1. Intensive Care Management of the Child with Acute Severe Asthma Dr Rachael Barber Co Lead Consultant North West & North Wales Paediatric Transport Service (NWTS)

2. Intensive Care Management of the Child with Acute Severe Asthma Pathophysiology of Severe Asthma Intubation - indications and difficulties Ventilation - difficulties and strategies Sedation and muscle relaxation Additional therapies

3. Inflammatory disease Lung mechanics Gas exchange Cardiovascular dynamics Metabolic effects Pathophysiology of Acute Severe Asthma

4. Three main components Airway inflammation and oedema Bronchial smooth muscle constriction Mucous plugging Asthma is primarily an inflammatory disease Episodic and variable obstruction of small and medium airways

5. Lung Mechanics Hyperinflation and air trapping Obstructed small airways causes premature airway closure with subsequent air trapping and hyperinflation Hypoxaemia Unequal distribution of affected areas leads to V/Q mismatch with increased shunting

6. Worsening airflow obstruction Severe airflow obstruction Incomplete exhalation Increased lung volume Expanded small airways Increased elastic recoil pressure Increased expiratory flow Decreased expiratory resistance Compensated hyperinflation and normocapnia Decompensated severe hyperinflation and hypercapnia

7. Dynamic Hyperinflation

8. Lung Mechanics Hyperinflation and air trapping Obstructed small airways causes premature airway closure with subsequent air trapping and hyperinflation Hypoxaemia Unequal distribution of affected areas leads to V/Q mismatch with increased shunting

9. Hypoxaemia and Shunting Hypoventilated areas of lung well perfused Hypoxic pulmonary vasoconstriction minimises extent of mismatch

10. Cardiovascular Dynamics Dynamic Hyperinflation High intrathoracic pressures High Lung Volumes Reduced venous return Increased Pulmonary vascular resistance Reduced cardiac output Reduced mixed venous saturations Increased RV afterload

11. Metabolic Effects V/Q mismatch Increased WOB Dehydration Hypoxia LactateKetones Metabolic Acidosis Iatrogenic

12. Indications for Intubation Cardiac or respiratory arrest Physical exhaustion Altered sensorium i.e. agitation, confusion, decreased conscious level Failure to maintain saturations with high flow oxygen Worsening respiratory acidosis despite treatment If in doubt seek advice from Paediatric Intensivist

13. Intubation High risk procedure Delayed gastric emptying High airway resistance Unable to achieve adequate pressures Patients often relatively dehydrated and hypovolaemic Patients usually in adrenergic state Most experienced operator Rapid sequence induction with cricoid Large endotracheal tube lowers resistance Cuffed ETT prevents leak Anticipate and rehydrate. Consider when giving anaesthetic agents. Relative hypotension is worrying sign of severe hyperinflation and impending cardiovascular collapse

14. Induction of Anaesthesia Drugs with bronchodilating effect: Propofol Ketamine - less vasodilation and cardiovascular effects Gaseous induction of anaesthesia Muscle relaxants Avoid atracurium as histamine release Sux/rocuronium for induction Maintenance whilst hypercapnoeic to prevent high respiratory drive

15. Anticipate difficulties Ventilation of the Asthmatic Child Difficulties Ventilation Strategies Complications

16. Principles of Ventilation of the Asthmatic Child Avoid dynamic hyperinflation and gas trapping Ensure oxygenation Permissive hypercapnia Ensure long expiratory time Consider manual decompression Paralyse and sedate initially Early and rapid wean once compliance improved

17. Asthma Morbidity Increased in mechanically ventilated patients

18. Modes of ventilation Allows for changes in lung compliance “Square wave” inspiratory flow Overdistension of compliant areas of lung Needs high flow rates to minimise I:E ratio Decelerating inspiratory flow More efficient at overcoming high resistance “Square wave” gas flow Use with set tidal volume Volume ControlPressure Control

19. High Inspiratory pressures Incomplete expirationIncomplete expiration Intrinsic PEEP Difficulties in Ventilation

20. High Inspiratory Pressures Airway obstruction generates very high resistance to airflow during inspiration May lead to development of barotrauma Pneumothorax, pneumomediastinum, subcutaneous emphysema Airway pressure dissipated along bronchial tree Alveolar pressures most predictive of barotrauma Associated with significant haemodynamic instability

21. High Inspiratory Pressures - Strategies Aim tidal volumes 5-7ml/kg Pressure ventilation with “square wave” Limit pressures P plat 30

22. High Inspiratory pressures Incomplete expiration Intrinsic PEEP Difficulties in Ventilation

23. Ventilator Flow-Time pressure cycles Incomplete expiration with gas trapping

24. Expiratory Pressures and Air Trapping Rigid obstruction to air flow slows expiration and leads to air trapping in alveolus at end of expiration Increase in respiratory rate shortens expiratory time preventing complete exhalation Forced expiratory effort increased pleural pressure compressing small airways

25. Incomplete Expiration - Strategies Long expiratory time Ensure complete expiration on flow loops Manual decompression if needed Permissive hypercapnoea

26. High Inspiratory pressures Incomplete expiration Intrinsic PEEP Difficulties in Ventilation

27. PEEP Intrinsic PEEP Difference between true alveolar end-expiratory pressure and airway pressure PEEPi creates positive pressure gradient from alveolus to mouth thereby increasing work of breathing for patient

28. Application of extrinsic PEEP Creates pressure equilibrium through airways reducing work of breathing for patient to near normal

29. Sedation and Muscle Relaxation Ketamine Midazolam Fentanyl Vecuronium Avoid morphine and atracurium as may promote histamine release Early wean of muscle relaxation once compliance improves to reduce risk of myopathy

30. Heliox Low density gas Pre-mixed - 80:20 Improves laminar flow Improves delivery of nebulised drugs Cochrane review showed no improvement in non-intubated patients

31. Inhaled Anaesthetic Agents All potent bronchodilators smooth muscle relaxation Case reports of use in status asthmaticus since 1930s Until recently difficulty with delivery in ICU AnaConDa device allows continuous delivery via ICU ventilator Modified HME filter with evaporator rod Dead space 100ml

32. Non-invasive Ventilation No strong supporting evidence Observational studies show improvement in clinical parameters and gas exchange Some evidence in use in A&E for moderate asthma May be poorly tolerated in young child

33. Case reports of use in patients Major air leak Cerebral compromise Novalung pumpless CO2 removal via percutaneous catheter ECMO

34. Maximal medical management Higher threshold for intubation and ventilation than other disorders Significant morbidity and mortality in ventilated asthmatics Permissive hypercapnia and other strategies to reduce complications Once compliance improves, rapid wean off ventilation Newer therapies Summary