Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2000-10-02
2003-09-23
Lo, Weilun (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S203120
Reexamination Certificate
active
06622725
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This Application claims priority to Canadian Application Serial No. 2304292 filed Mar. 31, 2000.
STATEMENT RE FEDERALLY SPONSORED RESEARCH/DEVELOPMENT
[Not Applicable]
BACKGROUND OF INVENTION
Physiology
Venous blood returns to the heart from the muscles and organs depleted of oxygen (O
2
) and full of carbon dioxide (CO
2
). Blood from various parts of the body is mixed in the heart (mixed venous blood) and pumped to the lungs. In the lungs the blood vessels break up into a net of small vessels surrounding tiny lung sacs (alveoli). The net sum of vessels surrounding the alveoli provides a large surface area for the exchange of gases by diffusion along their concentration gradients. A concentration gradient exists between the partial pressure of CO
2
(PCO
2
) in the mixed venous blood (PvCO
2
) and the alveolar PCO
2
. The CO
2
diffuses into the alveoli from the mixed venous blood from the beginning of inspiration until an equilibrium is reached between the PvCO
2
and the alveolar PCO
2
at some time during the breath. When the subject exhales, the first gas that is exhaled comes from the trachea and major bronchi which do not allow gas exchange and therefore will have a gas composition similar to the inhaled gas. The gas at the end of this exhalation is considered to have come from the alveoli and reflects the equilibrium CO
2
concentration between the capillaries and the alveoli; the PCO
2
in this gas is called end-tidal PCO
2
(PetCO
2
).
When the blood passes the alveoli and is pumped by the heart to the arteries it is known as the arterial PCO
2
(PaCO
2
). The arterial blood has a PCO
2
equal to the PCO
2
at equilibrium between the capillaries and the alveoli. With each breath some CO
2
is eliminated from the lung and fresh air containing little or no CO
2
(CO
2
concentration is assumed to be 0) is inhaled and dilutes the residual alveolar PCO
2
, establishing a new gradient for CO
2
to diffuse out of the mixed venous blood into the alveoli. The rate of breathing, or ventilation (V), usually expressed in L/min, is exactly that required to eliminate the CO
2
brought to the lungs and maintain an equilibrium PCO
2
(and PaCO
2
) of approximately 40 mmHg (in normal humans). When one produces more CO
2
(e.g., as a result of fever or exercise), more CO
2
is produced and carried to the lungs. One then has to breathe harder (hyperventilate) to wash out the extra CO
2
from the alveoli, and thus maintain the same equilibrium PaCO
2
. But if the CO
2
production stays normal, and one hyperventilates, then the PaCO
2
falls. Conversely, if CO
2
production stays constant and ventilation falls, arterial PCO
2
rises.
It is important to note that not all V contributes to blowing off CO
2
. Some V goes to the air passages (trachea and major bronchi) and alveoli with little blood perfusing them, and thus doesn't contribute to blowing off CO
2
. This V is termed “dead space” ventilation and gas in the lung that has not participated in gas exchange with the blood is called “dead space” gas. That portion of V that goes to well perfused alveoli and participates in gas exchange is called the alveolar ventilation (VA) and exhaled gas that had participated in gas exchange in the alveoli is termed “alveolar gas”.
Referring to the PCT Application No. WO98/41266 filed by Joe Fisher (WO98/41266), there is taught a method of accelerating the resuscitation of a patient having been anaesthetized by providing the patient with a source of fresh gas and a source of reserve gas. When the patient breathes at a rate less than or equal to the fresh gas flowing into the circuit, all of the inhaled gas is made up of fresh gas. When the patient's minute ventilation exceeds the fresh gas flow, the inhaled gas is made up of all of the fresh gas and the additional gas is provided by “reserve gas” consisting of a composition similar to the fresh gas plus CO
2
such that the concentration of CO
2
in the reserve gas of about 6% is such that its partial pressure is equal to the partial pressure of CO
2
in the mixed venous blood. At no time while using this method will the patient rebreathe gas containing anaesthetic. In order to accelerate the resuscitation of the patient, a source of fresh gas is provided for normal levels of minute ventilation, typically 5 L per minute and a supply of reserve gas is provided for levels of ventilation above 5 L per minute wherein the source of reserve gas includes approximately 6% carbon dioxide having a PCO
2
level substantially equal to that of mixed venous blood. It has been found that this method and various circuits and processes for implementing the method are advantageous not only for resuscitating individuals from surgery, but also to deal with carbon monoxide poisoning or the like as taught in the application. By allowing increased ventilation yet maintaining the PCO
2
level substantially equal to that prior to the increased ventilation, it has been found that in utilizing the method, maximum benefits of gas elimination are achieved without changing the CO
2
levels in the patient. However, one limitation is that a source of reserve gas and its delivery apparatus must be supplied to pursue the method and that the reserve gas must be at about 6% CO
2
concentration substantially having a PCO
2
equal to that of mixed venous blood or about 46 mmHg.
To simplify the circuit taught by Fisher (WO98/41266), the reserve gas can be replaced by previously exhaled gas. The gas at the end of exhalation has substantially equilibrated with mixed venous gas and thus has a PCO
2
substantially equal to it. However, if rebreathed gas is used instead of separately constituted reserve gas to prevent the decrease in PCO
2
with increased ventilation, the anesthetic and CO will also be rebreathed and their elimination will not be enhanced. There are other applications for a circuit that maintains PCO
2
constant with increased ventilation which are not invalidated by using exhaled gas as the reserved gas which are listed hereinafter.
Discussion of Prior Art Circuits Used for Rebreathing
Prior art circuits used to prevent decreased in PCO
2
resulting from increased ventilation, by means of rebreathing of previously exhaled gas are described according to the location of the fresh gas inlet, reservoir and pressure relief valve with respect to the patient. They have been classified by Mapleson and are described in Dorsch and Dorsch pg 168.
1. Maintenance of constant CO
2
with increased minute ventilation.
Mapleson A
The circuit comprises a pressure relief valve nearest the patient, a tubular reservoir and fresh gas inlet distal to the patient. In this circuit, on expiration, dead space gas is retained in the circuit, and after the reservoir becomes full, alveolar gas is lost through the relief valve. Dead space gas is therefore preferentially rebreathed. Dead space gas has a PCO
2
much less than mixed venous PCO
2
. This is less effective in maintaining PCO
2
than rebreathing alveolar gas, as occurs with the circuit of the present invention.
Mapleson B, C
The circuit includes a relief valve nearest the patient, and a reservoir with a fresh gas inlet at the near patient port. As with Mapleson A, dead space gas is preferentially rebreathed when minute ventilation exceeds fresh gas flow. In addition, if minute ventilation is temporarily less than fresh gas flow, fresh gas is lost from the circuit due to the proximity of the fresh gas inlet to the relief valve. Under these conditions, when ventilation once again increases there is no compensation for transient decrease in ventilation as the loss of fresh gas will prevent a compensatory decrease in PCO
2
.
With the present invention circuit, when minute ventilation temporarily is less than fresh gas flow, no fresh gas is lost from the circuit. Instead, the reservoir acts as a buffer, storing the extra fresh gas, and when ventilation increases once more, breathing the accumulated fresh gas allows PCO
2
to return to the previous level.
Mapleson D and E
Mapleson D consists
Fisher Joseph A.
Sasano Hiroshi
Tesler Janet
Vesely Alex
Volgyesi George
Lo Weilun
Mitchell Teena
Stetina Brunda Garred & Brucker
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