Exhausting Severe Asthma with a Deceptively Normal PaCO2

The gas is a warning, not a reassurance. A patient who has been working this hard for hours should be hyperventilating, with a PaCO2 of 25–35 mmHg. A PaCO2 of 44 mmHg with a pH of 7.34 in this clinical setting means she can no longer sustain the minute ventilation needed to keep her CO2 down. It is a 'normal' number produced by a failing respiratory pump. The PaO2 of 58 on 15 L reflects shunt and ventilation-perfusion mismatch, and the lactate reflects the work of breathing and possible salbutamol-driven glycolysis.

Add the clinical trajectory: a silent chest means airflow is too low to generate wheeze; a falling respiratory rate from 34 to 22 in a drowsy, exhausted patient is pre-arrest physiology, not improvement. The combination - silent chest, rising CO2, drowsiness, decreasing effort - is the classic picture immediately preceding hypoxic cardiac arrest in asthma.

Decision: intubate now, electively, while she still has a circulation. Waiting for the anaesthetist to arrive, or for a further dose of nebulised bronchodilator to work, risks a crash intubation in a hypoxic patient in a peri-arrest state, which carries a far higher mortality than a controlled rapid sequence induction.

Practical steps while waiting for the anaesthetist (who should be called immediately):

• Move to resus, full monitoring, capnography, difficult airway trolley, suction, two operators, and a plan for failed intubation including a supraglottic airway and front-of-neck access.
• Continue back-to-back salbutamol and ipratropium during preparation, keep magnesium running, and give intravenous hydrocortisone.
• Pre-oxygenate carefully; do not let the patient lie flat if that worsens distress.
• Do not use non-invasive ventilation as a substitute in an exhausted, drowsy patient - it delays the inevitable and can cause gastric distension and aspiration.

Preparation. Resuscitation bay, monitoring, capnography, suction, two skilled operators, drugs drawn up, and a pre-declared failed-airway plan. Give a 250–500 mL crystalloid bolus before induction: positive pressure ventilation plus auto-PEEP reduces venous return, and the asthmatic patient is often already volume-depleted.

Drugs. Ketamine 1–2 mg/kg is a good induction agent here - it provides bronchodilation and supports blood pressure; a small dose of midazolam may be added for an awake, agitated patient but increases hypotension risk. Rocuronium 1.2 mg/kg gives reliable, rapid paralysis and a long enough window to manage a difficult airway. Avoid morphine and atracurium (histamine release worsens bronchospasm).

Technique. Preoxygenate, then a controlled rapid sequence with the head elevated if tolerated. Use a 7.5 mm cuffed tube, confirm with waveform capnography, and secure early. Ventilate by hand slowly for the first minute, allowing full exhalation - the commonest cause of post-intubation collapse here is breath stacking, not a missed tube.

Ventilator settings and the reasoning behind each:

• Respiratory rate 8–10/min: a low rate allows long expiratory time and prevents air trapping.
• Tidal volume 6–8 mL/kg predicted body weight: small volumes limit plateau pressure and barotrauma.
• High inspiratory flow (80–100 L/min) and an inspiratory:expiratory ratio of 1:4 to 1:5: maximises expiratory time.
• Low PEEP (0–5 cmH2O) initially: high PEEP worsens hyperinflation in airflow obstruction.
• Plateau pressure below 30 cmH2O, and permissive hypercapnia down to a pH of about 7.15: accepting a high CO2 is safer than over-ventilating a stiff, obstructed lung.
• Continue salbutamol through the circuit, magnesium, steroids, and consider a ketamine infusion with paralysis for 24 hours.

Watch specifically for hypotension, rising peak pressures (pneumothorax, mucus plugging, tube obstruction) and loss of the capnography trace.

Think auto-PEEP first, then the mechanical causes.

1. Disconnect the circuit for 30–60 seconds and allow the chest to empty. A fall in blood pressure with a rise in airway pressure, in a ventilated asthmatic, is dynamic hyperinflation until proven otherwise; the delay in venous return is the mechanism of collapse. The blood pressure usually improves as the chest empties.
2. Reduce the respiratory rate and tidal volume and lengthen expiration; check the inspiratory:expiratory ratio. Hand-ventilate slowly if unsure.
3. Assess for pneumothorax: unilateral chest movement, tracheal shift, distended neck veins, loss of unilateral breath sounds, ultrasound (absent lung sliding) or immediate needle decompression if this is a tension pneumothorax after positive pressure.
4. Check the tube: position, patency, capnography waveform, suction for a mucus plug. A blocked or oesophageal tube mimics all of this.
5. Give fluid (250–500 mL) and start a vasopressor - noradrenaline is reasonable; if bronchospasm is severe and the pressure remains low, an adrenaline infusion combines bronchodilation with vasopressor support.
6. Sedation/paralysis: ensure adequate depth; coughing and straining against the ventilator worsens trapping. Ketamine infusion plus continued paralysis is often needed.
7. Continue bronchodilators through the circuit and consider intravenous salbutamol if available; magnesium and steroids continue.
8. Call ICU early. This patient needs a period of controlled hypoventilation and paralysis, and the most dangerous window is the first hour after intubation.

Document the sequence and the response, because these patients frequently deteriorate again during the next 24 hours as secretions and bronchospasm recur.