10 10
Correspondence
Pulse oximetry in pulseless patients We read with interest the case reported by Dawalibi et al. (Anaesthesia 1991; 46: 990-1) in which an oximeter (Ohmeda Biox 3700) continued to provide Spo, and heart rate readings from an organ donor for 15 min following cardiectomy. While we agree with the authors that pulse oximeter readings should not be accepted uncritically, we wish to report our experience of a similar case in which apparently reliable readings were obtained. Saturation was measured using a Nellcor N-200-E pulse oximeter (oximeter software version 2.9, powerbase version 2.7). Data from the oximeter communication port were stored on computer using methodology we have previously described [l], so we are able to reproduce the record here (Fig. I). Prior to cardiectomy, episodes of desaturation were recorded (0.4-0.6 h) which were associated with pulmonary oedema. Following this, Spo, was well maintained until cardiectomy. During cardiectomy, Spo, rapidly declined to around 85%, after which the oximeter displayed a saturation of zero and its pulse-search warning was illuminated. At this time there was a change in the monitor status of the oximeter from 11 (indicating pulse search status: locked) to 10 (indicating pulse search status: search) which coincided with the recorded time of clamp placement and cessation of ventilation. While no independent estimates of Spo, were obtained, all values provided by the oximeter are plausible in the light of the donor's condition. A possible explanation for the discrepancy between these results and those of Dawalibi et al. is use of a different type of oximeter. Our observation suggests that limitations of oximeters may differ ftom one brand to another. This is
100
00 60
' N
0
40 20
0 J 0
I
I
I
I
2
3
'0
Fig. 1.
consistent with the fact that different brands of oximeters use different algorithms [2] in the calculation of Spo,.
Flinders Medical Centre, Bedford Park. South Australia SO42
N. MACKEY J. PLUMMER A. ILSLEY H. OWEN
References [I] ILSLEY AH, OWENW, BROSEWG, WATSOND, FRONSKO RRL. A computerised monitoring system for studying
oxyhaemoglobin saturation in patients receiving postoperative analgesia. Anaesthesia Intensive Care I99 1; 1 9 294. MR,PARKERD, YOUNT [2] HUCHA, HUCHR, KONICV, NEUMAN J, LUBBERS D. Limitations of pulse oximetry. Lancet 1988; 1: 357-8.
Intra-aortic balloon pumps and the pulse oximeter The pulse oximeter has become an indispensable tool in operating theatres and intensive care units, because it provides an immediate and continuous measure of oxygen saturation. Many potential errors using pulse oximetry have been elucidated [l]. I present here another such error with an easy solution. A 58-year-old man required an intra-aortic balloon pump following aortic root replacement with coronary artery insertion. The abnormal pulse waveforms prevented correct functioning of the pulse oximeter. The radial arterial waveform had a triple peak, whilst distal to the balloon pump there was a double peak, which was palpable in the dorsalis pedis arteries. The three peaks of radial artery pressure ranged from 100 to 85 mmHg with a diastolic value of 60 mmHg and a heart rate of 120 beat.min-l. The pulse amplitude display of the Nellcor IOOE pulse oximeter showed these double and triple peaks, but it was unable to 'lock on' to them to produce any display of heart rate or saturation and there was no pulse tone. Neither pauses nor ratio changes in balloon pumping (being tried by the surgeon), nor changing the site of the probe or the oximeter averaging time, solved the problem. To overcome the problem the radial pulse was occluded by external finger pressure sufficient to remove any signal display on the oximeter. Intermittent release of this pressure in phase with the first peak of the pulse was performed using the rhythmic sound of the balloon pump and the arterial trace for timing. This produced a single pulse at the pulse oximeter probe and within a few seconds the audible pulse tone sounded, and saturation and heart rates corresponding to the ECG rate were displayed. Once 'locked on' the oximeter continued to correctly display heart rate once the external pressure was completely released. This was repeated three times to confirm the
findings, making sure to switch off the oximeter completely before each attempt. It was then performed twice on the great toe again overcoming the problem. The calculated saturation from an arterial sample was 97% and compared well with the oximeter value of 98%. In order to show that these findings were reproducible, a second patient with a n intra-aortic balloon pump was studied. There was a double pulse at the radial artery and a single pulse at the toe. The radial artery pressure was 145/80 mmHg with a heart rate of 115 beat.min-l. The single pulse in the foot was identified by the oximeter as a rate of 115 beat.min-' and saturation of 98%. The double pulse of the radial artery showed on the pulse amplitude display of the oximeter, with a saturation of 97% but a very unstable rate varying from 210 to 230 beat.min-' (about twice that of the heart rate). Arterial occlusion with intermittent release, as above, produced a saturation of 96% with a rate of 113 beat.min-' and this was maintained following release. The pulse oximeter calculates saturation from the pulsatile component of the red and infra-red absorption, and a pulse rate which correlates with the ECG heart rate is a useful way of validating the readings. It is important to allow a pause of about 20 s between releasing the occlusion and noting the saturation, in order to exclude any interference from venous pulsation caused by the intermittent occlusion of the pulse. It is also important to confirm with intra-arterial monitoring that the double or triple pulses shown on the oximeter are all arterial, as venous pulsations can cause erroneous saturations [2].
Queen Elizabeth Hospital. Birmingham BlS 2TH
T.C. SMITH
Correspondence
Pulse oximetry in pulseless patients We read with interest the case reported by Dawalibi et al. (Anaesthesia 1991; 46: 990-1) in which an oximeter (Ohmeda Biox 3700) continued to provide Spo, and heart rate readings from an organ donor for 15 min following cardiectomy. While we agree with the authors that pulse oximeter readings should not be accepted uncritically, we wish to report our experience of a similar case in which apparently reliable readings were obtained. Saturation was measured using a Nellcor N-200-E pulse oximeter (oximeter software version 2.9, powerbase version 2.7). Data from the oximeter communication port were stored on computer using methodology we have previously described [l], so we are able to reproduce the record here (Fig. I). Prior to cardiectomy, episodes of desaturation were recorded (0.4-0.6 h) which were associated with pulmonary oedema. Following this, Spo, was well maintained until cardiectomy. During cardiectomy, Spo, rapidly declined to around 85%, after which the oximeter displayed a saturation of zero and its pulse-search warning was illuminated. At this time there was a change in the monitor status of the oximeter from 11 (indicating pulse search status: locked) to 10 (indicating pulse search status: search) which coincided with the recorded time of clamp placement and cessation of ventilation. While no independent estimates of Spo, were obtained, all values provided by the oximeter are plausible in the light of the donor's condition. A possible explanation for the discrepancy between these results and those of Dawalibi et al. is use of a different type of oximeter. Our observation suggests that limitations of oximeters may differ ftom one brand to another. This is
100
00 60
' N
0
40 20
0 J 0
I
I
I
I
2
3
'0
Fig. 1.
consistent with the fact that different brands of oximeters use different algorithms [2] in the calculation of Spo,.
Flinders Medical Centre, Bedford Park. South Australia SO42
N. MACKEY J. PLUMMER A. ILSLEY H. OWEN
References [I] ILSLEY AH, OWENW, BROSEWG, WATSOND, FRONSKO RRL. A computerised monitoring system for studying
oxyhaemoglobin saturation in patients receiving postoperative analgesia. Anaesthesia Intensive Care I99 1; 1 9 294. MR,PARKERD, YOUNT [2] HUCHA, HUCHR, KONICV, NEUMAN J, LUBBERS D. Limitations of pulse oximetry. Lancet 1988; 1: 357-8.
Intra-aortic balloon pumps and the pulse oximeter The pulse oximeter has become an indispensable tool in operating theatres and intensive care units, because it provides an immediate and continuous measure of oxygen saturation. Many potential errors using pulse oximetry have been elucidated [l]. I present here another such error with an easy solution. A 58-year-old man required an intra-aortic balloon pump following aortic root replacement with coronary artery insertion. The abnormal pulse waveforms prevented correct functioning of the pulse oximeter. The radial arterial waveform had a triple peak, whilst distal to the balloon pump there was a double peak, which was palpable in the dorsalis pedis arteries. The three peaks of radial artery pressure ranged from 100 to 85 mmHg with a diastolic value of 60 mmHg and a heart rate of 120 beat.min-l. The pulse amplitude display of the Nellcor IOOE pulse oximeter showed these double and triple peaks, but it was unable to 'lock on' to them to produce any display of heart rate or saturation and there was no pulse tone. Neither pauses nor ratio changes in balloon pumping (being tried by the surgeon), nor changing the site of the probe or the oximeter averaging time, solved the problem. To overcome the problem the radial pulse was occluded by external finger pressure sufficient to remove any signal display on the oximeter. Intermittent release of this pressure in phase with the first peak of the pulse was performed using the rhythmic sound of the balloon pump and the arterial trace for timing. This produced a single pulse at the pulse oximeter probe and within a few seconds the audible pulse tone sounded, and saturation and heart rates corresponding to the ECG rate were displayed. Once 'locked on' the oximeter continued to correctly display heart rate once the external pressure was completely released. This was repeated three times to confirm the
findings, making sure to switch off the oximeter completely before each attempt. It was then performed twice on the great toe again overcoming the problem. The calculated saturation from an arterial sample was 97% and compared well with the oximeter value of 98%. In order to show that these findings were reproducible, a second patient with a n intra-aortic balloon pump was studied. There was a double pulse at the radial artery and a single pulse at the toe. The radial artery pressure was 145/80 mmHg with a heart rate of 115 beat.min-l. The single pulse in the foot was identified by the oximeter as a rate of 115 beat.min-' and saturation of 98%. The double pulse of the radial artery showed on the pulse amplitude display of the oximeter, with a saturation of 97% but a very unstable rate varying from 210 to 230 beat.min-' (about twice that of the heart rate). Arterial occlusion with intermittent release, as above, produced a saturation of 96% with a rate of 113 beat.min-' and this was maintained following release. The pulse oximeter calculates saturation from the pulsatile component of the red and infra-red absorption, and a pulse rate which correlates with the ECG heart rate is a useful way of validating the readings. It is important to allow a pause of about 20 s between releasing the occlusion and noting the saturation, in order to exclude any interference from venous pulsation caused by the intermittent occlusion of the pulse. It is also important to confirm with intra-arterial monitoring that the double or triple pulses shown on the oximeter are all arterial, as venous pulsations can cause erroneous saturations [2].
Queen Elizabeth Hospital. Birmingham BlS 2TH
T.C. SMITH
