Breathing circuits and oxygen flow rate

Using the correct breathing circuit and oxygen flow rate:

Most veterinarians and veterinary nurses are aware of the two different types of breathing circuits: non-rebreathing circuits and the circle system.  The circle system is the most often used but sometimes it is not the best solution to the problem of maintaining adequate ventilation under anesthesia.  To expand on this topic, let us consider forces that impair ventilation (surgical position, anesthetic drugs, endotracheal tubes), what tools we have to support ventilation (different sized breathing circuits, positive pressure ventilation either manually or from mechanical means) and how capnography fits into this discussion.

My opinion is that allowing an animal to ventilate spontaneously is preferable to positive pressure ventilation. But there are important caveats to that statement, the main one is that if the spontaneous ventilation is not sufficient to clear carbon dioxide and provide oxygen transportation to the tissues, we need to interfere. So why do we not always put every animal on a ventilator? Reviewing the physiology in brief, the pressure inside the thoracic cavity is negative at rest and more negative upon inspiration. This serves well to inflate the lungs. When the lungs inflate, they actually improve the pulmonary circulation and therefore enable respiration, filling the alveoli with gases and increasing the size of the capillary vessels by traction.[1]  The correlation to this is that the venous return, via the vena cavae to the right atrium of the heart, termed “preload” is also enhanced by this negative pressure[2]. Positive pressure ventilation, just like the name implies, pushes air into the lungs and changes the intrathoracic pressure from negative to positive and therefore reduces the preload to the heart. This is not a benefit to the animal and how well this is tolerated is multifactoral.  In summary, cardiac output gives us an idea of perfusion (with systemic vascular resistence which is another newsletter), and cardiac output depends on preload (need to put blood into the heart to get more out to systemic circulation) and positive pressure ventilation decreases cardiac output. How do we relate cardiac output to the veterinary patients? We frequently think of part of the equation, the mean arterial pressure. Positive pressure ventilation causes lower mean arterial blood pressure than spontaneous ventilation, with all other factors being the same.

To avoid interfering with spontaneous ventilation, we choose different size breathing circuits for different patients.  The non-rebreathing circuits, such as the Mapleson D, the Bain (a coaxial modification of the Mapleson-D), the Ayres T-Piece, and the Bickford, all bypass the carbon dioxide absorbant canister and the one way valves for inspiration and expiration.   These valveless systems have less resistance than the circle system. This allows an animals whose muscular strength might be reduced by anesthetic drugs and who is small to make each breath more effective at getting rid of carbon dioxide.  The textbooks recommend various size standards : all animals less than 10 kgs should be on it. Does that mean that animal who is 10.9 kgs should be on a circle system? Not necessarily. You can put a larger animal on and increase the oxygen flow rate. How do you know if the non-rebreathing system (NRB) is working? Measure the inspired and expired carbon dioxide with a capnometer or capnography device. If the inspired is zero, you have eliminated enough deadspace and if the end-tidal is 50 or less, you have made the correct choice.

So what is the oxygen flow rate in liters/min? The textbooks vary in suggestion, from 150 ml/kg [3]to 300 ml/kg[4] to 600 ml/kg[5] to prevent the rebreathing of carbon dioxide. The gold-standard way to do this is to use capnography as described above and use the lowest flow that prevents the rebreathing of carbon dioxide.  I recommend no less than ½ liter per min per patient and somewhere between 300-600 ml/kg for patients under 10 kg.  The higher the flow, the more oxygen and inhalant agent is used.

How should these be integrated with a circle anesthesia system?  The fresh gas line for the NRB should connect to the vaporizer outlet or the fresh gas line leading to the circle system but before it enters the system.  The waste gases should connect to and F-air canister or to a passive scavenger or to an active scavenger with an adjustable valve so that the gases are not pulled from the patients lungs.

Let us now consider the circle or rebreathing system. It consists of oxygen that flows to a vaporizer which connects to the inspiratory side of the system, usually just after the carbon dioxide absorbant canister. The patient inhales and the one-way valve lifts, gases flow into the tubing and out to the patients. The patient exhales and the expiratory one way valve lifts, the inspiratory valve stays seated to prevent the gases from flowing the wrong direction, and the waste gases enter the carbon dioxide absorbant canister where they react with the granules to remove CO2, add some water to the gases and then enter the inspiratory valve and the reservoir bag. By the way, the proper size of the reservoir bag is 60-90 mls/kg of animal (6 x the tidal volume; tidal volume is 10-15 ml/kg). What should the oxygen flow be? Considering that most general practices are not set up to do low flow or closed system anesthesia, I recommend anywhere from 33-66 ml/kg/min flow of oxygen. We know this meets the metabolic demands of most animals.

What other factors influence respiration?  Endotracheal tubes that are properly sized enable respiration.  Every tube lessens the airway diameter with reference to the equation for area of πr2. Therefore each size decrease in mm is that number squared decrease in area. Something to keep in mind and have different size tubes available should the patients airway be larger (or smaller) than you expect. It is better for the airway to put in the largest tube possible and use the cuff to form just enough seal to pass the leak test.  What other hindrences can be put on the tube? The elbows and bending connectors impair the flow of air, creating more resistence to breathing, so it’s better not to use them but we use them when positioning for the procedure would potentially cause a “kink” or collapse in the tube, obstructing the airway.    

Anesthetic drugs depress respiration, by depressing the innate drive to  breath in the repiratory centers of the central nervous system and by weakening the muscles of respiration.[6]  In addition, we position animals for surgeries or procedures often on their backs, with their limbs extended, and sometimes we cause their visceral to press on vessels and organs that are usually not feeling this weight, press on the diaphragm, and impair the chest and diaphragm expansion. So it is a three-fold attack: 1) reduce the nerve firings that tell the body to breath 2)reduce the strength of themuscle of breathing as a side effect of anesthetic drugs 3)position the animal so that the diaphragm cannot expand and the muscles are in a less than optimal arrangement for breathing.  This is why we don’t want to add any more impedances to small patients and so we turn to using NRBs. We want to eliminate the extra work added to breathing of the one way valves and the carbon dioxide absorbant canister.

[1] West, Pulmonary physiology

[2] Berne and Levy

[3] Manual

[4] Lumb and Jones

[5] BSAVA

[6] Lumb and Jones