Research film project 241, the Ventimeter.
Fifty years ago, Dr.
Francis Benedict introduced the recording spirometer, which made respiratory measurements practical for clinical use.
During the early 20s, Dr.
Brian Sword introduced the circle absorption system for use in anesthesia.
These two breathing systems, the spirometer and the anesthesia circle, are similar.
Both have separate tubes for inspiration and expiration.
Both have unidirectional valves.
A soda -lime canister and a reservoir to contain the respired deaths.
In fact, the only difference between the spirometer and the circle system is in the specific reservoirs used.
Each serves a special purpose.
The bag does not provide a suitable means for measurement of respiration.
The inverted bell of the spirometer does not provide a satisfactory means to breathe for the patient.
Why not combine the advantage of these two systems and have a simple continuous measurement of tidal volume during anesthesia? We record pulse and blood pressure regularly, but respiratory rate is the only routine respiratory observation during anesthesia.
To follow the patient's ventilation requires that we know tidal volume by some convenient means.
This is a tidal volume indicator developed at Roswell Park Memorial Institute for the purpose of continuous measurement of the anesthetized patient's respiration.
Whether the patient breathes spontaneously as he is now or whether we assist ventilation, we have continuous information as to tidal volume.
You'll note that we have it mounted on a two chamber absorber, which incidentally we have found to be more efficient than our original Roswell Park absorber.
In this circuit, the indicator has replaced the conventional pop -off valve.
This pop -off valve supplied by the manufacturer was removed and a simple P -tube used for mounting the indicator.
I'll show you the overflow valve.
It consists of a ball and chain linked to the dome of the bellows.
Here you can see an aluminum ball and a tapered seat.
As the bellows is extended, the ball is lifted off its seat and allows gas to overflow to the outside.
Now when we trigger the oxygen flush, note that the pressure in the circuit is not excessive.
It seldom exceeds three or four centimeters of water.
This then is a volume controlled pop -off valve.
It is phased to the end expiratory period.
The result is that with inspiration, this linkage slackens, allows the ball to fall into its seat and the valve becomes confident.
So whether the patient breathes spontaneously or whether we assist inspiration, gas does not leave the breathing circuit during inspiration.
Here in animation is a view to show how the ventilator works.
As the patient breathes gas from the bellows, a corresponding volume of gas in the bag enters the chamber which surrounds the bellows.
Each time the bellows moves down, this overflow valve is closed as the chain linkage slackens.
As ventilation is assisted or controlled, pressure applied to the bag is transferred inside the chamber to compress the bellows.
In other words, squeezing the bag indirectly squeezes the bellows by a volume which may be directly absorbed.
Excess gas cannot be introduced into the breathing circuit because of the action of the overflow valve.
For example, if the patient coughs or if the oxygen bypass is used, the overflow valve is lifted as soon as the bellows is fully extended.
This action is produced by the chain linkage and occurs normally with semi -closed system at the end of each expiration.
During inspiration, the breathing circuit remains closed whether a closed system or a high flow technique is used.
We can use the initial inspiratory excursion as a visual guide for assisting each breath.
For those who prefer to control the ventilation, the tidal volume indicator serves as a continuous monitor of the respiration by this technique.
Now we'd like to demonstrate another advantage of the indicator, namely the basis for judging the adjustment of gas flow from the gas machine and the discovery of leak from the circuit, a very common problem in breathing equipment.
Look at the end expiratory position of the bellows.
See that it tends to return each time to the same position of rest.
This indicates that we have exactly matched gas flow from the anesthesia machine with the patient's requirements.
Were this flow excessive, the end position would rise to the point of overflow.
We would increase gas flow into the circuit.
Now we are unable to see the full tidal volume.
Returning to a closed system with a matched flow will demonstrate the effect of leak by producing a leakage at the tracheal cuff.
Although one is still able to read the patient -inspired volume, the fall of the bellows leads to the quick discovery that a leak has occurred.
And as soon as it's corrected, we have that evidence as the return of the resting position of the bellows.
We have found at time the indicator quite helpful in discovering the kinking of a tracheal tube.
With kinkage, one will see that the volume delivered is immediately decreased.
And with correction of the kinking, again the patient's tidal volume returns to the previous level.
When we are using a closed system and with spontaneous breathing, no corrections are required for direct reading of the indicator scale.
We can take the figure of 500 cc here, represented by bellows travel, as this patient's tidal volume.
If we convert our circuit to our semi -closed system with a 5 liter flow rate, you will note that the excursions now are less.
The bellows travel reads about 400 cc.
Now let's discuss the correction required for the higher flow rate.
A correction for gas entering the circuit during inspiration.
This does not appear as a part of the travel of the bellows.
This correction amounts to 20 cc per liter of gas flow.
So that with a 5 liter flow, we may add 100 cc to the observed travel of the bellows.
This would then give us a tidal volume of slightly over 500 cc.
Now another correction is necessary.
That due to compression of gas in the breathing circuit and the stretch of the expansible component, especially the breathing tubes.
This correction amounts to about 10 cc of tidal volume per centimeter of water pressure.
So that with a peak inspiratory pressure of 10 centimeters of water, subtract 100 cc from the observed travel of the bellows.
The corrections due to gas flow from the anesthesia machine and the corrections necessary for pressure during controlled respiration are in the opposite direction.
In other words, they tend to cancel each other.
So that with a semi -closed technique and with the usual pressures required to inflate the anesthetized patient's lungs, we come out with a very close approximation of tidal volume without any correction.
To convert from a closed system to a non -rebreathing system.
To do this, we will occlude the inspiratory limb of the absorber, remove the valve Y piece, and we will install the double acting non -rebreathing Rubin valve.
Now the gas flow must exactly equal the patient's minute volume for a precise adjustment.
We can exceed this requirement of course, and again overflow will occur from the indicator.
Now we have approximately matched the requirements with the gas flow from the machine.
Again, we must subtract a volume due to the pressure built up in the system.
The corrected tidal volume here would be about 650 cc.
This device is not intended as a research instrument, but rather for clinical observation of the patient.
The ventilometer is actually a dry spirometer mounted in the breathing circuit so as not to rob the anesthetist of his favorite means of breathing for the patient, namely the familiar bag.
It incorporates an overflow valve system which replaces the conventional spring -loaded or weight -loaded pop -off.
This design improves the accuracy of estimating the patient's tidal volume and ensures safety when a mechanical ventilator is used.
With inspiratory pressures, gas is delivered to the patient and not through a spring -loaded or weight -loaded pop -off valve to the outside.
All flow to the outside occurs at one time in the cycle, namely immediately after expiration.
The ventilometer provides an objective figure of the patient's ventilation just as we record pulse and blood pressure.
