*4.1.2 Causes*

	- Airway disease- Chronic Obstructive Pulmonary Disease, Asthma, Cystic Fibrosis, Bronchiolitis Obliterans Syndrome
	- Interstitial lung disease- Interstitial Pulmonary Fibrosis, Sarcoidosis, Interstitial pneumonia e.g. Covid 19 pneumonia
	- Alveolar filling Pulmonary Edema, Left heart failure, Acute Lung Injury/ Acute Respiratory Distress Syndrome, Pneumonia, Trauma, Contusion, Alveolar hemorrhage/proteinosis, Transfusion Related Acute Lung Injury, Acute interstitial pneumonitis, Acute eosinophilic pneumonia, Bronchiolitis Obliterans Organizing Pneumonia/Cryptogenic Organizing Pneumonia, Aspiration, Upper airway obstruction, Near drowning

## *4.1.3 Evidences of NIV in acute hypoxemic respiratory failure*

Oxygen to improve hypoxia in acute hypoxic failure appears to be standard of care and in 2005 it was shown that NIV is better than oxygen in improving PaO2/ FiO2 by unloading of respiratory muscles [2]. In Earlier studies it was shown when used appropriately NIV is as effective as invasive mechanical ventilation in improving PaO2/FiO2 by Antonellii et al. thereby avoiding all complications related to endotracheal intubation and related ventilation [3]. But the most important aspect of success versus failure of NIV in acute hypoxic respiratory failure is 'Timing of initiation vis a vis progression of inciting disease as well as severity of respiratory failure" [4]. In Acute Hypoxic Respiratory Failure, NIV needs to be initiated for mild to moderate hypoxia and before the disease has progressed (a) as window of opportunity to use NIV is narrow (b) as once disease has progressed IMV is indicated.

Failure of NIV in acute hypoxic respiratory failure needs to be identified early to prevent higher mortality. Meta-analysis of NIV use in ALI/ARDS showed intubation rate of 46% (30–86%) and mortality of 35% (19–69%) and these widely variable results indicate that different diseases states causing hypoxic respiratory failure as well as baseline characteristics of individual patients along with threshold of intubation of the centre contribute to success or failure of NIV [5]. But studies are clear that milder is the hypoxia, lesser are the chances of failure indicating baseline PaO2 is one of the determinant of NIV outcome [6]. Further studies also showed earlier is the better as disease has potential for reversibility. Immunosuppressed patients also showed good success rates with use of NIV in acute hypoxic respiratory failure [7]. Hence, use of NIV in acute hypoxic respiratory failure is indicated as follows [8]:

Level 1: in acute cardiogenic pulmonary edema as well as immunocompromised patients with acute hypoxic respiratory failure.

Level 2: evidence is to use in post-operative hypoxic respiratory failure, COPD with community acquired pneumonia as well as to prevent hypoxic failure in acute severe asthma. However NIV in severe community acquired pneumonia without any underlying comorbidity to support use of NIV needs to be used with caution as in to prevent Extubation failure. Also in patients with do not intubate or resuscitate orders.

Level 3: in patients with thoracic trauma, upper respiratory tract obstruction, partial upper airway obstruction as well as treatment in acute severe asthma but with caution to use in severe ARDS.

Level 4: In very elderly (>75 years), obesity hypoventilation syndrome and in IPF with caution.

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

### **Figure 2.**

*Rationale of using NIV in acute hypoxic respiratory failure.*

### **Figure 3.**

*Showing modes of bi-level NIV used in hypoxic respiratory failure (spontaneous and spontaneous timed).*

Rationale of using NIV in acute hypoxic respiratory failure is that the primary or secondary lung failure, both lead to acute hypoxia which can be either due to abnormality in the exchange of gas or failure of the respiratory pump which leads to increase in minute ventilation, this is reversed by the use of NIV (**Figure 2**).

Modes of bi-level NIV used in hypoxic respiratory failure are defined by triggering: S-Spontaneous (a. patients efforts only triggering NIV), S/T- Spontaneous timed (b. patient's own as well as timed triggering to give back up RR in case patient has irregular and slow breathing pattern to safe guard adequate ventilation, misses a breath (**Figure 3**).

Helmet ventilation is the most secure form of NIV in severely hypoxic patients but claustrophobia may prohibit its use. It can be used in severe hypoxic respiratory failure and it's use in Covid 19 pneumonia and ARDS showed favorable results vis a vis oxygen therapy. However, cardiac instability is contraindication.

NIV is also used to support interventional procedures in patients with hypoxia while doing bronchoscopy or transesophageal Echocardiography.

However, in contrast to use of NIV in hypercapnic respiratory failure in hypoxic respiratory failure one needs to look beyond lungs into systemic component of disease process e.g. shock, acidosis, multi organ failure, etc. as these situations will favor invasive mechanical ventilation.

70% of failures of NIV occur within 48 hours, indicating need for intensive monitoring and identifying features of early failure proactively (**Figure 4**).

## **Figure 4.**

*Identify failing NIV early by bedside monitoring worsening of clinical features, gas exchange parameters and primary disease causing hypoxia.*

It has been shown that delaying intubation in such cases increases mortality and adds no benefit. Most common cause of failure is inability to correct hypoxia in 2/3rd of failed NIV's followed by intolerance, progression of disease with hemodynamic instability. Changing NIV machine to guaranteed volume mode (AVAP's or iVAP) can help to improve tidal volume and hypoxia. Hence all such cases should be managed in ICU settings only. Intolerance can be managed by using different types of masks or correcting into leaks. Use of sedation is strictly under observation with full facility and readiness to intubate.

NIV has penetrated deeply into the roots of medical management even in rural health facilities of India where a study conducted in rural India concluded that [9]:

