Cor J. Kalkman

Reduced renal failure following thoracoabdominal aortic aneurysm repair by selective perfusion

OBJECTIVESnRenal failure and visceral ischemia are feared complications following thoracoabdominal aortic aneurysm (TAAA) repair, significantly contributing to mortality. This prospective study describes volume- and pressure-controlled perfusion of the renal and visceral arteries during TAAA surgery.nnnMETHODSnIn 73 consecutive patients (mean age 59 years), TAAA repair (27 type I, 28 type II, 8 type III and 10 type IV) was performed, using retrograde and selective organ perfusion. Sixteen patients had impaired renal function with blood creatinine higher than 100 mmol/l. During the thoracic part of the procedure, the mean distal aortic pressure was kept above 60 mm Hg by means of left-heart bypass. After opening the abdominal aorta, the renal and visceral arteries were individually perfused by means of perfusion catheters (9 French) in the first 33 patients (group I). Volume flow through each catheter was assessed with ultrasound flow meters and maintained at least at 60 ml/min. In addition to volume flow measurements, catheters with pressure sensors were used in the last 40 patients (group II), allowing pressure-controlled selective perfusion. The extent of the aneurysm was comparable in both groups.nnnRESULTSnMean cross-clamp time for the thoracic part was 46 min, including proximal anastomosis and reattachment of intercostal arteries. Mean cross-clamp time for the abdominal part was 74 min, including re-implantation of intestinal and renal arteries and selective dacron grafts to the celiac-axis arteries (n = 5), superior mesenteric arteries (n = 8) and renal arteries (n = 25), through which the catheters guaranteed continuous perfusion during the time the anastomosis was performed. Urine output was uninterrupted in all patients, irrespective of cross-clamp time. In group I, one patient (3%) developed renal failure and three patients (9%) required temporary peritoneal dialysis. In group II, no patients developed renal failure and two patients (5%) required temporary peritoneal dialysis. Thirteen patients with pre-existing renal impairment did not deteriorate. No patients developed visceral ischemia or multiple-organ failure. Total in-hospital mortality was 6/73 (8%) and was related to cardiopulmonary complications.nnnCONCLUSIONSnRenal and visceral ischemia can be reduced significantly by continuous perfusion during cross-clamping in TAAA repair. Not only sufficient volume flow but also adequate arterial pressure appears to be essential in maintaining renal function.

Advanced pulse oximeter signal processing technology compared to simple averaging. i. effect on frequency of alarms in the operating room

STUDY OBJECTIVEnTo determine the effect of a new signal processing technique (Oxismart, Nellcor, Inc., Pleasanton, CA) on the incidence of false pulse oximeter alarms in the operating room (OR).nnnDESIGNnProspective observational study.nnnSETTINGnNonuniversity hospital.nnnPATIENTSn53 ASA physical status I, II, and III consecutive patients undergoing general anesthesia with tracheal intubation.nnnMEASUREMENTS AND MAIN RESULTSnIn the OR we compared the number of alarms produced by a recently developed third generation pulse oximeter (Nellcor Symphony N-3000) with Oxismart signal processing technique and a conventional pulse oximeter (Criticare 504). Three pulse oximeters were used simultaneously in each patient: a Nellcor pulse oximeter, a Criticare with the signal averaging time set at 3 seconds (Criticareaverage3s) and a similar unit with the signal averaging time set at 21 seconds (Criticareaverage21s). For each pulse oximeter, the number of false (artifact) alarms was counted. One false alarm was produced by the Nellcor (duration 55 sec) and one false alarm by the Criticareaverage21s monitor (5 sec). The incidence of false alarms was higher in Criticareaverage3s. In eight patients, Criticareaverage3s produced 20 false alarms (p < 0.01).nnnCONCLUSIONSnOur study did not show a beneficial effect in the OR on the incidence of false alarms of the Nellcor monitor with Oxismart signal processing compared with the Criticare monitor with the longer averaging time of 21 seconds.

LabVIEW: A software system for data acquisition, data analysis, and instrument control

Computer-based data acquisition systems play an important role in clinical monitoring and in the development of new monitoring tools. LabVIEW (National Instruments, Austin, TX) is a data acquisition and programming environment that allows flexible acquisition and processing of analog and digital data. The main feature that distinguishes LabVIEW from other data acquisition programs is its highly modular graphical programming language, “G,” and a large library of mathematical and statistical functions. The advantage of graphical programming is that the code is flexible, reusable, and self-documenting. Subroutines can be saved in a library and reused without modification in other programs. This dramatically reduces development time and enables researchers to develop or modify their own programs. LabVIEW uses a large amount of processing power and computer memory, thus requiring a powerful computer. A large-screen monitor is desirable when developing larger applications. LabVIEW is excellently suited by testing new monitoring paradigms, analysis algorithms, or user interfaces. The typical LabVIEW user is the researcher who wants to develop a new monitoring technique, a set of new (derived) variables by integrating signals from several existing patient monitors, closed-loop control of a physiological variable, or a physiological simulator.Computer-based data acquisition systems play an important role in clinical monitoring and in the development of new monitoring tools. LabVIEW (National Instruments, Austin, TX) is a data acquisition and programming environment that allows flexible acquisition and processing of analog and digital data. The main feature that distinguishes LabVIEW from other data acquisition programs is its highly modular graphical programming language, “G,” and a large library of mathematical and statistical functions. The advantage of graphical programming is that the code is flexible, reusable, and self-documenting. Subroutines can be saved in a library and reused without modification in other programs. This dramatically reduces development time and enables researchers to develop or modify their own programs. LabVIEW uses a large amount of processing power and computer memory, thus requiring a powerful computer. A large-screen monitor is desirable when developing larger applications. LabVIEW is excellently suited by testing new monitoring paradigms, analysis algorithms, or user interfaces. The typical LabVIEW user is the researcher who wants to develop a new monitoring technique, a set of new (derived) variables by integrating signals from several existing patient monitors, closed-loop control of a physiological variable, or a physiological simulator.

The influence of nitrous oxide to supplement fentanyl/low-dose propofol anesthesia on transcranial myogenic motor-evoked potentials during thoracic aortic surgery

OBJECTIVEnIntraoperative monitoring of myogenic motor evoked potentials to transcranial electrical stimulation (tc MEPs) is a new method to assess the integrity of the motor pathways. The authors studied the effects of 50% nitrous oxide (N2O) and a low-dose propofol infusion on tc MEPs paired electrical stimulation during fentanyl anesthesia with partial neuromuscular blockade.nnnDESIGNnCross-over study.nnnSETTINGnSt Antonius Hospital, Nieuwegein, The Netherlands.nnnPARTICIPANTSnTen patients scheduled to undergo surgery on the thoracoabdominal aorta were studied; 6 women aged 54 to 69 years and 4 men aged 68 to 77 years.nnnINTERVENTIONSnAfter achieving a stable anesthetic state and before surgery, tc MEPs were recorded during four 15-minute periods: (I) air/oxygen (O2; F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (II) N2O/O2 (F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (III) N2O/O2 (F(I)O2 = 50%; propofol target blood concentration, 1.0 microg/mL; and (IV) air/O2 (F(I)O2 = 50%); propofol target blood concentration, 1.0 microg/mL.nnnMEASUREMENTS AND MAIN RESULTSnTc MEPs were recorded from the right extensor digitorum communis muscle and the right tibialis anterior muscle. The right thenar muscle was used for recording the level of relaxation; the T1 response was maintained at 40% to 70% of the control compound muscle action potential. There was no significant difference in onset latency among the four phases. The addition of N2O and doubling the target propofol infusion to 1.0 microg/mL resulted in a 40% to 50% reduction of tc MEP amplitude recorded in the extensor digitorum communis muscle and tibialis anterior muscle (p < 0.01). During each phase, tc MEPs could be elicited and interpreted, except in one patient, in whom no tc MEPs could be elicited in the leg because of technical problems.nnnCONCLUSIONnThe data indicate that tc MEP monitoring is feasible during low-dose propofol, fentanyl/50% N2O in 02 anesthesia and partial neuromuscular blockade.

The influence of regional spinal cord hypothermia on transcranial myogenic motor-evoked potential monitoring and the efficacy of spinal cord ischemia detection

OBJECTIVEnMyogenic motor-evoked responses to transcranial electrical stimulation (transcranial myogenic motor-evoked potentials) can rapidly detect spinal cord ischemia during thoracoabdominal aortic aneurysm repair. Recent evidence suggests that regional spinal cord hypothermia increases spinal cord ischemia tolerance. We investigated the influence of subdural infusion cooling on transcranial myogenic motor-evoked potential characteristics and the time to detect spinal cord ischemia in 6 pigs.nnnMETHODSnRegional hypothermia was produced by subdural perfusion cooling. A laminectomy and incision of the dura were performed at L2 to advance 2 inflow catheters at L4 and L6, to cool the lumbar subdural space with saline solution. Two temperature probes were advanced at L3 and L5, and 1 cerebrospinal fluid pressure line was advanced at L4. Spontaneous cerebrospinal fluid outflow was allowed. Spinal cord ischemia was produced by clamping a set of critical lumbar arteries, previously identified by transcranial myogenic motor-evoked potentials and lumbar artery clamping. The time between the onset of ischemia and detection with transcranial myogenic motor-evoked potentials (amplitude < 25%) was determined at cerebrospinal fluid temperatures of 37 degrees C and 28 degrees C. Thereafter, the influence of progressive cerebrospinal fluid cooling on transcranial myogenic motor-evoked potential amplitude and latency was determined.nnnRESULTSnThe time necessary to produce ischemic transcranial myogenic motor-evoked potentials, after the clamping of critical lumbar arteries, was not affected at moderate subdural hypothermia (3.8 +/- 0.9 min) compared with subdural normothermia (3.2 +/- 0.5 min; P =.6). Thereafter, progressive cooling resulted in a transcranial myogenic motor-evoked potential amplitude increase at 28 degrees C to 30 degrees C and was followed by a progressive decrease. Response amplitudes decreased below 25% at 14.0 degrees C +/- 1.1 degrees C. The influence of cerebrospinal fluid temperature on transcranial myogenic motor-evoked potential amplitude was best represented by a quadratic regression curve with a maximum at 29.6 degrees C. In contrast, transcranial myogenic motor-evoked potential latencies increased linearly with decreasing subdural temperatures.nnnCONCLUSIONSnDetection of spinal cord ischemia with transcranial myogenic motor-evoked potentials is not delayed at moderate subdural hypothermia in pigs. At a cerebrospinal fluid temperature of 28 degrees C, transcranial myogenic motor-evoked potential amplitudes are increased. Further cerebrospinal fluid temperature decreases result in progressive amplitude decreases and latency increases.

Influence of Pulse Oximeter Settings on the Frequency of Alarms and Detection of Hypoxemia

Objective. The potential benefit of a reduced frequency of false pulse oximeter low oxyhemoglobin saturation (SpO2) alarms is that the attention of personnel is only directed to patients who experience hypoxemia. The present study was undertaken to better understand the effects of different settings of the pulse oximeter on false (artifact) and true (hypoxemia) alarms. Methods. Using the original SpO2 data of 200 postoperative patients, we calculated off-line the effects of five methods (artifact rejection, alarm delay (2–44 s, 2 s increments), averaging (10–90 s), median filtering (10–90 s) and decreasing the alarm limit from 90% to 85%) on the number of (true- and false) alarms. Results. 830 episodes of hypoxemia (SpO2 ≤ 90%) and 73 episodes of severe hypoxemia (SpO2 ≤ 85%) occurred. With a SpO2 alarm limit of 90%, the alarm was triggered 1535 times (830 true, 705 false). Artifact rejection reduced alarms by almost 50%. An alarm delay of 6 s or an averaging or median filtering epoch of 10 s resulted in an alarm reduction of almost 50%. No differences were found in the reduction of alarms between averaging and median filtering. Changing the alarm limit to 85% reduced the number of alarms by 82%. A similar reduction of alarms was obtained with either an alarm delay of 18 s or an averaging or median filtering epoch of 42 s. However, an alarm limit of 85% reduced the number of false alarms less than the other three algorithms (p < 0.01). Conclusions. The data from the present study suggest that in order to effectively suppress false alarms caused by pulse oximeter artifacts, it may be preferable to use a longer filtering epoch of approximately 40 s, rather than to decrease the lower alarm limit.

Development of Spinal Cord Ischemia After Clamping of Noncritical Segmental Arteries in the Pig

BACKGROUNDnBlood flow to the thoracolumbar spinal cord is thought to be critically dependent on the arteria radicularis magna. We investigated whether spinal cord blood supply becomes dependent on other, noncritical, segmental arteries if spinal cord perfusion pressure (SCPP) is decreased. The SCPP is equal to the mean arterial pressure (MAP) minus the cerebrospinal fluid (CSF) pressure (SCCP = MAP - CSF).nnnMETHODSnThe thoracoabdominal aorta was exposed in 10 pigs. Functional integrity of spinal cord motor pathways was assessed with myogenic motor-evoked potentials after transcranial electrical stimulation (tc-MEPs). Using this technique, a group of segmental arteries not critical for spinal cord blood supply was identified. Before, during, and after clamping of the noncritical segmental arteries, spinal cord ischemia was produced by decreasing SCPP by means of increasing CSF pressure, and the SCPP threshold at which tc-MEPs showed evidence of spinal cord ischemia was determined. Ischemic SCPP thresholds, obtained during and after clamping of the noncritical segmental arteries, were compared with the ischemic threshold obtained before clamping (control value).nnnRESULTSnBefore noncritical segmental arteries were clamped, ischemic tc-MEP changes occurred when the SCPP was below 15 +/- 5 (SD) mm Hg. With a total of 9 +/- 3 (SD) segmental arteries clamped, the ischemic SCPP threshold was 48 +/- 14 mm Hg (p < 0.01). After the release of all clamps, ischemia occurred at a SCPP of 15 +/- 5 (SD) mm Hg.nnnCONCLUSIONSnIn this porcine experiment, clamping of originally noncritical segmental arteries significantly reduced the tolerance of the spinal cord to a decrease in SCPP.

Epidural versus subdural spinal cord cooling: cerebrospinal fluid temperature and pressure changes

BACKGROUNDnRegional spinal cord cooling can increase the tolerable duration for spinal cord ischemia resulting from aortic clamping. We compared the efficacy of epidural and subdural cooling and the effect of the resulting cerebrospinal fluid-pressure (CSF) increases on spinal cord motor neuron function.nnnMETHODSnIn 8 pigs, CSF temperature and pressure were assessed in the subdural space at L4, T15, and T7. Saline was infused at 333, 666, and 999 ml/h at four consecutive locations: L4 subdural, L4 epidural, T15 subdural, and T15 epidural. First, the influence of CSF-pressure increases during normothermic infusion on transcranial motor evoked potentials (tc-MEPs) was assessed. Then, hypothermic infusion (4 degrees C) was performed to assess CSF-temperature changes.nnnRESULTSnDuring normothermic infusion, baseline CSF pressures increased uniformly from 6 +/- 4 mm Hg to 34 +/- 18, 42 +/- 17, and 50 +/- 18 mm Hg with increasing infusion rates (p < 0.001), and did not differ between epidural or subdural infusion. Tc-MEPs indicated spinal cord ischemia in 6 animals when CSF pressures reached 65 +/- 11 mm Hg. During hypothermic infusion, CSF temperatures decreased from 37 degrees to 35 +/- 1.2 degrees, 31 +/- 2.2 degrees, and 28 +/- 2.8 degrees C, but increasing CSF-temperature gradients were observed between the infusion location and distant segments. Subdural cooling resulted in lower CSF temperatures (p < 0.001), but caused larger CSF-pressure increases (p < 0.001).nnnCONCLUSIONSnSubdural and epidural infusion cooling produce localized spinal cord hypothermia in pigs. The concurrent pressure increases, however, are uniformly distributed and can result in tc-MEP evidence of ischemia.

Advanced pulse oximeter signal processing technology compared to simple averaging. II. effect on frequency of alarms in the postanesthesia care unit

STUDY OBJECTIVEnTo determine the effect of a new pulse oximeter (Nellcor Symphony N-3000, Pleasanton, CA) with signal processing technique (Oxismart) on the incidence of false alarms in the postanesthesia care unit (PACU).nnnDESIGNnProspective study.nnnSETTINGnNonuniversity hospital.nnnPATIENTSn603 consecutive ASA physical status I, II, and III patients recovering from general or regional anesthesia in the PACU.nnnINTERVENTIONSnWe compared the number of alarms produced by a recently developed third-generation pulse oximeter (Nellcor Symphony N-3000) with Oxismart signal processing technique and a conventional pulse oximeter (Criticare 504, Waukesha, WI). Patients were randomly assigned to either a Nellcor pulse oximeter or a Criticare with the signal averaging time set at either 12 or 21 seconds. For each patient the number of false (artifact) alarms was counted.nnnMEASUREMENTS AND MAIN RESULTSnThe Nellcor generated one false alarm in 199 patients and 36 (in 31 patients) loss of pulse alarms. The conventional pulse oximeter with the averaging time set at 12 seconds generated a total of 32 false alarms in 17 of 197 patients [compared with the Nellcor, relative risk (RR) 0.06, confidence interval (CI) 0.01 to 0.25] and a total of 172 loss of pulse alarms in 79 patients (RR 0.39, CI 0.28 to 0.55). The conventional pulse oximeter with the averaging time set at 21 seconds generated 12 false alarms in 11 of 207 patients (compared with the Nellcor, RR 0.09, CI 0.02 to 0.48) and a total of 204 loss of pulse alarms in 81 patients (RR 0.40, CI 0.28 to 0.56). The lower incidence of false alarms of the conventional pulse oximeter with the longest averaging time compared with the shorter averaging time did not reach statistical significance (false alarms RR 0.62, CI 0.3 to 1.27; loss of pulse alarms RR 0.98, CI 0.77 to 1.3).nnnCONCLUSIONSnTo date, this is the first report of a pulse oximeter that produced almost no false alarms in the PACU.

Motor evoked potentials

During the last decade, somatosensory evoked potentials (SSEP) have become established as a practical method for monitoring the spinal cord during various surgical procedures where there is a risk of paraplegia, e.g., scoliosis surgery, thoracic aortic surgery, and neurosurgical procedures upon the spinal cord. However, it has also become apparent that SSEP have limitations concerning their ability to monitor the entire spinal cord. SSEP travel exclusively in ascending sensory pathways (dorsal columns and posterolateral tracts). Accordingly, selective injury to the more anteriorly located motor tracts and motor neuronal systems in the central gray matter and anterior horn may go undetected. A number of case reports have described false negative results with SSEP monitoring, i.e., postoperative paraplegia despite unaltered intraoperative SSEP [4,27,38]. A recent survey by the Scoliosis Research Society among physicians performing intraoperative SSEP monitoring during spinal surgery revealed that five out of 27 major neurological complications (17%) that occurred with monitoring in place were not diagnosed by changes in SSEP [8]. Even if technical errors or lack of experience are taken into account that may have hampered the acquisition of reliable SSEP waveforms in some of these cases, this figure suggests that injury to the spinal cord is sometimes limited to the motor pathways. Given the differences in blood supply to the anterior and posterior spinal cord, there are several clinical situations where selective ischemia of the anterior part of the cord may ensue. This is particularly true for the thoracic spinal cord, where in some patients the anatomical variation of the anterior spinal artery may be such that interruption of a single intercostal or lumbar feeder vessel will result in spinal cord ischemia.