capacity and opening up the upper airway to prevent apnea as well as hypopnea. It also counterbalances the intrinsic positive end expiratory pressure in patients suffering from chronic obstructive pulmonary disease.

## **2.2 Initiation of NIPPV**

A portable ventilator can be used to initiate NIPPV. First, there needs to be setting up of volume targeted strategy and the tidal volumes need to be higher than in invasive ventilation. A tidal volume of 10 to 15 cc/kg is used. This can compensate for the leak of air through the mouth as well as around the mask. Respiratory rate can be decided and chosen as in standard ventilation. Adequacy of ventilation/oxygenation should be checked through the means of arterial blood gas. Tidal volume or respiratory rate can be increased if the minute ventilation needs to be increased. Similarly,

## *Non-Invasive Ventilation in Acute Hypoxemic Respiratory Failure DOI: http://dx.doi.org/10.5772/intechopen.104720*

in an over ventilated lung, respiratory rate or tidal volume may be decreased. Oxygen supplementation is provided in line with the circuit.

BiPAP uses a pressure targeted strategy for ventilation. Inspiratory pressure or IPAP can be chosen from 8 to 20 cm H2O pressure. It can be thought of as pressure support. As the pressure increases it will be more uncomfortable for the patient. Generally, BiPAP is started between a range of 8 to 11 cm H2O.Expiratory pressure or EPAP is set at 3 to 5 cm H2O.It can be thought of as PEEP. Difference obtained from subtracting EPAP from IPAP is the amount of support being provided to the patient. In case the patient required further ventilation, IPAP level can be increased gradually. A back up rate can be set by the ventilator rate which can be chosen as a value below the patient's spontaneous rate to assure that the patient does not develop apnea. A higher ventilator rate may be chosen in order to prevent periods of prolonged apnea and in order to allow resting of the respiratory muscles. If there needs to be improvement in oxygenation, amount of oxygen can be increased in the circuit or EPAP level may be increased. An Increase in EPAP level leads to decrease in tidal volume. To counter this, IPAP level can be increased in the same increment as the increase in EPAP.

Before Weaning one must consider to check if the patient has improved oxygen saturation at low flow oxygen rate, respiratory rate of below twenty four breaths per minute and ensure that there is interruption of positive pressure ventilation for short duration of time to ensure talking, eating, drinking and assess tolerance and gradually increase these pauses.

Drawbacks in the use of positive pressure ventilation are difficulties arising from the discomfort of the mask, headgear or straps and air leaks. Patients also complain of nasal pain, erythema or breakdown of skin due to use of mask. There can also be nasal congestion or dryness as well as ulceration of the nasal bridge with long duration of mask usage. Eye irritation due to air leak blowing into the eyes, gastric distention and aspiration are other few problems encountered in the use of positive pressure ventilation.

It is important to remember that NIPPV when initiated can induce anxiety in the patients making them uncomfortable. In order to make the patients acclimatized to the technique of NIPPV the patients require a 1:1 assistance by respiratory therapist who also makes fine adjustments in the flow rates and pressures depending on the requirement of the patient. On an average it may take about an hour for patient to become comfortable with NIPPV. IT is crucial to monitor the respiratory rate, heart rate as well as arterial blood gas to detect the effectiveness of NIPPV in correcting acute respiratory failure. At any point, if the patient deteriorated on NIPPV, conversion to endotracheal tube in order to ensure proper oxygenation should be considered.
