Jeremy Lieberman

Critical Oxygen Delivery in Conscious Humans Is Less Than 7.3 ml O2· kg−1· min−1

Background The “critical” level of oxygen delivery (DO2) is the value below which DO2 fails to satisfy the metabolic need for oxygen. No prospective data in healthy, conscious humans define this value. The authors reduced DO2 in healthy volunteers in an attempt to determine the critical DO2. Methods With Institutional Review Board approval and informed consent, the authors studied eight healthy, conscious volunteers, aged 19–25 yr. Hemodynamic measurements were obtained at steady state before and after profound acute isovolemic hemodilution with 5% albumin and autologous plasma, and again at the reduced hemoglobin concentration after additional reduction of DO2 by an infusion of a &bgr;-adrenergic antagonist, esmolol. Results Reduction of hemoglobin from 12.5 ± 0.8 g/dl to 4.8 ± 0.2 g/dl (mean ± SD) increased heart rate, stroke volume index, and cardiac index, and reduced DO2 (14.0 ± 2.9 to 9.9 ± 2.0 ml O2 · kg−1 · min−1; all P < 0.001). Oxygen consumption (VO2; 3.0 ± 0.5 to 3.4 ± 0.6 ml O2 · kg−1 · min−1;P < 0.05) and plasma lactate concentration (0.50 ± 0.10 to 0.62 ± 0.16 mM;P < 0.05; n = 7) increased slightly. Esmolol decreased heart rate, stroke volume index, and cardiac index, and further decreased DO2 (to 7.3 ± 1.4 ml O2 · kg−1 · min−1; all P < 0.01 vs. before esmolol). VO2 (3.2 ± 0.6 ml O2 · kg−1 · min−1;P > 0.05) and plasma lactate (0.66 ± 0.14 mM;P > 0.05) did not change further. No value of plasma lactate exceeded the normal range. Conclusions A decrease in DO2 to 7.3 ± 1.4 ml O2 · kg−1 · min−1 in resting, healthy, conscious humans does not produce evidence of inadequate systemic oxygenation. The critical DO2 in healthy, resting, conscious humans appears to be less than this value.

Fresh Blood and Aged Stored Blood Are Equally Efficacious in Immediately Reversing Anemia-induced Brain Oxygenation Deficits in Humans

Background:Erythrocytes are transfused to treat or prevent imminent inadequate tissue oxygenation. 2,3-diphosphoglycerate concentration decreases and oxygen affinity of hemoglobin increases (P50 decreases) with blood storage, leading some to propose that erythrocytes stored for 14 or more days do not release sufficient oxygen to make their transfusion efficacious. The authors tested the hypothesis that erythrocytes stored for 3 weeks are as effective in supplying oxygen to human tissues as are erythrocytes stored for less than 5 h. Methods:Nine healthy volunteers donated 2 units of blood more than 3 weeks before they were tested with a standard, computerized neuropsychological test (digit–symbol substitution test [DSST]) on 2 days, 1 week apart, before and after acute isovolemic reduction of their hemoglobin concentration to 7.4 and 5.5 g/dl. Volunteers randomly received autologous erythrocytes stored for either less than 5 h (“fresh”) or 3 weeks (“stored”) to return their hemoglobin concentration to 7.5 g/dl (double blinded). Erythrocytes of the alternate storage duration were transfused on the second experimental day. The DSST was repeated after transfusion. Results:Acute anemia slowed DSST performance equivalently in both groups. Transfusion of stored erythrocytes with decreased P50 reversed the altered DSST (P < 0.001) to a time that did not differ from that at 7.4 g/dl hemoglobin during production of acute anemia (P = 0.88). The erythrocyte transfusion–induced DSST improvement did not differ between groups (P = 0.96). Conclusion:Erythrocytes stored for 3 weeks are as efficacious as are erythrocytes stored for 3.5 h in reversing the neurocognitive deficit of acute anemia. Requiring fresh rather than stored erythrocytes for augmentation of oxygen delivery does not seem warranted.

Electrocardiographic ST-segment changes during acute, severe isovolemic hemodilution in humans.

BackgroundControversy exists regarding the lowest blood hemoglobin concentration that can be safely tolerated. The authors studied healthy resting humans to test the hypothesis that acute isovolemic reduction of blood hemoglobin concentration to 5 g/dl would produce an imbalance in myocardial oxygen supply and demand, resulting in myocardial ischemia. MethodsFifty-five conscious healthy human volunteers were studied. Isovolemic removal of aliquots of blood reduced blood hemoglobin concentration from 12.8 ± 1.2 to 5.2 ± 0.5 g/dl (mean ± SD). Removed blood was replaced simultaneously with intravenous fluids to maintain constant isovolemia. Hemodynamics and arterial oxygen content (Cao2) were measured before and after removal of each aliquot of blood. Electrocardiographic (ECG) changes were monitored continuously using a Holter ECG recorder for detection of myocardial ischemia. ResultsDuring hemodilution, transient, reversible ST-segment depression developed in three subjects as seen on the electrocardiogram during hemodilution. These changes occurred at hemoglobin concentrations of 5–7 g/dl while the subjects were asymptomatic. Two of three subjects with ECG changes had significantly higher heart rates than those without ECG changes at the same hemoglobin concentrations. When evaluating the entire study period, the subjects who had ECG ST-segment changes had significantly higher maximum heart rates than those without ECG changes, despite having similar baseline values. ConclusionWith acute reduction of hemoglobin concentration to 5 g/dl, ECG ST-segment changes developed in 3 of 55 healthy conscious adults and were suggestive of, but not conclusive for, myocardial ischemia. The higher heart rates that developed during hemodilution may have contributed to the development of an imbalance between myocardial supply and demand resulting in ECG evidence of myocardial ischemia. However, these ECG changes appear to be benign because they were reversible and not accompanied by symptoms.

Women are at greater risk than men for malpositioning of the endotracheal tube after emergent intubation

ObjectivesTo investigate the occurrence of endotracheal tube malpositioning after emergent intubation in critically ill adults and to determine the need for a routine postintubation chest radiograph to assess endotracheal tube position. DesignProspective study. SettingAll adult critical care and acute care units of a 560-bed university teaching hospital. PatientsStudy of 297 consecutive intubations (185 intubations in males and 112 intubations in females) in 238 adult patients. MethodsEmergent endotracheal intubations were performed by resident physicians with supervision from an intensive care unit (ICU) or anesthesia attending physician or an anesthesia resident. After intubation, proper positioning of the endotracheal tube was verified by the intubating physician using clinical criteria, including auscultation of bilateral breath sounds, symmetric chest expansion, and palpation of the endotracheal tube cuff in the suprasternal notch. The endotracheal tube position relative to the lower anterior incisors or alveolar ridge was recorded using the centimeter markings printed on the endotracheal tube. A chest radiograph was obtained after intubation to verify endotracheal tube position. Appropriate endotracheal tube position on chest radiograph was defined as between >2 and ≤6 cm above the carina. Measurements and Main ResultsOf the 297 intubations, 26 were excluded from analysis because a chest radiograph was not obtained or the patient was not of normal stature. For the remaining 271 intubations, 42 (15.5%) endotracheal tubes were inappropriately placed, according to the radiographic assessment. The percentage of malpositioned endotracheal tubes was significantly higher in women than in men (61.9% vs. 38.1%, respectively; chi-square: p < .001). Thirty-three (78.6%) of 42 malpositioned endotracheal tubes were placed <2 cm from the carina, with the highest occurrence (24/33) of proximal malposition occurring in women. Positioning of endotracheal tubes using the centimeter markings printed on the tube referenced to the lower incisors did not accurately identify malposition as documented by chest radiograph. ConclusionsEmergent endotracheal intubations result in a significant occurrence of malpositioned endotracheal tubes that are undetected by clinical evaluation. Malpositioning is not detected by routine clinical assessment, but only by chest radiograph. Women are at greater risk than men for endotracheal tube malpositioning after emergent intubation; in women, the endotracheal tube is more likely to be positioned too close to the carina. A chest radiograph for confirmation of endotracheal tube position after emergent intubation should remain the standard of practice. (Crit Care Med 1994; 22:1127–1131)

Optimal Head Rotation for Internal Jugular Vein Cannulation When Relying on External Landmarks

External anatomic landmarks have traditionally been used to approximate the location of the neck blood vessels to optimize central venous cannulation of the internal jugular vein (IJV) while avoiding the common carotid artery (CCA). Head rotation affects vessel orientation, but most landmark techniques do not specify its optimal degree. We simulated catheter insertion via both an anterior and central approach to the right IJV using an ultrasound probe held in the manner of a syringe and needle in 49 volunteers. Increased head rotation from 0°, 15°, 30°, 45°, and 60° to the left of midline was associated with higher probability of a simulated needle contacting the IJV and the CCA. For both approaches, the risk of CCA contact was <10% for head rotations of ≤45°. Increased body surface area (BSA) and body mass index (BMI) were associated with more CCA contact at head rotations of 45° or 60°. To optimize IJV contact while reducing the likelihood of inadvertent contact with the CCA, the head should be rotated no more than 30° in patients with high BMI or BSA, but it may be turned to 60° if BMI or BSA is low.

Heart rate increases linearly in response to acute isovolemic anemia

BACKGROUND : The cardiovascular response to acute isovolemic anemia in humans is thought to differ from that of other species. Studies of anesthetized humans have found either no change or a decreased heart rate. A previous study showed that in 32 healthy unmedicated humans, heart rate increased during acute isovolemic anemia. The hypothesis that heart rate in humans increases in response to acute isovolemic anemia and that the increase is affected by gender was tested.

The efficacy of motor evoked potentials in fixed sagittal imbalance deformity correction surgery.

Study Design. Retrospective analysis of Transcranial Motor Evoked potential (TcMEP) responses and clinical outcome. Objective. To determine the sensitivity and specificity of TcMEPs to detect and predict isolated nerve root injury in selected patients having complex lumbar spine surgery. Summary of Background Data. The surgical correction of fixed sagittal plane deformity involves posterior-based osteotomies and significant changes in the length of and space for the neural elements. The role of transcranial motor-evoked potential (TcMEP) monitoring in osteotomies below the conus has not been established. The purpose of this paper is to describe the relationship between neural complications from surgery and intraoperative TcMEP changes. Methods. We retrospectively studied 35 consecutive patients in a single center treated with posterior-based osteotomies for the correction of fixed sagittal plane deformity. Transcranial motor-evoked potentials, free-running and evoked electromyography data were assessed for each case. Analysis includes description of the intraoperative changes observed, and a correlation of changes with postoperative clinical findings. Results. Thirty-five consecutive patients underwent surgery for fixed sagittal plane deformity with complete neuromonitoring data. Twenty-five patients (71%) had an episode of greater than 80% reduction in MEP amplitude to at least 1 muscle. Fifteen of 25 had improvement of TcMEPs after repositioning of the legs (1), additional surgical decompression (4), or volume and pharmacologic resuscitation (10). All 15 of these awoke with no detectable neurologic injury. Ten patients (29%) had reduced TcMEP signals that did not improve despite further decompression and manipulation of the osteotomy site. All 10 had a greater than 67% drop in TcMEPs for at least 1 muscle persisting at the end of the case, and all had a postoperative neurologic deficit. The TcMEP changes in patients who demonstrated nerve injury postoperativelywere observed most often during osteotomy closure or sustained dural retraction. 9 patients had weakness involving the iliopsoas or quadriceps; 1 patient had isolated unilateral dorsiflexion weakness. Monitoring TcMEPs in multiple muscle groups was both highly sensitive and specific for predicting injury. Nine patients had recovered motor function completely by discharge, and all but 1 patient (grade 4/5) had a normal motor examination at 6-week follow-up. Conclusion. The use of TcMEPs is sensitive and specific to change in neural function. No patients had a false negative test. The rate of neural deficits is consistent with previous literature, suggesting that TcMEP monitoring may not prevent neural injury. However, there were several cases in which intraoperative intervention resulted in recovery of TcMEPs, and none of these patients sustained any postoperative neural deficit. The severity of neural deficits in this series was minor and the duration was limited. TcMEPs may contribute to calling attention to the need for intraoperative corrections including widening decompressions, improving perfusion, and limiting deformity correction so that more severe neural compromise may be prevented.

The effect of age on motor evoked potentials in children under Propofol/Isoflurane Anesthesia

Intraoperative transcranial motor evoked potential (MEP) monitoring may help prevent neurologic injury during spine surgery. This type of monitoring may be difficult in the pediatric population under general anesthesia. We retrospectively reviewed data from 56 children, aged 2 to 18 yr, who were to undergo surgical correction of idiopathic scoliosis with MEP monitoring. Under combined isoflurane-propofol general anesthesia, before incision, we examined the minimum stimulating threshold voltage required to achieve a 50-microvolt or greater MEP response amplitude. Younger age was associated with an increase in the threshold voltage needed to elicit a sufficient MEP response. In addition, younger age was associated with longer stimulating pulse trains and greater need to adjust stimulating scalp electrodes. Body surface area, height, weight, and body mass index were also significant factors, but they were not independent predictors, after adjusting for age. Younger children received significantly lower levels of isoflurane and comparable doses of propofol, compared with older patients. Stronger stimulation needed to produce MEP responses in younger patients may reflect immaturity of their central nervous system, specifically conduction by the descending corticospinal motor tracts. Greater attention must be given to optimizing physiologic variables, limiting depressant anesthetics, and selecting the most favorable stimulating conditions in children, especially those <10 yr old.

Fresh and stored red blood cell transfusion equivalently induce subclinical pulmonary gas exchange deficit in normal humans.

BACKGROUND: Transfusion can cause severe acute lung injury, although most transfusions do not seem to induce complications. We tested the hypothesis that transfusion can cause mild pulmonary dysfunction that has not been noticed clinically and is not sufficiently severe to fit the definition of transfusion-related acute lung injury. METHODS: We studied 35 healthy, normal volunteers who donated 1 U of blood 4 weeks and another 3 weeks before 2 study days separated by 1 week. On study days, 2 U of blood were withdrawn while maintaining isovolemia, followed by transfusion with either the volunteers autologous fresh red blood cells (RBCs) removed 2 hours earlier or their autologous stored RBCs (random order). The following week, each volunteer was studied again, transfused with the RBCs of the other storage duration. The primary outcome variable was the change in alveolar to arterial difference in oxygen partial pressure (AaDO2) from before to 60 minutes after transfusion with fresh or older RBCs. RESULTS: Fresh RBCs and RBCs stored for 24.5 days equally (P = 0.85) caused an increase of AaDO2 (fresh: 2.8 mm Hg [95% confidence interval: 0.8–4.8; P = 0.007]; stored: 3.0 mm Hg [1.4–4.7; P = 0.0006]). Concentrations of all measured cytokines, except for interleukin-10 (P = 0.15), were less in stored leukoreduced (LR) than stored non-LR packed RBCs; however, vascular endothelial growth factor was the only measured in vivo cytokine that increased more after transfusion with LR than non-LR stored packed RBCs. Vascular endothelial growth factor was the only cytokine tested with in vivo concentrations that correlated with AaDO2. CONCLUSION: RBC transfusion causes subtle pulmonary dysfunction, as evidenced by impaired gas exchange for oxygen, supporting our hypothesis that lung impairment after transfusion includes a wide spectrum of physiologic derangements and may not require an existing state of altered physiology. These data do not support the hypothesis that transfusion of RBCs stored for >21 days is more injurious than that of fresh RBCs.

Strategies for managing decreased motor evoked potential signals while distracting the spine during correction of scoliosis.

Surgical correction of kyphoscoliosis may result in spinal cord injury and neurologic deficits. Monitoring somatosensory evoked potentials (SSEPs) and transcranial motor evoked potentials (MEPs) intraoperatively may allow for early detection and reversal of spinal cord injury. Controlled hypotension and isovolemic hemodilution are often used during these cases to reduce blood loss and transfusion. However, these physiologic parameters may affect the quality of SSEP and MEP signals. Acute reduction or loss of MEP or SSEP signals during spinal distraction presents a crisis for the operative team: should distraction be immediately relieved? The authors describe three patients who showed a decrease in evoked potential signals under hypotensive, hemodiluted conditions at the stage of spinal distraction. Each case illustrates a different strategy for successful management of these patients.