Controlling arterial blood gas concentration
A system for controlling an amount of at least one gas X in a subject's lung to target at least one end tidal partial pressure of at least one gas X (PetX T ) uses a control system for controlling the gas delivery device, wherein the control system implements a sequential gas delivery system and a feedback algorithm which compares a PetX T for a respective breath of variable size and preferably a respective current PetX value measured by a measurement system, to obtain an error signal, the feedback algorithm adapted for generating a control signal based on the error signal, the control signal determining the amount of gas X to be inspired by the subject in at least a first portion of a respective ensuing respective inspiratory cycle to target PetX T for the respective interval.
1. A system for targeting an end tidal partial pressure of at least one gas X (PetX T ) in a mammalian subject's lung, comprising:
(1) a gas delivery device configured for connection to a breathing circuit;
(2) a control system;
(3) a measurement system configured to obtain a value which represents an approximation of the partial pressure of the at least one gas X in a subject's arterial blood after gas exchange in a previous breath [i−1];
wherein the control system is configured, with respect to a series of respective breaths [i] of respective sizes determined by the subject spontaneously, to:
(A) make available for inspiration to the subject, a first gas in a first part of a respective breath [i], the volume of the first gas selected such that a neutral second gas is arranged for inspiration in a second part of the respective breath [i], wherein the volume of neutral gas arranged for inspiration in the respective breath [i] at least equals and preferably exceeds the subject's anatomic dead space volume; and
(B) use a feedback control algorithm to:
(i) obtain, on a breath by breath basis, the measured value at least approximately corresponding to the arterial partial pressure of gas X after gas exchange in a previous breath ([i−1]);
(ii) based on the measured value, compute an error signal with respect to the PetX T for the respective breath [i]; and
(iii) generate a control signal based on the error signal that controls an amount of gas X delivered for inspiration by the subject in the first gas to target PetX T for the respective breath [i], the control signal signalling the gas delivery apparatus to deliver for inspiration in the first part of the respective breath [i], the amount of gas X that targets PetX T for the respective breath [i].
2. A system according to claim 1 , wherein the measured value is the concentration of the at least one gas X in a subject's expired end tidal gas after gas exchange in breath [i−1].
3. A system according to claim 1 , wherein the neutral gas has a partial pressure of gas X that equals the end tidal partial pressure of gas X targeted for the respective breath [i] or the end tidal partial pressure of gas X measured or computed for a previous breath, preferably an immediately preceding breath [i−1].
4. A system according to claim 1 , wherein the control system is implemented by a computer, the computer configured to provide control signals to one or more rapid flow controllers.
5. A system according to claim 1 , wherein the gas delivery device is a gas blender and wherein the measurement system includes at least one flow sensor positioned to measure flow of an inspiratory gas stream to the subject, and wherein the gas blender is controlled to add a varying amount of gas X to the inspiratory gas stream based on measurements obtained from the flow sensor to deliver the respective amounts of gas X required to be inspired by the subject to target the respective PetX T for the series of respective breaths [i].
6. A system according to claim 1 , wherein the breathing circuit is a sequential gas delivery (SGD) circuit.
7. A system according to claim 1 , wherein the control system implements a virtual SGD circuit.
8. A system according to claim 1 , wherein the gas X is carbon dioxide or oxygen.
9. A system according to claim 1 , wherein the control system is configured to contemporaneously target for the respective breath [i], a target end tidal concentration of gas X and at least a gas Y.
10. A system as claimed in claim 9 , wherein the gas X is carbon dioxide and the gas Y is oxygen.
11. A system according to claim 1 , wherein the system is configured to be used when the subject is hyperventilating.
12. A system according to claim 1 , wherein the amount of gas X required to be delivered in the first inspired gas to target the end tidal partial pressure of gas X for the respective breath [i] is determined prospectively based on a predictive algorithm, and wherein the error signal is added to the signal generated by the predictive algorithm.
13. A system according to claim 1 , wherein the feedback control algorithm is selected from a group comprising a PD, a PI and a PID control algorithm.