Arterial CO.sub.2 Monitor and closed loop controller

• US 5,402,796 A
• Filed: 04/21/1993
• Issued: 04/04/1995
• Est. Priority Date: 09/19/1990
• Status: Expired due to Term

• Current Assignee: University of Melbourne
• Original Assignee: University of Melbourne
• Inventors: Packer, John S., Law, Eng-Boon L., Cade, John F.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): Casler, Brian L.
• Application Number:
  US08/030,123
• Time in Patent Office: 713 Days
• Field of Search: 128/716, 128/719, 128/200.24, 128/204.21-204.24, 73/23.3, 422/84
• US Class Current: 600/532
• CPC Class Codes: A61B 5/0836 Measuring rate of CO2 production
• CPC Class Codes: A61B 5/091 Measuring volume of inspired or expired gases, e.g. to determine lung capacity
• CPC Class Codes: A61M 16/0051 with alarm devices
• CPC Class Codes: A61M 16/026 specially adapted for predicting, e.g. for determining an information representative of a flow limitation during a ventilation cycle by using a root square technique or a regression analysis
• CPC Class Codes: A61M 2230/202 partial carbon oxide pressure, e.g. partial dioxide pressure (P-CO2)

Abstract

The arterial CO₂ monitor and closed loop controller for use with a ventilator monitors a patient'"'"'s breath and determines PaCO₂ based upon a determination of a deadspace ratio, which is the ratio of the alveolar deadspace to alveolar tidal volume. The method generally comprises the steps of continuously monitoring measurable parameters of a patient'"'"'s breath; obtaining an input value of PaCO₂ from a blood sample of the patient and using the patients breath parameters and the input value to calculate the deadspace ratio; and continuously determining PaCO₂ based on the assumption that the deadspace ratio subsequently remains constant. Decision rules obtained from other measurable data are preferably also used to identify the onset of changes in the deadspace ratio, and a new deadspace ratio is then determined from the patient'"'"'s breath parameters and further input value of PaCO₂ from the patient'"'"'s blood sample.

Claims

1. A method of continuously and non-invasively monitoring arterial blood CO₂ partial pressure of artificially ventilated patients by monitoring a patient'"'"'s breath, and determining PaCO₂ based upon a determination of a deadspace ratio, comprising the steps of:

a) continuously monitoring measurable parameters of a patient'"'"'s breath;

b) measuring the PaCO₂ from a blood sample of the patient and obtaining an input value;

calculating the deadspace ratio using the patient'"'"'s breath parameters and said input value; and

c) continuously determining PaCO2 based on an assumption that the deadspace ratio subsequently remains constant.

2. The method of claim 1, wherein said step of continuously monitoring measurable parameters of a patient'"'"'s breath comprises monitoring mixed-expired CO2 partial pressure, inspired CO2 partial pressure, and alveolar tidal volume.

3. The method of claim 2, wherein said step of continuously monitoring measurable parameters of a patient'"'"'s breath further comprises monitoring alveolar deadspace.

4. The method of claim 1, wherein said step of continuously determining PaCO₂ is based upon the equation

5. The method of claim 1, further comprising the steps of identifying an onset of changes in the deadspace ratio by decision rules obtained from other measurable data;

and determining a new deadspace ratio from the patient'"'"'s breath parameters and a further input value of PaCO2 from the patient'"'"'s blood sample.

6. The method of claim 5, wherein said step of identifying an onset of changes in the deadspace ratio by decision rules obtained from other measurable data comprises monitoring parameters related to lung mechanics and changes in CO₂ production as measured by PE.CO₂ partial pressure from the alveolar tidal volume.

7. The method of claim 1, further comprising the step of adjusting patient ventilation based on the determined value of PaCO₂.

8. An apparatus for continuously and non-invasively monitoring arterial blood CO2 partial pressure of artificially ventilated patients, comprising:

a) means for monitoring continuously measurable parameters relevant to a patient'"'"'s breath and providing data relating thereto;

b) means for obtaining an input value corresponding to the PaCO₂ of a blood sample from the patient;

means for determining a deadspace ratio, connected to receive said input value and the patient'"'"'s breath parameter data, wherein said means for determining calculates the deadspace ratio from the input value and the patient'"'"'s breath parameter data; and

c) means for continuously determining PaCO2 based on the deadspace ratio, and an assumption that the deadspace ratio remains subsequently constant.

9. The apparatus of claim 8, wherein said means for monitoring continuously measurable parameters relevant to a patient'"'"'s breath and providing data relating thereto comprises a capnograph.

10. The apparatus of claim 8, wherein said means for determining a deadspace ratio comprises a microprocessor.

11. The apparatus of claim 8, further including means for identifying an onset of changes in the deadspace ratio to thereby signal a need for a further blood sample to re-calculate the deadspace ratio, based upon decision rules.

12. The apparatus of claim 8, further including means for controlling the operation of a mechanical ventilator based upon the values of PaCO₂ determined by the apparatus.