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In this post, we’ll be talking about breathing circuits.  It’s not uncommon for us to get a call from a panicked customer who needs circuits because they are out, they need a specialty circuit, they need to conform to a new Infection Control guideline, or they’re simply furious with the current circuit supplier.  Regardless of the reason,   not just any old circuit will suffice.  In many cases, not just any circuit will do!

Basic Use

The purpose of a basic circle-system circuit is to create a closed pathway from the anesthesia gas machine to the patient and back to the machine.  The machine cycles pressure and volume of a combination of oxygen, anesthetic, and the patient’s re-breathed gas, which ventilates and maintains a level of sedation during surgery.  It is critical that the circuit does not leak and properly protect the patient from potential infectious pathogens that may be present in anesthesia machine.

Components of a breathing circuit

The basic circle system circuit consists of:

Limbs: Single (limb within a limb), Double, corrugated or non-expandable, expandable.  Regardless of limb configuration, there are two- inspiratory (delivers inspired gas) and expiratory (retrieves expired gas).  

In a Single Limb Configuration, the Inspiratory Limb is inside the Expiratory limb.  The reason is that the warm expired gas heats the incoming cold Inspired gas.  A Single Limb circuit comes in expandable and, more typically, non expandable.  When evaluating a single limb circuit, insure that the inner tube does not disconnect from each end of circuit.  This caused what’s called ‘dead space’; our anecdotal experience tells us that some anesthesiologists and CRNAs are particular about this point, others are not. 

In a Double, or Dual Limb configuration, there is a dedicated tube for the inspiratory and one for the inspiratory.  The can be non-expandable or expandable.  When evaluating a single limb circuit, check for leakage.  Leakage occurs from pinholes that are a consequence of deficiencies in the manufacturing process.  Leakage is a problem for two reasons; it releases anesthetic gas into the operating room and it may cause the anesthesia machine to alarm due to having mismatches in the inspired vs. expired gas volume.  The last thing an anesthetist needs is a distraction from a bogus alarm.

Limbs come in different sizes and lengths.  There are pediatric and adult limbs that are from 60 inches up to 108 inches and longer. 

Wyes and Elbows: The end of the circuit that actually attaches to the patient is called the elbow.  The part that connects to the elbow to the dual limbs is called a wye.  A wye can swivel or be fixed; a swivel wye helps relieve stress on the endotracheal tube and subsequently, the patient’s airway.   A single limb circuit doesn’t have a wye.  

Gas sampling port: CO2 is sampled in order to verify that the patient is processing oxygen.  The CRNA or Anesthesiologist relies on the accuracy this reading to manage the ventilation of the patient; particularly in infants, pediatrics, and patients with low lung capacity.  Typically, the clinician samples at the elbow because it is closest to the patient.  Therefore, there will be a gas sampling port, on the elbow.  In pediatric circuits, clinicians prefer to have a port on the elbow as well as the wye.  This gives the option of removing the elbow but still allowing for sampling.  When evaluating a circuit, insure that it comes with a Gas Sampling Line (a tube that connects the circuit to the monitor that reads CO2).  These tubes have male or female fittings so be sure to choose the one that matches the monitor.

Filters: Most circuits come with a filter; some with two and still others with three.  The filter configuration is designed to protect the patient from anything that may be in the anesthesia machine and/or to protect the machine from anything from the patient.  With a single filter, the clinician attaches the filtered limb to the inspiratory port of the machine.  This protects the patient from inspiring contaminants.   Sometimes, the limbs are reversed and for this reason, some clinicians prefer two filters to protect the patient in case this reversal happens.

Heat/Moister Exchanger (HME):  This item is typically placed between the Wye (if there is one) and the elbow.  Its purpose is to collect moisture and heat as the patient expires and warms the gas as the patient inspires.  There are low resistance flow HMEs, mini-HMEs and HMEs that also contain a filter (HMEF).  Customers who use the mini-HMEs like the fact that they are lightweight.  The trade-off is that the resistance is a little higher than the larger, bulkier HME. 

Breathing Bag: A bag is a bag, right?  First, the bag should be latex free.   Non-latex bags are typically blue.  All green bags are latex.  Some manufactures offer colored bags, particularly in pediatric sizes.  The bag has a particular tactile feel that allows the CRNA and Anesthesiologist to feel the patient’s respiration.  Some are more particular about this feel than others.  All of them are sensitive, though, when a bag does not stay connected to the machine.  Some bags come with a plastic grommet that creates a very secure connection to the machine. 

Cuffs: The cuffs are at the machine end of the circuit.  Good quality plastic cuffs insure a secure fit.  Circuits without these cuffs are prone to sliding off the machine. 



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