258
Correspondence
gas-tight seal when anaesthesia was administered by mask. It occurred to me that the old reassurance could be restored by simple electronic means: a small microphone could be inserted into the system near the valve. This works very well applied to the Bain apparatus by placement in the bag mount (Fig. 1). We use an electronic device (Cortest Monitoring System)* originally intended for an oesophageal stethoscope, but simple components can be obtained from ordinary radio stores, as shown in the Figure (Tandy: Electret Tie Clip Microphone and Mini audio amplifier). Probably one of the amplifier/loudspeakers available for personal hifi systems could also be used. Royal Victoria InJirrnary, Newcastle upon Tyne NEI 4LP
B.E. WELSH
*Diagnostic Instrument Corporation, Littleton, Mass 01460. USA.
Fig. 1.
Ephedrine and hypotension Drs Wood, Foley and Lawler are to be congratulated for their bravery in publishing their letter (Anaesthesia 1989; 4 4 869). Eleven litres of intravenous fluid seems to underline what is known to be true: fluid given to attempt to correct hypotension after epidural top-ups is rarely successful. Ephedrine would appear to be the treatment of choice.
Given incrementally there is usually an effect on the blood pressure but little on the fetus. Perhaps it would have been wiser to give more than 19 mg rather than further fluid? County Hospital. Hereford HRI 2ER
J.H.W. BALLANCE
Erroneous explanation for an erroneous pulse oximeter reading We cannot allow Dr Hopkins’ comments on pulse oximetry to go unchallenged (Anuesthesia 1989; 4 4 868). Clinical signs of hypoxaemia are unreliable. It was demonstrated in 1947 that the majority of clinicians cannot detect cyanosis until the oxygen saturation is below 80X.l I t is therefore not surprising that Dr Hopkins did not notice cyanosis when the oximeter read 90%. This clearly demonstrates the increased margin for safety inherent in the use of pulse oximetry. The basic principle of pulse oximetry is that the oximeter differentiates between the constant absorption of light by immobile substances and the pulsatile absorption associated with blood flow underneath the probe.* One would therefore expect that the constant absorption of light by dried blood on the patient’s skin would not affect the pulse oximeter reading. We carried out a simple experiment. We both demonstrated that a pulse oximeter gave identical readings from both o u r right and left index fingers. We then liberally coated the index finger of one hand with autologous blood,
and allowed it to dry. We compared the pulse oximeter readings from the right and left index fingers. The presence of external dried blood made no difference to the reading. Dr Hopkins should look elsewhere for the explanation of the patient’s desaturation. We suggest that the probe might have become displaced when the patient was turned prone, or that rubbing the digit during cleaning improved tissue perfusion. Mount Sinai Hospital. Toronto, Canada
J. OYSTON A. ORDMAN
References I . COMKOE JH, BOTELHOS. The unreliability of cyanosis in the recognition of arterial anoxemia. American Journal of Medicul Sciences 1947; 2 1 4 1-6. 2. ALEXANDER C M , TELLER LE, GROSSJB. Principles of pulse oximetry: theoretical and practical considerations. Anc,sthesia and Analgesia 1989; 68: 368-76.
An unusual occlusion of an anaesthetic system by a polythene drape An otherwise fit 46-year-old woman was anaesthetised for the operation of myringoplasty. She was covered with an adhesive polythene drape (Steridrape). The patient’s head was moved during the procedure and, shortly after this, a disconnexion of the system at the catheter mount was detected. It proved impossible to inflate the patient at all after reconnexion under the drapes. The drapes were
removed rapidly and the system dismantled to reveal that during reconnexion a portion of the polythene drape had been punched out, and formed a diaphragm across the lumen. No ill effects were suffered by the patient. The Middlesex Hospital, London W I N 8 A A
S.K. ANDERSON
Correspondence
gas-tight seal when anaesthesia was administered by mask. It occurred to me that the old reassurance could be restored by simple electronic means: a small microphone could be inserted into the system near the valve. This works very well applied to the Bain apparatus by placement in the bag mount (Fig. 1). We use an electronic device (Cortest Monitoring System)* originally intended for an oesophageal stethoscope, but simple components can be obtained from ordinary radio stores, as shown in the Figure (Tandy: Electret Tie Clip Microphone and Mini audio amplifier). Probably one of the amplifier/loudspeakers available for personal hifi systems could also be used. Royal Victoria InJirrnary, Newcastle upon Tyne NEI 4LP
B.E. WELSH
*Diagnostic Instrument Corporation, Littleton, Mass 01460. USA.
Fig. 1.
Ephedrine and hypotension Drs Wood, Foley and Lawler are to be congratulated for their bravery in publishing their letter (Anaesthesia 1989; 4 4 869). Eleven litres of intravenous fluid seems to underline what is known to be true: fluid given to attempt to correct hypotension after epidural top-ups is rarely successful. Ephedrine would appear to be the treatment of choice.
Given incrementally there is usually an effect on the blood pressure but little on the fetus. Perhaps it would have been wiser to give more than 19 mg rather than further fluid? County Hospital. Hereford HRI 2ER
J.H.W. BALLANCE
Erroneous explanation for an erroneous pulse oximeter reading We cannot allow Dr Hopkins’ comments on pulse oximetry to go unchallenged (Anuesthesia 1989; 4 4 868). Clinical signs of hypoxaemia are unreliable. It was demonstrated in 1947 that the majority of clinicians cannot detect cyanosis until the oxygen saturation is below 80X.l I t is therefore not surprising that Dr Hopkins did not notice cyanosis when the oximeter read 90%. This clearly demonstrates the increased margin for safety inherent in the use of pulse oximetry. The basic principle of pulse oximetry is that the oximeter differentiates between the constant absorption of light by immobile substances and the pulsatile absorption associated with blood flow underneath the probe.* One would therefore expect that the constant absorption of light by dried blood on the patient’s skin would not affect the pulse oximeter reading. We carried out a simple experiment. We both demonstrated that a pulse oximeter gave identical readings from both o u r right and left index fingers. We then liberally coated the index finger of one hand with autologous blood,
and allowed it to dry. We compared the pulse oximeter readings from the right and left index fingers. The presence of external dried blood made no difference to the reading. Dr Hopkins should look elsewhere for the explanation of the patient’s desaturation. We suggest that the probe might have become displaced when the patient was turned prone, or that rubbing the digit during cleaning improved tissue perfusion. Mount Sinai Hospital. Toronto, Canada
J. OYSTON A. ORDMAN
References I . COMKOE JH, BOTELHOS. The unreliability of cyanosis in the recognition of arterial anoxemia. American Journal of Medicul Sciences 1947; 2 1 4 1-6. 2. ALEXANDER C M , TELLER LE, GROSSJB. Principles of pulse oximetry: theoretical and practical considerations. Anc,sthesia and Analgesia 1989; 68: 368-76.
An unusual occlusion of an anaesthetic system by a polythene drape An otherwise fit 46-year-old woman was anaesthetised for the operation of myringoplasty. She was covered with an adhesive polythene drape (Steridrape). The patient’s head was moved during the procedure and, shortly after this, a disconnexion of the system at the catheter mount was detected. It proved impossible to inflate the patient at all after reconnexion under the drapes. The drapes were
removed rapidly and the system dismantled to reveal that during reconnexion a portion of the polythene drape had been punched out, and formed a diaphragm across the lumen. No ill effects were suffered by the patient. The Middlesex Hospital, London W I N 8 A A
S.K. ANDERSON
