An 82-year-old woman is mechanically ventilated for acute respiratory failure following acute intracerebral hemorrhage. Her FiO2 has been 30% with an arterial blood gas showing adequate ventilation and oxygenation for the last 24 hours (7.43/37/89/25). Suddenly, the ventilator alarms for low exhaled tidal volume. On bedside evaluation, her SpO2 is 84%, respiratory rate 20 breaths per minute, HR 124 beats per minute and blood pressure 105/65 mm Hg. Her ventilator graphics before and after the alarm are depicted below. What mode of mechanical ventilation is she receiving and what triggered the alarm? 

1. The mode is: 

(a) Pressure control

(b) Pressure support

(c) Volume control

(d) Volume SIMV 

2. The alarm was triggered by: 

(a) Acute pulmonary edema

(b) Endotracheal tube cuff leak

(c) Endotracheal tube dislodgement into the right mainstem

(d) Tension pneumothorax

(e) Mucus plugging of the endotracheal tube 

Answer

1. (a) Pressure control 

2. (e) Mucus plugging of the endotracheal tube 

The mode is pressure control as each breath is time-triggered, pressure targeted and time cycled. You can see that each breath is delivered at a constant pressure of 20 cm H20 with a constant inspiratory time. In a pressure control breath, the tidal volume will vary with changes in lung or chest wall compliance, variation in patient effort, changes in endotracheal tube or tracheobronchial tree resistance or development of a cuff leak. In this case, the tidal volume suddenly dropped from 400 mL to 200 mL. You can rule out a leak, because the expiratory limb of the volume tracing returns to zero baseline each breath. Neither of the breaths shown are patient-triggered, so variation in patient effort cannot explain the decrease in tidal volume. To determine whether the tidal volume drop is due to a reduction in compliance (e.g. pulmonary edema or pneumothorax) or an increase in resistance (e.g. mucus plug or bronchospasm), you must examine the expiratory flow wave tracing. Before the alarm fires, the patient has a rapid expiratory flow at a peak rate of over 40 L/min. After the alarm fires, you can see that the expiratory flow rate drops in half and has a slow return to baseline suggestive of a sudden increase in resistance. Furthermore, the inspiratory flow rate is also reduced from 40 L/min to 20 L/min, confirming a rise in resistance.   

So, what do you do next? 

First, increase the FiO2 on the ventilator to 100% to resolve her hypoxemia. Next, perform in-line suctioning to remove a mucus plug. If the exhaled minute ventilation remains low despite suctioning and there is no obstruction of the ETT, the next step would be to remove the patient from the ventilator and assist ventilation with a bag-valve mask. Other causes of a sudden increase in resistance that you can troubleshoot include a kink in the ventilator circuit, bronchospasm, fluid pooling in the circuit tubing or a foreign body. When the ventilator alarms, always remember to examine your patient and evaluate the ventilator graphics to quickly determine the problem. Identifying the mode of mechanical ventilation also helps, because changes in compliance or resistance affect pressure, flow and volume differently in volume control versus pressure control modes. If the patient above was receiving volume control ventilation, the tidal volume would have remained constant, but the increase in resistance would have caused a spike in peak inspiratory pressure without a change in plateau pressure.