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Loss of a right radial arterial pressure tracing during thoracic aortic aneurysm repair
Loss of a Right Radial Arterial Pressure Tracing During Thoracic Aortic Aneurysm Repair W. S h e r m a n Turnage, MD, FCCP, and Richard J. Laborde, MD
O N T I N U A L measurement of the arterial blood pressure is an essential component of monitoring during anesthesia. 1 In most cardiovascular procedures, effective control of the hemodynamic state requires contmuous measurement of the arterial blood pressure by direct cannulation of a peripheral artery. In thoracic aortic surgery, the challenge is to find a peripheral artery that will provide for continuous arterial blood pressure during the period the aorta will be cross-clamped. Generally, the right radial artery is preferred during the repair of descending aortic aneurysms because it may be necessary to crossclamp the aorta above the origin of the left subclavian artery, thereby eliminating blood flow to the left radial artery. 2 The authors encountered a patient who had a right radial arterial catheter in place during a descending thoracic aortic aneurysm repair, but the pressure tracing was lost during clamping of the aorta proximal to the left subclavian artery. In this report, the possible causes of this phenomenon and the management are described.
C
CASE REPORT
A 72-year-old man was admitted to this institution for the evaluation of a left hilar lung mass, incidentally discovered on a routine chest radiograph done during an annual physical examination. The mass was 4 cm in diameter and well circumscribed in the left hilar region. Further outpatient evaluation had failed to yield a diagnosis. His past medical history was unremarkable, his laboratory studies were within normal limits, and he took no medications. Physical examination was remarkable only for a blood pressure of 150 to 180/86-92 mmHg. An electrocardiogram showed normal sinus rhythm, and the only abnormality was evidence of borderline left ventricular hypertrophy. A mediastinoscopy was performed but yielded no diagnosis. Therefore, the patient was brought to the operating room for a left thoracotomy and possible left upper lobe resection. In the preoperative preparation room, a 20-g right radial arterial catheter and 14-g intravenous catheter were inserted. He was then taken to the operating room, where general anesthesia was uneventfully induced with fentanyl, sodium thiopental, and vecuronium. A 37F double-lumen endotracheal tube was placed and appropriately positioned by auscultation and fiberoptic endoscopy, and the patient was turned to the right lateral decubitus position An axillary roll was placed, and the right radial arterial catheter and right index finger pulse oximeter probe confirmed the patency of the right axillary artery. On entering the left thorax, the surgeon discovered the hilar mass was actually a 6-cm descending thoracic aortic aneurysm. He elected to proceed with repair of the aneurysm; therefore, a 8.5F sheath-introducer was placed via the left internal jugular vein. A pulmonary artery catheter was passed into the right atrium via the sheath-introducer, but the catheter could not be advanced into the pulmonary artery and was therefore left in the right atrial position. An additional 14-g intravenous catheter was Inserted into a
peripheral vein, the head was packed in Ice, and the surgeon cannulated the left atrium and femoral artery for partial left heart bypass. When partial left heart bypass was established, the surgeon isolated the aortic arch vessels and examined the aorta to identify the proximal neck of the aneurysm. As the proximal neck of the aneurysm involved the origin of the left subclavian artery, the surgeon first placed a clamp across the aorta immediately proximal to the left subclavian artery. The patient's direct right radial arterial blood pressure had been stable at 130/70 mmHg before aortic cross-clamping; however, just after the clamp was applied, the arterial pressure tracing on the monitor changed from a pulsatlle waveform to a straight line registering a mean pressure of 30 mmHg (aortic crossclamp position 1 in Fig 1). The arterial pressure transducer, intra-arterlal catheter, and monitor were all inspected for proper functioning, but no malfunction of the monitoring system could be identified. The surgeon removed the aortic cross-clamp, and the right radial arterial pressure tracing immediately returned to normal. After reldentification of the aortic arch vessels to ensure the clamp had not been occluding the brachlocephalic artery, the clamp was reapplied. Again, the right radial arterial pressure tracing became a straight line at around 30 mmHg. The surgeon then removed the clamp and began to systematically apply the clamp at different points along the aortic arch to verify correct identification of anatomy. When the clamp was removed, the right radial arterial pressure waveform was pulsatIle, and registered a pressure of 140/80 mmHg. When the clamp was applied to the brachiocephalic artery (aortic cross-clamp position 2 in Fig 1), the right radial arterial pressure tracing immediately damped to a straight line, registering a pressure of 10 mmHg. When the clamp was removed, the arterial waveform returned to normal. The clamp was then apphed to the aortic arch between the left carotid artery and left subclavian artery, and the right radial arterial pressure tracing again damped to a straight line with a mean pressure of around 30 mmHg. While the clamp was left in place, low compliance pressure tubing and a 20-g needle were passed in sterile fashion to the surgeon, and a direct blood pressure measurement of the brachlocephalic artery was obtained wa direct arterial puncture. This waveform was normal with a pressure of 180/90 mmHg; however, the right radial arterial catheter continued to display a straight line at a pressure of 30 mmHg (Fig 2). The
From the Department of Anestheslology, Umverslty of South Florida College of Medtcme, Tampa, FL Address repnnt requests to W Sherman Turnage, MD, FCCP, Department of Anesthestology, Untverstty of South Flonda College of Medicine, 12901 Bruce B. Downs Blvd, MDC Box 59, Tampa, FL 33612 Copyright © 1995 by W B Saunders Company 1053-0770/95/0904-001553 00/0 Key words Direct measurement, radtal artenal, anesthesta, momtormg
Journal of Cardlothoracm and VascularAnesthesla, Vol 9, No 4 (August), 1995: pp 431-434
431
432
TURNAGE AND LABORDE
Right Radial Arterial Pressure Waveform 180 150 01
"IE E
h
120 90
h
J
60 30 aortic cross-clamp applied
Left Common Carotid a. Brachiocephalic a. aorticcross-clampposition2
. ,AI~ "~ ~
aortic cross-clamp position 1
/
/
Left Subclavian a.
/~'4P"
/
/
Fig 1. Arterialcatheterwaveformassociatedwith aorticcross-clampingThe positionof the aorticcross-clampduringattemptsto localizethe reasonfur lossof the arterialcatheterwaveformis shown. surgeon then proceeded with repair of the aneurysm, and while the aorta was cross-clamped, he periodically reinserted the needle into the brachiocephalic artery to measure the blood pressure. Titration of a sodium nitroprusside infusion was guided by the intermittent arterial pressure measurements, and the pressure kept between 140 to 180/80 to 90 mmHg. At the conclusion of the procedure, the patient was transported to the cardiac surgical intensive care unit. He remained hemodynamically stable throughout his postoperative course and awakened the evening of surgery. He was extubated in the early morning on the first postoperative day and was later discharged home in good condition. DISCUSSION
There are two possible explanations for the interruphon of pulsatile flow though the right radial artery after occlusion of blood flow through the left subclavian artery: subclavian steal syndrome or aberrant origin of the right subclavian artery. Subclavian steal syndrome occurs when there is an obstructive atherosclerotic leszon at the origin of one subclavian artery. Collateral flow to the subclavian artery distal to the obstruction is provided via the vertebrobasllar circulation. Because the origin of the vertebral
artery is frequently distal to the obstructive subclavian arterial lesion, flow can be diverted from the contralateral subclavian artery into the vertebral artery through the Circle of Willis then retrograde into the vertebral artery of the obstructed subclavian artery. This collateral subclavian arterial circulation will increase during exerose of the affected arm, increasing blood flow to the arm muscles at the expense of stealing blood flow from the basilar arterial circulation. If the subclavian flow is sufficiently greater, the patient may develop symptoms of cerebral ischemia. 3 In this patient, if the right subclavian artery had been obstructed--and the right subclavian artery depended on collateral circulation from the left subclavian artery for adequate blood flow--the pulsatile pressure would have ceased when the left subclavian artery was excluded from the circulation. This hypothesis would explain the ability to record a pulsatile pressure more proximally in the brachiocephalic artery, but not in the distal circulation of the arm when the cross-clamp was applied to the aorta proximal to the left subclavian artery. The loss of pulsatile pressure in the arterial catheter when only the brachiocephalic artery was occluded suggests that distal perfuslon to the right arm was dependent on some degree of blood flow through the right subclavian artery, and not entirely on flow from
RADIAL ARTERIAL PRESSURE DURING TAA REPAIR
433
Brachiocephalic Arterial Pressure Waveform
Right Radial Arterial Pressure Waveform 180
180
150
150
\
o~ 120 -rEE 90 Jkdv
~ 120 ~~.~
,oj
60
6O
30 aortic cross-clamp applied
aortic cross clamp apphed
LeftC o m m o n Brachiocephalic
a.
.
Carotid
a.
, ~ " L e f t S u b c l a v i a n a.
•
.
•
aortic cross clamp posl
.
Fig 2. Arterial catheter and direct brachiocephalic artery pressure waveforms during aortic cross-clamping The change in the artertal catheter and direct brachiocephalic arterial pressure waveforms is shown as the clamp is applied to the aorta just proximal to the left subclavian artery
collateral circulation from the left subclavian, because otherwise pulsatile flow would have been observed on the right radial arterial catheter tracing during the period of brachlocephalic artery occlusion• An aberrant origin of the right subclavian artery is present when the right subclavian artery arises from the descending portion of the aortic arch, frequently posterior to the origin of the left subclavian artery (Figs 3 and 4). This anomaly occurs in about 0.5% of the population and is commonly an asymptomatic incidental finding on routine
diagnostic barium swallow studies of the esophagus or on angiography. Some patients will present with complaints of dysphagia or wheezing caused by compression of the esophagus or trachea by the aberrant vessel. Most anomalous origins of the right subclavian artery occur in association with a left aortic arch, but other anomalies of the aortic arch may also include anomalous origins of the right subclavian artery. 4 In this case, clamping the aorta so as to exclude the left subclavian artery from the circulation would also exclude the right subclavian circulation. R. C• C. L . C C. LC•C. L. Subd.
L Subd.
R Subcl•
Fig 3 Common anatomic variations of the aortic arch The common variations of aortic arch anatomy and their frequency are shown, including the anomalous origin of the right subclavian artery. (Reprinted with permission from Grant's Atlas of Anatomy [ed 7], © 1971 by Williams & WUkins )
O.
434
TURNAGE AND LABORDE
Fig 4, Anomalous origin of the right subclavmn artery. Cephalad view of the anomalous r~ght subclavmn artery showing compression of the esophagus. (Reprinted with permission from Grant's Atlas of Anatomy (ed 7), © 1971 by Williams & Wilkins.)
It is also possible that the surgeon placed the cross-clamp in such a fashion as to inadvertently clamp a portion of the brachlocephalic trunk in the tip of the clamp. This cause was considered during efforts to identify the problem with the arterial pressure tracing, and both surgeons palpated the tip of the cross-clamp, confirming it to be free. Because it was necessary to clamp the aorta proximal to the left subclavian artery, and doing so repeatedly caused loss of the pressure waveform from the right radial arterial catheter, the only other means of obtaining an artenal blood pressure measurement proximal to the aortic crossclamp was to intermittently measure the arterial pressure by direct puncture of the brachlocephahc artery. This method proved satisfactory under the circumstances, but given the goal of maintaining an adequate proxamal perfuslon pressure for collateral blood flow to the spinal cord while preventing severe hypertension and its attendant nsk of intracramal bleedmg, it is clearly preferable to continu-
ously monitor the arterial blood pressure. The surgeon tried initially to leave the needle and pressure tubing in place to measure the brachiocephalic artenal pressure, but it was essential to position the lung retractor for optimal surgzcal exposure in such a way that the pressure tubing was consistently kinked. The authors compromised by titrating sodium nitroprusside much less aggressively than usual, and they had the surgeon perform direct puncture arterial blood pressure measurements as often as he reasonably could. The &fficulty encountered with this patient suggests that all patients presenting for major thoracic or vascular surgical procedures should have the arterial blood pressure measured in both arms during the preoperative evaluation. If a significant difference is found between arms, the diagnosis of subclavian artery stenosis should be considered and the patient's pnmary physician should be consulted regarding appropriate further evaluation. Because most patients presenting for thoracic aortic aneurysm repairs have had a dmgnosis established by aortography, it is frequently possible to review the angiogram with the radiologist for evidence of subclavian artery stenosls. In this patient, anglography had not been performed because an aneurysm had not been suspected preoperatively. Unfortunately, the patient did not receive an angiogram postoperatively to estabhsh whether he had a subclavian artery stenosis or aberrant origin of the right subclavian artery. In summary, an unusual case is presented where there was loss of a pulsatile pressure tracing during &rect arterial blood pressure monitoring for thoracic aortic aneurysm repair. The authors suggest that a hemodynamically significant subclavian artery stenosis may have been responsible for the loss of pulsatlle blood flow through the right radial artery when the aorta was cross-clamped immediately proximal to the left subclavian artery. Direct arterial puncture provided an adequate means of intermittently measuring the arterial blood pressure during the period of aortic cross-clamping and may prove to be a satisfactory alternative means of blood pressure measurement in similar circumstances.
REFERENCES
1 ASA Standards for basic lntraoperatlve monitoring Am Soc Anestheslol (December): 1986 2. Reich DL, Kaplan JA: Hemodynamlcmonitoring, in Kaplan JA (ed)' CardiacAnesthesla Philadelphia, Saunders, 1993,pp 263-266 3 Stoeltlng RK, Dlerdorf SF' Peripheral vascular disease, in:
Anesthesia and Coexzsting Disease New York, Churchill Lwmgstone, 1993, pp 129-130 4 Morrow WR. Aortic arch and pulmonary artery anomalies, in Oskl FA, DeAngehs CD, Felgln RD, et al (eds)" Prlnczples and Practice of Pediatrics Philadelphia, Llpplncott, 1990
O N T I N U A L measurement of the arterial blood pressure is an essential component of monitoring during anesthesia. 1 In most cardiovascular procedures, effective control of the hemodynamic state requires contmuous measurement of the arterial blood pressure by direct cannulation of a peripheral artery. In thoracic aortic surgery, the challenge is to find a peripheral artery that will provide for continuous arterial blood pressure during the period the aorta will be cross-clamped. Generally, the right radial artery is preferred during the repair of descending aortic aneurysms because it may be necessary to crossclamp the aorta above the origin of the left subclavian artery, thereby eliminating blood flow to the left radial artery. 2 The authors encountered a patient who had a right radial arterial catheter in place during a descending thoracic aortic aneurysm repair, but the pressure tracing was lost during clamping of the aorta proximal to the left subclavian artery. In this report, the possible causes of this phenomenon and the management are described.
C
CASE REPORT
A 72-year-old man was admitted to this institution for the evaluation of a left hilar lung mass, incidentally discovered on a routine chest radiograph done during an annual physical examination. The mass was 4 cm in diameter and well circumscribed in the left hilar region. Further outpatient evaluation had failed to yield a diagnosis. His past medical history was unremarkable, his laboratory studies were within normal limits, and he took no medications. Physical examination was remarkable only for a blood pressure of 150 to 180/86-92 mmHg. An electrocardiogram showed normal sinus rhythm, and the only abnormality was evidence of borderline left ventricular hypertrophy. A mediastinoscopy was performed but yielded no diagnosis. Therefore, the patient was brought to the operating room for a left thoracotomy and possible left upper lobe resection. In the preoperative preparation room, a 20-g right radial arterial catheter and 14-g intravenous catheter were inserted. He was then taken to the operating room, where general anesthesia was uneventfully induced with fentanyl, sodium thiopental, and vecuronium. A 37F double-lumen endotracheal tube was placed and appropriately positioned by auscultation and fiberoptic endoscopy, and the patient was turned to the right lateral decubitus position An axillary roll was placed, and the right radial arterial catheter and right index finger pulse oximeter probe confirmed the patency of the right axillary artery. On entering the left thorax, the surgeon discovered the hilar mass was actually a 6-cm descending thoracic aortic aneurysm. He elected to proceed with repair of the aneurysm; therefore, a 8.5F sheath-introducer was placed via the left internal jugular vein. A pulmonary artery catheter was passed into the right atrium via the sheath-introducer, but the catheter could not be advanced into the pulmonary artery and was therefore left in the right atrial position. An additional 14-g intravenous catheter was Inserted into a
peripheral vein, the head was packed in Ice, and the surgeon cannulated the left atrium and femoral artery for partial left heart bypass. When partial left heart bypass was established, the surgeon isolated the aortic arch vessels and examined the aorta to identify the proximal neck of the aneurysm. As the proximal neck of the aneurysm involved the origin of the left subclavian artery, the surgeon first placed a clamp across the aorta immediately proximal to the left subclavian artery. The patient's direct right radial arterial blood pressure had been stable at 130/70 mmHg before aortic cross-clamping; however, just after the clamp was applied, the arterial pressure tracing on the monitor changed from a pulsatlle waveform to a straight line registering a mean pressure of 30 mmHg (aortic crossclamp position 1 in Fig 1). The arterial pressure transducer, intra-arterlal catheter, and monitor were all inspected for proper functioning, but no malfunction of the monitoring system could be identified. The surgeon removed the aortic cross-clamp, and the right radial arterial pressure tracing immediately returned to normal. After reldentification of the aortic arch vessels to ensure the clamp had not been occluding the brachlocephalic artery, the clamp was reapplied. Again, the right radial arterial pressure tracing became a straight line at around 30 mmHg. The surgeon then removed the clamp and began to systematically apply the clamp at different points along the aortic arch to verify correct identification of anatomy. When the clamp was removed, the right radial arterial pressure waveform was pulsatIle, and registered a pressure of 140/80 mmHg. When the clamp was applied to the brachiocephalic artery (aortic cross-clamp position 2 in Fig 1), the right radial arterial pressure tracing immediately damped to a straight line, registering a pressure of 10 mmHg. When the clamp was removed, the arterial waveform returned to normal. The clamp was then apphed to the aortic arch between the left carotid artery and left subclavian artery, and the right radial arterial pressure tracing again damped to a straight line with a mean pressure of around 30 mmHg. While the clamp was left in place, low compliance pressure tubing and a 20-g needle were passed in sterile fashion to the surgeon, and a direct blood pressure measurement of the brachlocephalic artery was obtained wa direct arterial puncture. This waveform was normal with a pressure of 180/90 mmHg; however, the right radial arterial catheter continued to display a straight line at a pressure of 30 mmHg (Fig 2). The
From the Department of Anestheslology, Umverslty of South Florida College of Medtcme, Tampa, FL Address repnnt requests to W Sherman Turnage, MD, FCCP, Department of Anesthestology, Untverstty of South Flonda College of Medicine, 12901 Bruce B. Downs Blvd, MDC Box 59, Tampa, FL 33612 Copyright © 1995 by W B Saunders Company 1053-0770/95/0904-001553 00/0 Key words Direct measurement, radtal artenal, anesthesta, momtormg
Journal of Cardlothoracm and VascularAnesthesla, Vol 9, No 4 (August), 1995: pp 431-434
431
432
TURNAGE AND LABORDE
Right Radial Arterial Pressure Waveform 180 150 01
"IE E
h
120 90
h
J
60 30 aortic cross-clamp applied
Left Common Carotid a. Brachiocephalic a. aorticcross-clampposition2
. ,AI~ "~ ~
aortic cross-clamp position 1
/
/
Left Subclavian a.
/~'4P"
/
/
Fig 1. Arterialcatheterwaveformassociatedwith aorticcross-clampingThe positionof the aorticcross-clampduringattemptsto localizethe reasonfur lossof the arterialcatheterwaveformis shown. surgeon then proceeded with repair of the aneurysm, and while the aorta was cross-clamped, he periodically reinserted the needle into the brachiocephalic artery to measure the blood pressure. Titration of a sodium nitroprusside infusion was guided by the intermittent arterial pressure measurements, and the pressure kept between 140 to 180/80 to 90 mmHg. At the conclusion of the procedure, the patient was transported to the cardiac surgical intensive care unit. He remained hemodynamically stable throughout his postoperative course and awakened the evening of surgery. He was extubated in the early morning on the first postoperative day and was later discharged home in good condition. DISCUSSION
There are two possible explanations for the interruphon of pulsatile flow though the right radial artery after occlusion of blood flow through the left subclavian artery: subclavian steal syndrome or aberrant origin of the right subclavian artery. Subclavian steal syndrome occurs when there is an obstructive atherosclerotic leszon at the origin of one subclavian artery. Collateral flow to the subclavian artery distal to the obstruction is provided via the vertebrobasllar circulation. Because the origin of the vertebral
artery is frequently distal to the obstructive subclavian arterial lesion, flow can be diverted from the contralateral subclavian artery into the vertebral artery through the Circle of Willis then retrograde into the vertebral artery of the obstructed subclavian artery. This collateral subclavian arterial circulation will increase during exerose of the affected arm, increasing blood flow to the arm muscles at the expense of stealing blood flow from the basilar arterial circulation. If the subclavian flow is sufficiently greater, the patient may develop symptoms of cerebral ischemia. 3 In this patient, if the right subclavian artery had been obstructed--and the right subclavian artery depended on collateral circulation from the left subclavian artery for adequate blood flow--the pulsatile pressure would have ceased when the left subclavian artery was excluded from the circulation. This hypothesis would explain the ability to record a pulsatile pressure more proximally in the brachiocephalic artery, but not in the distal circulation of the arm when the cross-clamp was applied to the aorta proximal to the left subclavian artery. The loss of pulsatile pressure in the arterial catheter when only the brachiocephalic artery was occluded suggests that distal perfuslon to the right arm was dependent on some degree of blood flow through the right subclavian artery, and not entirely on flow from
RADIAL ARTERIAL PRESSURE DURING TAA REPAIR
433
Brachiocephalic Arterial Pressure Waveform
Right Radial Arterial Pressure Waveform 180
180
150
150
\
o~ 120 -rEE 90 Jkdv
~ 120 ~~.~
,oj
60
6O
30 aortic cross-clamp applied
aortic cross clamp apphed
LeftC o m m o n Brachiocephalic
a.
.
Carotid
a.
, ~ " L e f t S u b c l a v i a n a.
•
.
•
aortic cross clamp posl
.
Fig 2. Arterial catheter and direct brachiocephalic artery pressure waveforms during aortic cross-clamping The change in the artertal catheter and direct brachiocephalic arterial pressure waveforms is shown as the clamp is applied to the aorta just proximal to the left subclavian artery
collateral circulation from the left subclavian, because otherwise pulsatile flow would have been observed on the right radial arterial catheter tracing during the period of brachlocephalic artery occlusion• An aberrant origin of the right subclavian artery is present when the right subclavian artery arises from the descending portion of the aortic arch, frequently posterior to the origin of the left subclavian artery (Figs 3 and 4). This anomaly occurs in about 0.5% of the population and is commonly an asymptomatic incidental finding on routine
diagnostic barium swallow studies of the esophagus or on angiography. Some patients will present with complaints of dysphagia or wheezing caused by compression of the esophagus or trachea by the aberrant vessel. Most anomalous origins of the right subclavian artery occur in association with a left aortic arch, but other anomalies of the aortic arch may also include anomalous origins of the right subclavian artery. 4 In this case, clamping the aorta so as to exclude the left subclavian artery from the circulation would also exclude the right subclavian circulation. R. C• C. L . C C. LC•C. L. Subd.
L Subd.
R Subcl•
Fig 3 Common anatomic variations of the aortic arch The common variations of aortic arch anatomy and their frequency are shown, including the anomalous origin of the right subclavian artery. (Reprinted with permission from Grant's Atlas of Anatomy [ed 7], © 1971 by Williams & WUkins )
O.
434
TURNAGE AND LABORDE
Fig 4, Anomalous origin of the right subclavmn artery. Cephalad view of the anomalous r~ght subclavmn artery showing compression of the esophagus. (Reprinted with permission from Grant's Atlas of Anatomy (ed 7), © 1971 by Williams & Wilkins.)
It is also possible that the surgeon placed the cross-clamp in such a fashion as to inadvertently clamp a portion of the brachlocephalic trunk in the tip of the clamp. This cause was considered during efforts to identify the problem with the arterial pressure tracing, and both surgeons palpated the tip of the cross-clamp, confirming it to be free. Because it was necessary to clamp the aorta proximal to the left subclavian artery, and doing so repeatedly caused loss of the pressure waveform from the right radial arterial catheter, the only other means of obtaining an artenal blood pressure measurement proximal to the aortic crossclamp was to intermittently measure the arterial pressure by direct puncture of the brachlocephahc artery. This method proved satisfactory under the circumstances, but given the goal of maintaining an adequate proxamal perfuslon pressure for collateral blood flow to the spinal cord while preventing severe hypertension and its attendant nsk of intracramal bleedmg, it is clearly preferable to continu-
ously monitor the arterial blood pressure. The surgeon tried initially to leave the needle and pressure tubing in place to measure the brachiocephalic artenal pressure, but it was essential to position the lung retractor for optimal surgzcal exposure in such a way that the pressure tubing was consistently kinked. The authors compromised by titrating sodium nitroprusside much less aggressively than usual, and they had the surgeon perform direct puncture arterial blood pressure measurements as often as he reasonably could. The &fficulty encountered with this patient suggests that all patients presenting for major thoracic or vascular surgical procedures should have the arterial blood pressure measured in both arms during the preoperative evaluation. If a significant difference is found between arms, the diagnosis of subclavian artery stenosis should be considered and the patient's pnmary physician should be consulted regarding appropriate further evaluation. Because most patients presenting for thoracic aortic aneurysm repairs have had a dmgnosis established by aortography, it is frequently possible to review the angiogram with the radiologist for evidence of subclavian artery stenosls. In this patient, anglography had not been performed because an aneurysm had not been suspected preoperatively. Unfortunately, the patient did not receive an angiogram postoperatively to estabhsh whether he had a subclavian artery stenosis or aberrant origin of the right subclavian artery. In summary, an unusual case is presented where there was loss of a pulsatile pressure tracing during &rect arterial blood pressure monitoring for thoracic aortic aneurysm repair. The authors suggest that a hemodynamically significant subclavian artery stenosis may have been responsible for the loss of pulsatlle blood flow through the right radial artery when the aorta was cross-clamped immediately proximal to the left subclavian artery. Direct arterial puncture provided an adequate means of intermittently measuring the arterial blood pressure during the period of aortic cross-clamping and may prove to be a satisfactory alternative means of blood pressure measurement in similar circumstances.
REFERENCES
1 ASA Standards for basic lntraoperatlve monitoring Am Soc Anestheslol (December): 1986 2. Reich DL, Kaplan JA: Hemodynamlcmonitoring, in Kaplan JA (ed)' CardiacAnesthesla Philadelphia, Saunders, 1993,pp 263-266 3 Stoeltlng RK, Dlerdorf SF' Peripheral vascular disease, in:
Anesthesia and Coexzsting Disease New York, Churchill Lwmgstone, 1993, pp 129-130 4 Morrow WR. Aortic arch and pulmonary artery anomalies, in Oskl FA, DeAngehs CD, Felgln RD, et al (eds)" Prlnczples and Practice of Pediatrics Philadelphia, Llpplncott, 1990