# Acute Respiratory Failure and Mechanical Ventilation Across Ages

## Introduction

Acute respiratory failure is among the most common reasons for ICU admission in both pediatric and adult populations. The med-peds physician must understand the **physiologic differences** in respiratory mechanics across ages, recognize respiratory failure early, and be competent in initiating and managing **mechanical ventilation** in neonates, children, and adults.

## Definitions

**Type 1 (Hypoxemic)**: PaO2 <60 mmHg on room air; failure of oxygenation. **Type 2 (Hypercapnic)**: PaCO2 >50 mmHg; failure of ventilation. **Type 3 (Perioperative)**: Atelectasis-related. **Type 4 (Shock-related)**: Hypoperfusion of respiratory muscles.

## Age-Specific Respiratory Physiology

### Neonates and Infants
Higher metabolic rate and oxygen consumption per kilogram; **Smaller functional residual capacity (FRC)** relative to closing capacity; Compliant chest wall with less elastic recoil; Obligate nasal breathers (neonates); Diaphragm-dependent breathing with fatigue-prone type II muscle fibers; Desaturate rapidly due to lower oxygen reserve.

### Children
Transition to more adult-like physiology by school age. Smaller airways increase resistance (resistance proportional to 1/r^4) Lower airway disease (bronchiolitis, croup) is a frequent cause of respiratory failure.

### Adults
Greater respiratory reserve and FRC. COPD, pneumonia, ARDS are leading causes. Comorbidities (obesity, heart failure) compound respiratory failure.

## Common Etiologies by Age

| Age Group | Common Causes |
|---|---|
| Neonates | RDS, meconium aspiration, TTN, congenital anomalies |
| Infants | Bronchiolitis, pertussis, foreign body aspiration |
| Children | Asthma, pneumonia, ARDS, neuromuscular disease |
| Adults | COPD exacerbation, pneumonia, ARDS, PE, heart failure |

![Illustration comparing pediatric vs adult airway anatomy](images/pediatric-adult-airway.jpg)

## Assessment and Recognition

### Clinical Signs of Impending Respiratory Failure
Tachypnea (age-appropriate rates); Accessory muscle use: nasal flaring, intercostal/subcostal retractions, head bobbing (infants) Grunting (neonates and infants); Altered mental status (agitation progressing to lethargy); Cyanosis (late sign); Declining SpO2 despite supplemental oxygen.

### Arterial Blood Gas Interpretation
Assess oxygenation (PaO2), ventilation (PaCO2), and acid-base status. Calculate **P/F ratio** (PaO2/FiO2): <300 = ALI; <200 = ARDS; <100 = severe ARDS. **Oxygenation index (OI)** used in pediatrics: (FiO2 x MAP x 100) / PaO2.

## Non-Invasive Support

### High-Flow Nasal Cannula (HFNC)
Provides heated, humidified oxygen at high flow rates. Generates modest positive pressure; washes out dead space. Increasingly used as first-line in bronchiolitis and adult hypoxemic failure. Pediatric flows: 1-2 L/kg/min; Adult flows: 30-60 L/min.

### Non-Invasive Positive Pressure Ventilation (NIPPV)
**CPAP**: Continuous positive airway pressure; splints airways, improves FRC. **BiPAP**: Adds inspiratory pressure support for work of breathing. Effective in COPD exacerbation, cardiogenic pulmonary edema, and pediatric respiratory distress. Contraindicated in facial trauma, inability to protect airway, hemodynamic instability.

## Invasive Mechanical Ventilation

### Indications for Intubation
Failure of non-invasive support; Inability to protect airway (GCS 8 or less); Severe respiratory acidosis or refractory hypoxemia; Anticipated clinical course (e.g., burn airway, impending deterioration).

### Endotracheal Tube Selection
**Neonates/Infants**: Uncuffed ETT traditionally used; cuffed tubes increasingly accepted. Tube size: (age/4) + 3.5 (uncuffed) or (age/4) + 3 (cuffed) **Adults**: Cuffed ETT; 7.0-7.5 mm for women, 7.5-8.0 mm for men.

### Initial Ventilator Settings

| Parameter | Pediatric | Adult |
|---|---|---|
| Mode | Pressure control or SIMV+PS | Volume control (AC) or SIMV+PS |
| Tidal volume | 6-8 mL/kg IBW | 6-8 mL/kg IBW |
| Rate | Age-appropriate (neonate 30-40; child 15-25) | 12-20 breaths/min |
| PEEP | 5-8 cmH2O (titrate) | 5-10 cmH2O (titrate per ARDSNet) |
| FiO2 | Start 100%, wean to SpO2 target | Start 100%, wean rapidly |

![Table of initial ventilator settings by age group](images/ventilator-settings-by-age.jpg)

## Lung-Protective Ventilation

### ARDS Management
**Low tidal volume ventilation**: 6 mL/kg ideal body weight (ARDSNet protocol) **Plateau pressure** <30 cmH2O. **Driving pressure** <15 cmH2O (Pplat - PEEP) **PEEP titration**: Use ARDSNet PEEP/FiO2 tables. **Prone positioning**: Improves oxygenation and mortality in moderate-severe ARDS (PROSEVA trial); also used in pediatric ARDS. **Permissive hypercapnia**: Accept elevated PaCO2 to avoid ventilator-induced lung injury.

### Pediatric ARDS (PALICC Guidelines)
Similar lung-protective principles apply; OI used for severity stratification; Tidal volume 5-8 mL/kg; exhaled tidal volume monitoring critical in smaller patients. ECMO considered for refractory cases.

## Weaning and Extubation

### Readiness Assessment
Improvement or resolution of underlying cause; Adequate oxygenation on FiO2 40% or less and PEEP 8 or less; Hemodynamic stability without high-dose vasopressors; Intact cough and gag reflex.

### Spontaneous Breathing Trial (SBT)
T-piece or pressure support (5-8 cmH2O) for 30-120 minutes. Monitor for tachypnea, tachycardia, desaturation, accessory muscle use. **Cuff leak test** in pediatrics to assess for post-extubation stridor.

### Post-Extubation
Plan for potential reintubation. Consider dexamethasone pre-treatment to reduce laryngeal edema (especially in children) HFNC or NIPPV as bridge in high-risk patients.

![Algorithm for ventilator weaning and extubation readiness](images/extubation-algorithm.jpg)

## Clinical Pearls

Infants and neonates desaturate faster than adults due to lower FRC and higher oxygen consumption; preoxygenation is critical before intubation. Low tidal volume ventilation (6 mL/kg IBW) is the standard of care for ALL ventilated patients with ARDS, regardless of age. Prone positioning reduces mortality in moderate-to-severe ARDS and should be initiated early. In pediatric patients, pressure-control modes may be preferred for better compensation of air leaks around uncuffed tubes. The med-peds physician managing ventilators on both pediatric and adult services must remember to adjust all parameters for ideal body weight and age-appropriate physiology.

## References

1. Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and ARDS. *N Engl J Med*. 2000;342(18):1301-1308.
2. Guerin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). *N Engl J Med*. 2013;368(23):2159-2168.
3. Khemani RG, Smith LS, Zimmerman JJ, et al. Pediatric acute respiratory distress syndrome: Definition, incidence, and epidemiology (PALICC). *Pediatr Crit Care Med*. 2015;16(5 Suppl 1):S99-S117.
4. Rochwerg B, Brochard L, Elliott MW, et al. Official ERS/ATS clinical practice guidelines: Noninvasive ventilation for acute respiratory failure. *Eur Respir J*. 2017;50(2):1602426.