Pediatric Acute Respiratory Distress Syndrome (PARDS)

Table of content

Definition and criteria

Pediatric ARDS (PARDS) definition

Pediatric Acute Lung Injury Consensus Conference (PALICC) has provided the first pediatric-specific definition for ARDS.

Berlins definition

This is an update of the AECC definition and similar to the PARDS definition, In fact, PARDS is a Berlins modification for children. The only difference is, that it uses PaO2/FiO2 ratio for classifying mild, moderate, and severe ARDS in place of OI and SF ratio as in PARDS definition.

AECC definition

This is the older definition, that needs a mention to complete the list, you can also see how the criteria to define ARDS evolve and compare them. There was no pediatric definition at this time. How tragic?

AECC (American European Consensus Criteria) criteria for diagnosis of ALI/ARDS comprises of

1. Acute onset. (No defined period)
2. Severe arterial hypoxemia resistant to oxygen therapy alone (PaO2/FIO2  ratio ≤200 for ARDS and ≤300 for ALI). (Two different names, just to add to the confusion)
3. Diffuse pulmonary inflammation (bilateral infiltrates on chest radiograph) (non-specific)
4. No evidence of left atrial hypertension.

The Murray Lung Injury Score for ARDS

Murray Lung Injury Score utilizes lung compliance, PaO2/FiO2 ratio, degree of alveolar consolidation, and level of positive end-expiratory pressure for defining ARDS.

Etiology

Direct Injury	Indirect Injury
Common	Common
Pneumonia	Sepsis
Aspiration pneumonia	Severe trauma
	Multiple transfusions of blood products
Uncommon	Uncommon
Inhalational injury	Acute pancreatitis
Pulmonary contusion	Drug overdose
Near drowning	DIC
Reperfusion injury	Burns
	Head injury

Pathophysiology of ARDS in children

ARDS Pathophysiology: Source 1

Stages 

During the course of injury, lung parenchyma usualy goes through following 3 phases

• a. Exudative phase
• b. Proliferative phase
• c. Fibrotic phase

Therapeutic strategies in ARDS

Definitive care

1. Control of causative factors (sepsis, shock, etc.)

Ventilation

1. Non-invasive ventilation in mild cases
2. Mechanical ventilation
   1. Controlled oxygen exposure (FiO2)
   2. Avoiding volutrauma (Low Tv), barotrauma and atelectorauma (appropriate PEEP)
3. Non-conventional ventilation
   1. High-frequency ventilation
   2. Liquid ventilation (just to complete the list)
4. Proning
5. Combination of proning with mechanical ventilation
6. ECMO

Drug-based therapies

1. Nitric oxide
2. Surfactant
3. Corticosteroids and other anti-inflammatory agents

Non-respiratory support

1. Avoiding fluid overload
2. Supportive therapy
3. Analgesia and sedation
4. Nutrition

Psychosocial support

Management

Control of causative factor

1. Identification of the trigger source and source control is the first important step in the management of ARDS.
2. Sepsis being a common trigger for ALI/ARDS, early antibiotic therapy is recommended in suspected infection.
3. Intravascular volume expansion with crystalloids and vasopressors is used to manage patients with shock in the initial stages.

Respiratory Support

Respiratory support for patients with ALI/ARDS ranges from supplemental oxygen to assisted ventilatory support.

Respiratory support is complicated by ventilator-induced lung injury resulting from

• Alveolar over-distention (volutrauma).
• Barotrauma due to low lung compliance and high ventilatory pressures
• Repeated alveolar collapse and re-expansion (atelectotrauma),
• and oxygen toxicity.

The goal of ventilating patients with ALI/ARDS is to maintain adequate gas exchange with minimal ventilator-induced lung injury.

A. Oxygenation strategies

For children with ALI/ARDS PaO2 of 60 to 80 mmHg is usually considered safe.

High FiO2 should be avoided to minimize the risk of direct cellular toxicity and avoid reabsorption atelectasis.

Decrease FiO2 below 0.6 as soon as possible.

B. Ventilation strategies

Permissive hypercapnia during mechanical ventilation has led to a significant decrease in ARDS mortality.

Target arterial pH levels in children with ALI/ARDS are anything above 7.2

C. Tidal Volume (Tv)

• Target Tv to <6 mL/Kg and plateau pressure <30 cmH2O. 
• Combining lower tidal volumes with higher PEEP improves outcomes.

D. Positive End-Respiratory Pressure (PEEP)

Current clinical practice in absence of routine static PV (pressure-volume) curve measurement is to keep PEEP between 8 cm H2O and 20 cm H2O.

PEEP should be progressively increased by 2–3 cm H2O increments to maintain saturation between 90 and 95% with FiO2<0.6.

The child should be monitored for any evidence of cardiovascular compromise and hyperinflation.

If facilities for monitoring pressure-volume loops are available, then it is desirable to keep the PEEP above the lower inflection point.

Here is more on lover and upper inflection point/pressure-volume loop

E. Inspiratory Time

1. Inspiratory time depends on the time constant of alveoli. In ARDS this increases, hence an optimal Ti is necessary, Low Ti will affect oxygenation.
2. This can be increased further to keep the IE ratio of at least.
3. When Ti is increased more than Te to improve oxygenation, it is called inverse ratio ventilation. However, this is not used routinely since it may cause Co2 retention.

F. Mode of Ventilation

1. It is the optimal ventilator settings and monitoring that is important than a particular mode of ventilation. In most cases, pressure-controlled ventilation is used.
2. The preferred mode is a time-cycled, pressure regulated, volume-controlled mode to limit plateau pressure to ≤30 cm H2O.

G. Recruitment Maneuvers

• Recruitment maneuver in a child who desaturates either due to worsening lung condition or after disconnection of a circuit for suctioning can be tried is tried.
• In this, the PEEP is incrementally increased to very high levels for a short period and then brought down at a point of maximum saturation, tidal volume, etc.
• Before starting the maneuver rule out AIR LEAK.

H. Weaning

1. During weaning, the ventilator’s contribution to total ventilation is gradually reduced.
2. In volume-controlled ventilation, the VT is usually reduced to about 4–6 ml/kg.
3. In pressure-controlled ventilation, the PIP is gradually reduced in steps of 1–2 cm H2O.
4. PEEP and FiO2 are reduced while monitoring the saturations and PaO2.
5. As the amount of mandatory support reduces, PSV can eventually replace the controlled mode of ventilation.

I. Extubation

Extubation is planned once the child’s respiratory condition improves allowing a decrease in ventilator settings to minimal- FiO2 of less than 40%, PEEP of 4–5 cm H2O, PIP of less than 15 cm H2O.

The child is hemodynamically stable and the sensorium is normal/near normal with the presence of protective reflexes.

Dexamethasone may be used prior to extubation in the case of prolonged ventilation to reduce airway edema.

J. High-Frequency Ventilation

Indication

1. Children who fail to improve on conventional ventilation or deteriorate.
2. HFV is particularly useful in children with air leaks- pneumothorax, bronchopleural fistulae.

Mechanism

High-frequency oscillatory ventilation uses very-low tidal volumes at a very high rate (frequency is the term used) and laminar airflow to protect the lung.

It was found that high-frequency oscillatory ventilation is associated with higher mean airway pressures, improved oxygenation, and a reduced need for supplemental oxygen.

Other Therapies to Improve Oxygenation

Prone position

Over the recent few years, proning or prone positioning has gained a lot of attention in pediatric ICUs. Proning combined many mechanisms, to improve oxygenation and ventilation without causing additional barotrauma or volutrauma. Consider prone positioning in children who do not respond to conventional ventilation strategies.

ECMO

Extracorporeal membrane oxygenation is increasingly used where conventional ventilatory support and proning fail.

Pharmacotherapy to improve ventilation in ARDS

Nitric Oxide

Inhaled nitric oxide is a potent pulmonary vasodilator and doses as low as 1 ppm can improve oxygenation in ALI/ARDS.

It may be used in patients for temporary rescue where hypoxemia is refractory to more conventional interventions.

Prostacyclin

Aerosolized prostacyclin may improve oxygenation in children with ALI/ARDS.

Surfactant

Surfactant has been tried since the ARDS pathophysiology involves a decrease in surfactant, however, there is no robust evidence to use it on a routine basis.

Corticosteroids

1. Corticosteroids decrease the production of a number of inflammatory and profibrotic mediators by many mechanisms.
2. However, the evidence is not robust enough for routine use.
3. They are useful in the early course of the disease.

Summary of respiratory support in a Pediatric ARDS

Non-respiratory Supportive Care

Fluid Administration

Restrict the fluid intake to about 2/3rd of maintenance once the child is hemodynamically stable.

Fluid restriction should only be implemented after children have been resuscitated adequately from septic shock.

Once hemodynamic stability is achieved, fluid administration should be limited in an effort to minimize the capillary leak and control pulmonary edema.

Blood Transfusion

Try to maintain hemoglobin >7 g/dL in children with shock or profound hypoxia.

Nutrition

Initiate enteral nutrition as early as possible.

In children who are unlikely to tolerate enteral feeds for a few days, parenteral nutrition is initiated, and Omega 3 fatty acid supplementation improves clinical outcomes.

Analgesic and Sedation

Analgesics and sedation should be used to minimize physical and mental discomfort.

The use of muscle relaxants in children with ALI/ARDS should be limited to difficult and asynchronised ventilation. Do not use muscle relaxants routinely.

Prevention of nosocomial infections

Prevention of VAP, central line infection, and UTI is very important to reduce mortality and morbidity.

Glycemic control

Monitor the glucose values regularly and consider the use of insulin if the values are persistently above target values.

Coagulopathy and mechanical ventilation are risk factors for clinically important gastrointestinal bleeding in children. Use stress ulcer prophylaxis with intravenous H2 antagonist or proton pump inhibitor.

Approach to case management of ARDS in children

Image attribution - Source1:Biologically Variable or Naturally Noisy Mechanical Ventilation Recruits Atelectatic Lung August 2000; American Journal of Respiratory and Critical Care Medicine 162(1):319-23 DOI:10.1164/ajrccm.162.1.9903120

Author

Ranjith C S. | DNB (Pediatrics), DM (Medical Oncology)

Ranjith has completed Pediatric Residency from Kanchi Kamakoti Childs Trust Hospital and further trained in Medical Oncology from JIPMER

Update

Ajay Agade | DNB(Pediatrics), FNB(Pediatric Intensive Care), Fellowship in Pediatric pulmonology and LTV

Ajay is a Paediatric Intensivist, currently working in Pediatric Pulmonology & LTV at Great Ormond Street Hospital NHS, London