1016
Methods and Devices TRANSCUTANEOUS MEASUREMENT OF ARTERIAL BLOOD-GAS TENSIONS BY MASS SPECTROMETRY D. PARKER D. DELPY Department of Medical Physics, University College Hospital, London WC1E 6JA
lately been shown in the non-invasive of analysis blood-gases, especially for continuous moniof oxygen tension in sick babies.1 Miniature toring polarographic electrodes have been developed to measure oxygen tension transcutaneously,2,3 and electrodes of this type are now available commercially. A device for monitoring carbon-dioxide tension has also been reported,4 based on a modified pH electrode, but this electrode is unsuitable for routine clinical use, being relatively large and heavy. All these electrodes include some means of heating the surrounding tissue to 42-45OC since under certain conditions this heating causes sufficient local hyperaemia for the gas levels measured to reflect true arterial values closely. In the clinical situation it would be more beneficial to be able to monitor both Po2 and Pco2 continuously. To do this electrochemically would require two separate MUCH interest has
10
.30
20
40
Minutes Fig. 3-Traces obtained with probe at 44°C and using increased sensitivity on mass spectrometer. Between (A) and (B) the cuff was applied to cut off blood-flow. (C) The subject took*six breaths of pure oxygen. (D) Rebreathing was started.
(E) Rebreathing ceased.
probes and measuring systems, and this is impracticable We have attempted to measure Po2 and use. the gas diffusing through the skin- using by detecting PC02 for routine
a mass
spectrometer. This would have
advantages over the electrochemical method: only one probe would need
two to
be attached to the
skin,
and the
analyse any gas spectrometer which diffuses through the skin (e.g., anaesthetic gases or even emitted
mass
Fig. 1—Transcutaneous probe incorporating heating coil and thermistor
probe temperature
at
to
keep
about 43 °C.
can
vapours). The probe consists of a short cylindrical body over the open end of which is sealed a gas-permeable membrane supported on a porous substrate (.fig. 1). A heating coil and thermistor together keep the temperature of the probe at around 43 °C. A small flexible tube connects the inner chamber to the mass spectrometer, and the probe is attached to the skin using a ring of double-sided adhesive tape (as used with electrocardiograph electrodes). The traces illustrated here were obtained using a commercial medical
spectrometer (’Medspect MS-8’,’, Scientific Research Instruments Corporation) with the probe placed on the forearm. So far only Po2 and Pco2 have been monitored, and figs 2 and 3 summarise the effect on these of various manoeuvres. The mass spectrometer was calibrated in vitro by placing the heated probe in atmospheres of known oxygen and carbon-dioxide concentrations.
mass
Minutes Fig. 2-These traces show effect on PaO, and PaCOe of breathing pure rebreathing, and hyperventilation. (A) Blood-flow to arm cut off by applying pressure to cuff on upper arm. (B) Pressure released. (C) Few breaths of pure oxygen taken.
oxygen,
(D) Rebreathing into a large bag. (E) Rebreathing ceased. (F) Cuff reapplied, but this time the arm was exercised while the blood-flow was cut off; this led to a more rapid drop in Po. and a higher rise in Pco,. (G) Cuff released. Between (H) and (J) cuff was reapplied, but at a pressure sufficient only to prevent venous
flow.
(K) Few breaths of oxygen. (L) Subject started to hyperventilate. (M) Stopped hyperventilating.
REFERENCES
Lancet, 1974, ii, 32. Huch, R., Lubbers, D. W., Huch, A. Pflugers Arch. 1972, 337, 185. 3. Eberhard, P., Mindt, W., Jann, F., Hammacher, K. Adv. exp. Med. Biol. 1973, 37B, 1097. 4. Huch, A., Lubbers, D. W., Huch, R. Anæsthetist, 1973, 22, 379. 1. 2.
Methods and Devices TRANSCUTANEOUS MEASUREMENT OF ARTERIAL BLOOD-GAS TENSIONS BY MASS SPECTROMETRY D. PARKER D. DELPY Department of Medical Physics, University College Hospital, London WC1E 6JA
lately been shown in the non-invasive of analysis blood-gases, especially for continuous moniof oxygen tension in sick babies.1 Miniature toring polarographic electrodes have been developed to measure oxygen tension transcutaneously,2,3 and electrodes of this type are now available commercially. A device for monitoring carbon-dioxide tension has also been reported,4 based on a modified pH electrode, but this electrode is unsuitable for routine clinical use, being relatively large and heavy. All these electrodes include some means of heating the surrounding tissue to 42-45OC since under certain conditions this heating causes sufficient local hyperaemia for the gas levels measured to reflect true arterial values closely. In the clinical situation it would be more beneficial to be able to monitor both Po2 and Pco2 continuously. To do this electrochemically would require two separate MUCH interest has
10
.30
20
40
Minutes Fig. 3-Traces obtained with probe at 44°C and using increased sensitivity on mass spectrometer. Between (A) and (B) the cuff was applied to cut off blood-flow. (C) The subject took*six breaths of pure oxygen. (D) Rebreathing was started.
(E) Rebreathing ceased.
probes and measuring systems, and this is impracticable We have attempted to measure Po2 and use. the gas diffusing through the skin- using by detecting PC02 for routine
a mass
spectrometer. This would have
advantages over the electrochemical method: only one probe would need
two to
be attached to the
skin,
and the
analyse any gas spectrometer which diffuses through the skin (e.g., anaesthetic gases or even emitted
mass
Fig. 1—Transcutaneous probe incorporating heating coil and thermistor
probe temperature
at
to
keep
about 43 °C.
can
vapours). The probe consists of a short cylindrical body over the open end of which is sealed a gas-permeable membrane supported on a porous substrate (.fig. 1). A heating coil and thermistor together keep the temperature of the probe at around 43 °C. A small flexible tube connects the inner chamber to the mass spectrometer, and the probe is attached to the skin using a ring of double-sided adhesive tape (as used with electrocardiograph electrodes). The traces illustrated here were obtained using a commercial medical
spectrometer (’Medspect MS-8’,’, Scientific Research Instruments Corporation) with the probe placed on the forearm. So far only Po2 and Pco2 have been monitored, and figs 2 and 3 summarise the effect on these of various manoeuvres. The mass spectrometer was calibrated in vitro by placing the heated probe in atmospheres of known oxygen and carbon-dioxide concentrations.
mass
Minutes Fig. 2-These traces show effect on PaO, and PaCOe of breathing pure rebreathing, and hyperventilation. (A) Blood-flow to arm cut off by applying pressure to cuff on upper arm. (B) Pressure released. (C) Few breaths of pure oxygen taken.
oxygen,
(D) Rebreathing into a large bag. (E) Rebreathing ceased. (F) Cuff reapplied, but this time the arm was exercised while the blood-flow was cut off; this led to a more rapid drop in Po. and a higher rise in Pco,. (G) Cuff released. Between (H) and (J) cuff was reapplied, but at a pressure sufficient only to prevent venous
flow.
(K) Few breaths of oxygen. (L) Subject started to hyperventilate. (M) Stopped hyperventilating.
REFERENCES
Lancet, 1974, ii, 32. Huch, R., Lubbers, D. W., Huch, A. Pflugers Arch. 1972, 337, 185. 3. Eberhard, P., Mindt, W., Jann, F., Hammacher, K. Adv. exp. Med. Biol. 1973, 37B, 1097. 4. Huch, A., Lubbers, D. W., Huch, R. Anæsthetist, 1973, 22, 379. 1. 2.
