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The Truth About Dead Space in Breathing Circuits in veterinary anesthesia
Comprehension of what exactly is dead space
A lot of confusion exists in veterinary practices as to the definition of dead space in a breathing circuit.
There are 2 types of dead space:
- Anatomical dead space
- Mechanical dead space
Nothing can be done about anatomical dead space, as this is the patient.
Mechanical dead space, however, must be kept to a minimum. The dead space is the space in the breathing circuit where both inhalation and exhalation passes through a common path. Because the patient breathes in and out of the same tube and he can possibly inhale some unfiltered (containing CO2) exhaled gas.
- The portion of the endotracheal tube which extends out of the trachea (from the mouth to the breathing circuit)
- The elbow on the breathing circuit
- Any connector used between the endo tube and the breathing circuit (ex: CO2 adaptors, apnea alarm adaptors etc)
- The Y piece at the end of a Y circuit
Y Circuit
The tubes on the Y circuit do not cause dead space because. The check valves on the absorber the inhalations of the patient passes through one tube and exhalation through the other. Using tubes with a smaller diameter "does not reduce dead space". This may actually "increase breathing resistance". However "shorter" tubes reduce breathing resistance.
Bain circuit
In addition to the dead space shown in the picture, if insufficient flow is used with a Bain tube then the patient will be re-breathing CO2. The fresh gas flow must be greater than the minute volume that the patient breathes otherwise there will be CO2 buildup and the difference between minute volume and fresh gas flow will essentially be additional dead space.
Universal F
The tubes on the Y circuit do not cause dead space because of the check valves on the absorber "the inhalations of the patient pass through one tube and exhalation through the other. Using tubes with a smaller diameter "does not reduce dead space". This may actually "increase breathing resistance". However "shorter" tubes reduce breathing resistance.
Coaxial Circuit
The Coaxial tube used on re-breathing circuits (not to be confused with Bain tubes) is similar to the Universal F but without the down side. It has the same breathing resistance as the standard Y (length for length). It has an attached inhalation tube and cannot be connected backwards. It was designed as an improvement to the Universal F.
