Artificial manual breathing unit (“Ambu”) bags are seen throughout healthcare to facilitate mask ventilation or ventilation through an indwelling endotracheal/tracheostomy tube.
Pictured is an adult Ambu® SPUR® II resuscitator bag. Based on the manufacturer’s specifications, this Ambu bag has a resuscitator (“bag”) volume of ~1547 mL (~600 mL delivered with one hand, ~1000 mL delivered with two hands), a pressure-limiting valve of 40 cm H2O, and a reservoir volume of ~2,600 mL. The oxygen reservoir fills the resuscitator bag. Therefore, if the oxygen flow rate cannot keep the reservoir full due to high minute ventilation, the reservoir bag will empty, delivering less oxygen.
Let’s look back at those volumes. How often have you been in trauma, codes, or patient transports where an Ambu bag is forcefully squeezed rapidly? How often are we delivering more than a liter of tidal volume to our patients? Unlike a ventilator where expired tidal volume can actually be measured, we’re “guesstimating” Ambu volumes based on the degree to which we squeeze the bag. In patients who are under-resuscitated or tenuous from a hemodynamic standpoint, the additional intrathoracic pressure may cause further derangements.
Often when I’m inducing and intubating unstable patients, I’ll start with “ZEEP” (zero PEEP), be mindful of how often and to what extent I’m squeezing the bag, and slowly dial up intrathoracic pressure as tolerated.