ABC... As Easy as 1-2-3? Was a Hit Song. A–F Changed Anesthesia Systems.
- Tim Keohane
- 18 hours ago
- 7 min read
AI Summary
This article explores how the alphabet became important in anesthesia long before The Jackson 5 made "ABC" famous. It tells the story of William Mapleson and his A–F classification of anesthesia breathing systems, explaining how different circuit designs affect fresh gas flow, resistance, and the patient's work of breathing. The article also compares rebreathing and non-rebreathing systems and introduces A.M. Bickford's PUSH framework as a simple way to evaluate which system may be best suited for a particular patient.
ABC... As Easy as 1-2-3? Was a Hit Song. A–F Changed Anesthesia Systems.
In 1970, the Motown group, The Jackson 5, released the hit pop song “ABC... as easy as 1-2-3.” The song made learning and falling in love feel simple, just follow the letters. But long before The Jackson 5 raced up the charts, a British anesthetist was using the alphabet for a different purpose: to classify breathing systems and organize how anesthetic gases flow and how patients breathe under anesthesia. The standardized system allowed anesthetists to easily understand and compare breathing circuits for safer and more effective patient care.
William Mapleson was born on August 2, 1926, in West Ealing, London. He earned his bachelor’s degree in Physics from the University of Durham in 1947 and spent much of his career at the Welsh National School of Medicine. He published more than 100 papers in mechanical ventilation and anesthetic pharmacology.
Building on his research, he developed the Mapleson classification, a practical system that organizes anesthetic breathing circuits (A–F) based on fresh gas flow efficiency. Using the alphabet, he labeled circuits A through F according to how fresh gas enters the system and how exhaled gases are cleared. He once joked, “I seem to have become the only person to make his reputation in anesthesia on the strength of his knowledge of the alphabet.”
The ABC's of Anesthesia
Clinically, each classification differs in how efficiently it supports spontaneous versus controlled ventilation:
Mapleson A (Magill) – Most efficient for spontaneous breathing, where the patient breathes on their own. Requires fresh gas flow approximately equal to the patient’s minute ventilation, minimizing rebreathing.
Mapleson B & C – Compact systems with no clear efficiency advantage. Both require high fresh gas flows and are generally considered inefficient, which is why they are rarely used in modern practice.
Mapleson D (Bain) – Most efficient for controlled ventilation, where breathing is assisted or delivered mechanically. Fresh gas is delivered near the patient, allowing effective clearance of exhaled gases.
Mapleson E (Ayre’s T-piece) – A simple, low-resistance system ideal for small patients. It has no valves or reservoir bag, making it suitable for spontaneous breathing but requiring high fresh gas flows to prevent rebreathing.
Mapleson F (Jackson-Rees) – A modification of the T-piece that includes a reservoir bag. This allows for assisted or controlled ventilation while maintaining low resistance, making it widely used in pediatric anesthesia.
Mapleson’s alphabet classification makes breathing systems easier to organize on paper. By labeling circuits A through F based on fresh gas flow and how exhaled gases are cleared, he created a simple way to compare different breathing systems. More than 70 years later, the Mapleson classification is still used in both human and veterinary anesthesia because it helps clinicians understand how different systems work and when they should be used.
But Mapleson’s real contribution was not just organizing systems with the alphabet. His work highlighted how circuit design directly affects efficiency and the patient’s work of breathing. Using the alphabet sounds simple. But in the anesthesia room, the decision is not really about letters. It is about how hard the patient has to work to take each breath.
Why Resistance Matters in Anesthesia Systems
Resistance is what makes breathing easier or harder. In any anesthesia circuit, resistance comes from the anesthesia machine and its components like tubing, valves, and CO₂ absorbent. The more the patient has to push through, the harder each breath becomes. A system that works well for one patient may create unnecessary effort for another.
A healthy 70-lb Labrador Retriever undergoing a routine procedure can usually overcome the resistance of a traditional circle system without difficulty. Larger patients generate greater tidal volumes and inspiratory force, allowing them to move gases through unidirectional valves, CO₂ absorbent, and longer breathing pathways more effectively.
A 6-lb kitten, however, does not breathe with the same strength or volume. For very small patients, even modest resistance can increase the work of breathing during anesthesia. That is why low-resistance non-rebreathing systems are commonly used for patients under approximately 15 lb. By minimizing resistance and reducing the effort required to breathe, these systems help support smoother and more efficient ventilation in pediatric and small animal patients.
That is where clinical judgment begins. The choice of breathing system is not simply about memorizing the alphabet. It is about understanding resistance, efficiency, and choosing the system that best fits the patient in front of you. In practice, that decision usually comes down to two options: rebreathing systems and non-rebreathing systems.
How AM Bickford and PUSHes forward with Anesthesia technology
At A.M. Bickford, we use the acronym PUSH as a simple educational framework to help explain some of the factors veterinarians may consider when evaluating breathing systems. It is not intended to replace clinical judgment, patient assessment, manufacturer instructions, or established anesthesia protocols.
Patient size: Smaller patients generally have less reserve and are more sensitive to resistance.
Underlying condition: Respiratory compromise, weakness, or illness may reduce the patient’s ability to tolerate added effort.
System resistance: The circuit design changes how much work is required to move gas.
How Hard Must This Patient Work to Breathe? Ask whether this patient can comfortably breathe through this system, not just whether the system is commonly used.
The PUSH framework is intended as an educational tool to encourage consideration of factors that may influence breathing system selection. Final equipment selection should always be based on the veterinarian's professional judgment and the specific needs of the patient and procedure.
What Is a Rebreathing System?
A rebreathing system, often called a circle system, is an anesthesia breathing system that allows a patient to reuse a portion of the gases they exhale. Instead of sending all exhaled gas out of the system, carbon dioxide is removed by a CO₂ absorber and the remaining oxygen and anesthetic gas are recirculated back to the patient. This design reduces the amount of fresh gas needed and helps conserve heat and moisture within the breathing circuit.
Rebreathing systems are commonly used for medium to large patients and are the standard configuration found on most veterinary anesthesia machines. While the breathing circuit, valves, and carbon dioxide absorber add some resistance to breathing, most patients tolerate it well. Proper sizing of the breathing circuit, reservoir bag, and other components helps ensure the system works efficiently and comfortably for the patient.
How a Rebreathing System Works?
In a rebreathing system, the patient must move gas through tubing, unidirectional valves, and a CO₂ absorber. This includes components like standard adult breathing circuits, such as the A.M. Bickford 52019.
This adds resistance and increases the work of breathing. Most patients tolerate this without difficulty. But as resistance increases, each breath requires more effort. If that effort exceeds what the patient can manage, ventilation becomes less effective, and the patient can rebreathe carbon dioxide.
What Is a Non-Rebreathing System?
A non-rebreathing system is an anesthesia breathing system that does exactly what its name suggests—the patient does not breathe the same gas twice. Instead of recycling exhaled gas through a carbon dioxide absorber like a circle system, a non-rebreathing system uses a continuous flow of fresh oxygen and anesthetic gas to carry exhaled carbon dioxide away from the patient.
These systems are commonly used for small patients, birds, and exotic animals because they create very little resistance to breathing. When every breath counts, reducing the effort required to move air through the system can be an important advantage. Common examples include Bain circuits and Jackson-Rees style systems, both of which are designed to deliver anesthesia while making breathing as easy as possible for the patient.
Non-Rebreathing System
In a non-rebreathing system the breathing path is simple. Fresh gas flows directly to the patient without passing through the valves and absorber found in a circle system. Fewer components in the breathing path generally means less resistance and less work required to move gas. For smaller patients, that difference is one of the reasons veterinarians often consider a non-rebreathing system.
A.M. Bickford PC-2 non-rebreathing systems are built to perform. Designed to minimize resistance and simplify the breathing path, PC-2 systems are available in multiple configurations: Bain Circuit, PC-2A, PC-2B, and PC-2C—to match different setups and patient needs.
At the center of the system, the PC-2 valve provides smooth, one-hand control while keeping the system lightweight and responsive. Its design supports quick adjustments without adding unnecessary resistance to the breathing path.
Integrated waste gas management through compatibility with F/Air scavenging systems helps direct anesthetic gas away from the patient and staff, improving safety in the operating room.
When resistance matters, and it does for small, compromised, or low-reserve patients, the PC-2 system removes unnecessary effort from every breath.
Bain Circuit & Bain Block
The Bain circuit is one of the most recognized non-rebreathing systems in veterinary anesthesia. Its coaxial design places the fresh gas delivery tube inside the larger breathing hose, allowing fresh gas from the anesthesia machine and vaporizer to flow directly to the patient through a lightweight, streamlined circuit.
Many veterinarians pair the Bain circuit with a Bain block to bring the reservoir bag, pop-off valve, and manometer into one convenient location. The pop-off valve allows adjustment of system pressure, while the manometer provides a visual indication of airway pressure during anesthesia and assisted ventilation. Together, the Bain circuit and Bain block create a simple, lightweight breathing system.
Summary and Conclusion
The Jackson 5 made the alphabet sound easy. William Mapleson made it practical. But the alphabet was never the point, both are still remembered because they simplified something complex. Mapleson used the alphabet to organize breathing systems. The P.U.S.H acronym helps remind us how much work is required to breathe.
Because in the operating room, the real question isn’t how well you know A through F. It’s whether the patient can overcome the resistance in front of them.
And when every breath matters, making the right decision should feel as easy as 1-2-3.