Asthma in the intensive care unit

Asthma in the Intensive Care Unit Sean M Caples Assistant Professor of Medicine College of Medicine Mayo Clinic Overview • Definitions • Pathophysiology • Differential Diagnosis • Clinical Features • Management Acute exacerbation of asthma Pathophysiology 15% develop sudden-onset ( 110 Resp rate > 25-30 Pulsus paradoxus > 25 mmHg Limited ability to speak – FEV1 < 50% predicted (or peak flow) The Response to β-agonists Shortcomings • Their presence or absence NOT predict outcomes • About half of those considered to have “lifethreatening attacks” were discharged from the emergency department • Severity may best be based upon outcomes rather than presentation Differential Diagnosis (Misdiagnosis: – 25% of admissions) • COPD • Cardiogenic pulmonary edema • Bronchiectasis • PE • Endobronchial / tracheal pathology • Pneumonia • Foreign body • Glottic dysfunction ICU Admissions • About 20 papers over 25 years • Criteria for admission rarely stated • Wide range (3 to 70%) reported on need for ventilatory assistance; estimated about one-third NPPV • A single randomized, placebo controlled trial (used sham NPPV mask set at IPAP / EEP 1); not fully blinded • Use of hours of Bi-level in the ED in those with severe asthma (FEV1 < 40%), in addition to standard therapy • Improved FEV1 and rate of hospitalization Soroksky, Chest, 2003 Invasive Mechanical Ventilation Absolute indications: mental status changes impending respiratory arrest Larger diameter tube preferred to minimize resistance to airflow (≥ 8.0) About 4% of hospital admissions Post-intubation hypotension • Common: at least one-third • Contributors – – – – Effects of sedation on vascular tone Hypovolemia Worsening hyperinflation Tension pneumothorax (barotrauma) Ventilator Settings Key Concepts • Peak airway pressures – a function of flow characteristics – likely to be elevated early – can be aggravated by high inspiratory flow rates, dried secretions in tube, dys-synchrony / biting • Plateau pressure measured with end-inspiratory breath hold • Threshold level (i.e < 30 to 35 cm H2O) not consistently correlated with outcomes • Give adequate exhalation time – Respiratory rate / minute ventilation, flow rates Peak pressure Plateau pressure Reasonably reliable in the non-paralyzed patient Auto-PEEP measured here Lower inspiratory flow rates (100 L/min to 40) decrease expiratory time causes increase in end expiratory volume (VEE) • No consensus on ventilator mode – AC vs SIMV – Probably best to avoid pressure control early since, with high airway pressures, minute ventilation will be erratic – Apply extrinsic PEEP during spontaneous modes to overcome intrinsic PEEP (not helpful during AC) • Risk-benefits of sedation / paralysis – Daily interruption • Low tidal volume ventilation (6 to ml/kg) • Permissive hypercapnia Permissive Hypercapnia • A “consequence” of low tidal volume ventilation • May have therapeutic role (anti-inflammatory, antioxidative) in research models • Slow rise in PaCO2 well tolerated Laffey et al, Lancet 1999 • Little data to support buffering (bicarbonate or THAM) but it is probably not uncommon • Theoretical risk of worsening CO2 with Bicarbonate HCO3- + H+ H2CO3 H2O + CO2 (THAM is a non-bicarbonate buffer) • Might avoid hypercapnia in brain injury and myocardial depression One proposed algorithm Corbridge and Corbridge Use of Bronchodilators with Ventilator • NO controlled trials to support recommendations • MV patients tend to have higher dose requirements (may relate to their disease or to technical considerations) • MDI use – – – – Activate close to circuit near patient Use a spacer Temporarily turn humidifier off Temporarily turn down flow rate to reduce turbulence Refractory cases • Ketamine – Sedative, analgesic, bronchodilator • General anesthetics – Halothane, enflurane – High risk; very short acting • Heliox [...]... 8.1% in those intubated • In the USA, minorities living in large cities are disproportionately at risk of morbidity and mortality Clinical Features • No sign or symptom is uniformly present • Wheezing absent 5% (a concerning finding) • Dyspnea absent up to 20% of the time Impaired ventilatory response to hypoxia associated with near-fatal cases P0.1 airway occlusion pressure 0.1 second after the start... With monotherapy, two-thirds of patients in Emerg Dept are discharged • 2.5mg nebulized every 20 minutes (can be given continuously) • Tremor and tachycardia usually mild • Subcutaneous epinephrine or terbutaline of little added benefit Additive effects of ipratropium bromide (anticholinergic) in more severe disease with prolonged symptoms The effects of ipratropium are not always replicated in other studies... – Increased oxygen consumption of respiratory muscles – Aggressive sympathomimetic use Management • Pharmacologic agents • Mechanical ventilation – NPPV – Invasive • Risks • Beside monitoring • Adjusting the knobs Oxygen • To maintain saturations >90% • Enhances oxygen delivery to peripheral tissues (including respiratory muscles) • Reverses hypoxic pulmonary vasoconstriction β-Agonists (Short-acting)... start of inspiration against an occluded airway Impaired perception of dyspnea associated with near-fatal cases Pathophysiology Increased airway resistance (non-uniform) Diminished flow Air-trapping / Hyperinflation Increased work of breathing Changes in elastic recoil (Muscle weakness / fatigue not common) • Mild hypoxemia due to V/Q mismatch • Slow to resolve • Marked hypoxemia is uncommon Hyperinflation... Corticosteroids • Conflicting results over whether these result in physiologic changes in first 6 hrs • May improve outcome (rates of hospitalization) when used early Cochrane Systematic Review: Reduces Hospitalization, especially in those with more severe disease, not already on steroids 9 Trials have compared dose of drug in severe asthma: No evidence for an Optimal (Or higher) Dose National Institutes of Health... methylpred, prednisolone) in 3 or 4 divided doses for 48 hours then 60 – 80mg/day until peak flows improve • Oral dosing probably as good as IV if no GI upset and intubation not planned • Inhaled corticosteroids may have some added benefit Theophylline / Methylxanthines • No positive impact on multiple outcomes (peak flow, hospitalization) dependent of use of steroids • May increase adverse effects:... hypoxemia is uncommon Hyperinflation • Quantitated only in small studies • Residual volume 400% normal • Functional residual capacity 200% normal • Total lung capacity slightly increased Hyperinflation Auto-PEEP – Increases inspiratory threshold for airflow – Decreased radius of curvature puts diaphragm at mechanical disadvantage – At some point, deflation no longer passive— accessory expiratory muscles... Cardiovascular Consequences • Decreased preload • Increased afterload • Pulsus paradoxicus Decreased cardiac output Blood Gases • Hypocapnia • Mild hypoxemia • Respiratory alkalosis Blood Gases • CO2 retention in about 10% – Modest: 10-15 mmHg over normal – Indicates FEV1 < 20% – May recover without intubation • Normocarbia: 15 to 20% cases – FEV1 20 to 30% – Impending respiratory failure Blood Gases • Metabolic... increase adverse effects: GI, tremor, arrhythmia • May be some benefit in children Magnesium Sulfate • Conflicting study results • May have bronchodilatory properties via effects on smooth muscle • IV MgSO4 (2 to 10gm) modestly improves spirometry in those with severe asthma but no impact found on admission rates • Adverse effects IV: flushing, hypotension Nebulized MgSO4 may have additive bronchodilatory