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Hypoventilation

Hypoventilation can be estimated by observing respiratory rate and depth (very subjective) and can be quantified using capnometry. Hypoventilation can cause hypercarbia, with subsequent respiratory acidosis, and hypoxemia. Thus, hypoventilation should be corrected.

ETCO2 is ~35–45 mm Hg in awake patients and ~40– 50 (up to 55) mm Hg in patients in an appropriate surgical plane of anesthesia. To correct increasing CO2, first ensure that the cause is not excessive anesthetic depth by checking the vaporizer setting and evaluating indicators of the patient’s anesthetic plane. Initiate PPV if ETCO2 is >60 mm Hg (hypercapnia). The anesthetist can deliver breaths by manually squeezing the reservoir bag while occluding the adjustable pressure limiting valve, taking great care to not leave the valve closed except when delivering a breath. A safety pop-off relief valve will prevent this complication. 

Bain non-rebreathing circuit (NRC) adapter with in-circuit manometer (white arrow) and safety pop-off valve (black arrow).  Courtesy of Heidi Shafford, published in the Journal of Feline Medicine and Surgery, 2018;20:602-34
Bain non-rebreathing circuit (NRC) adapter with in-circuit manometer (white arrow) and safety pop-off valve (black arrow).  Courtesy of Heidi Shafford, published in the Journal of Feline Medicine and Surgery, 2018;20:602-34

 

A mechanical ventilator can be used if the anesthetist is knowledgeable and comfortable with ventilator use.  Prior to instituting PPV, the hemodynamic status of the patient should be stable, if possible, as PPV can negatively affect cardiac output through impaired venous return. Recheck BP after starting PPV. If BP declines, decrease peak airway pressure and consider a fluid bolus (5–20 mL/kg) if there is the potential that the patient is hypovolemic. PPV can also cause barotrauma, so ventilator settings should start conservatively and be adjusted based on ETCO2.

If hypercapnia persists, investigate causes of increased inspired CO2 including excessive dead space, exhausted CO2 absorbent or one-way valves not functioning properly in an RC, or inadequate oxygen flow in an NRC. If machine malfunction is suspected, it may be prudent to quickly replace the machine with a different machine.

These guidelines are supported by generous educational grants from IDEXX Laboratories, Inc., Midmark, and Zoetis Petcare.