Changes in hematocrit based on incremental blood sampling: mathematical models perform poorly.

PURPOSE: Excessive blood sampling, with its inherent risks, is of growing concern among clinicians. We performed this study to measure the changes in hematocrit (Hct) during a laboratory investigation where multiple blood samples are collected. The performance of a simple mathematical model, used in clinical practice to predict Hct changes, is evaluated. METHODS: Eight healthy male volunteers participated in this study. The equation Hct(f) = Hct(i)*(EBV-BL)/EBV is used to predict changes in Hct. Where Hct(f) and Hct(i) are, respectively, the final and initial Hct, EBV is the estimated blood volume and BL is the blood loss. RESULTS: Thirty-five pharmacokinetic samples per subject were collected totalling 314 mL of BL. The Hct decreased from 44.2% +/- 2.2% to 39.9% +/- 2.5% (P = 0.001). On average, model predictions tended to have a discrete tendency to underestimate the Hct changes (-0.5% points of bias). While the predictions of the Hct were very accurate in 50% of the subjects, the discrepancy of the Hct predictions was clinically significant in the other 50% of the subjects. CONCLUSION: Consistent with the model prediction, this study demonstrated a significant reduction in the Hct values in healthy subjects undergoing incremental phlebotomy. On average, the model successfully predicted the decrease in Hct. However, the inter- and intra-individual variabilities in the Hct changes are clinically significant. In clinical settings, which are not well controlled environments, the variability is likely to be greater and the clinical use of the model cannot replace the need to monitor the Hct.

Dexmedetomidine pharmacodynamics: part I: crossover comparison of the respiratory effects of dexmedetomidine and remifentanil in healthy volunteers.

BACKGROUND: Dexmedetomidine, a highly selective alpha2-adrenoceptor agonist used for short-term sedation of mechanically ventilated patients, has minimal effect on ventilation. METHODS: This study compared the respiratory effect of dexmedetomidine to that of remifentanil. The authors measured and compared respiratory responses of six healthy male volunteers during (1) a stepwise target-controlled infusion of remifentanil, (2) a stepwise target-controlled infusion of dexmedetomidine, and (3) a pseudonatural sleep session. RESULTS: Compared with baseline, remifentanil infusions resulted in respiratory depression as evidenced by a decrease in respiratory rate and minute ventilation, respiratory acidosis, and apnea episodes resulting in desaturations. Remifentanil disturbed the natural pattern of breathing and flattened the distribution of ventilatory timing (inspiratory time/ventilatory cycle time). The respiratory effects of dexmedetomidine markedly contrasted with those of remifentanil. When compared with baseline, during dexmedetomidine infusions, the respiratory rate significantly increased, and the overall apnea/hypopnea index significantly decreased. The distribution of inspiratory time/ventilatory cycle time showed an increased peak. In addition, dexmedetomidine seemed to mimic some aspect of natural sleep. While the subjects were breathing a 5% CO2 mixture, hypercapnic arousal phenomena (documented by the Bispectral Index, the electroencephalogram, and sudden increase in the minute ventilation) were observed during dexmedetomidine infusions. Similar phenomena during natural sleep have been reported in the literature. CONCLUSIONS: In comparison with remifentanil, dexmedetomidine infusions (1) did not result in clinically significant respiratory depression, (2) decreased rather than increased the apnea/hypopnea index, and (3) exhibited some similarity with natural sleep.

Dexmedetomidine pharmacodynamics: Part II: Crossover comparison of the analgesic effect of dexmedetomidine and remifentanil in healthy volunteers.

BACKGROUND: Dexmedetomidine is a highly selective alpha2-adrenoceptor agonist used for short-term sedation of mechanically ventilated patients. The analgesic profile of dexmedetomidine has not been fully characterized in humans. METHODS: This study was designed to compare the analgesic responses of six healthy male volunteers during stepwise target-controlled infusions of remifentanil and dexmedetomidine. A computer-controlled thermode was used to deliver painful heat stimuli to the volar side of the forearms of the subjects. Six sequential 5-s stimuli (ranging from 41 degrees to 50 degrees C) were delivered in random order. The recorded visual analog scale was used to fit an Emax model. RESULTS: Compared to baseline, remifentanil infusions resulted in a right shift of the sigmoid curve (increased T50, the temperature producing a visual analog scale score of 50% of the maximal effect, from 46.1 degrees C at baseline to 48.4 degrees and 49.1 degrees C during remifentanil infusions) without a change of the steepness of the curve (identical Hill coefficients gamma during baseline and remifentanil). Compared to baseline, dexmedetomidine infusions resulted in both a right shift of the sigmoid curve (increased T50 to 47.2 degrees C) and a decrease in the steepness of the curve (decreased gamma from 3.24 during baseline and remifentanil infusions to 2.45 during dexmedetomidine infusions). There was no difference in the pain responses between baseline and after recovery from remifentanil infusions (identical T50 and gamma). CONCLUSION: As expected, dexmedetomidine is not as effective an analgesic as the opioid remifentanil. The difference in the quality of the analgesia with remifentanil may be a reflection of a different mechanism of action or a consequence of the sedative effect of dexmedetomidine.

Intraoperative fluid management during orthotopic liver transplantation.

OBJECTIVE: To assess clinical safety of a low central venous pressure (CVP) fluid management strategy in patients undergoing liver transplantation. DESIGN: Retrospective record review comparing 2 transplant centers, one using the low CVP method and the other using the normal CVP method. SETTING: University-based, academic, tertiary care centers. PARTICIPANTS: Patients undergoing orthotopic cadaveric liver transplantation. INTERVENTIONS: Each center practiced according to its own standard of care. Center 1 maintained an intraoperative CVP <5 mmHg using fluid restriction, nitroglycerin, forced diuresis, and morphine. If pressors were required to maintain systolic arterial pressure >90 mmHg, phenylephrine or norepinephrine was used. At center 2, CVP was kept 7 to 10 mmHg and mean arterial pressure >75 mmHg with minimal use of vasoactive drugs. MEASUREMENTS AND MAIN RESULTS: Data collected included United Network for Organ Sharing status, surgical technique, intraoperative transfusion rate, preoperative and peak postoperative creatinine, time spent in intensive care unit and hospital, incidence of death, and postoperative need for hemodialysis. Principal findings include an increased rate of transfusion in the normal CVP group but increased rates of postoperative renal failure (elevated creatinine and more frequent need for dialysis) and 30-day mortality in the low CVP group. CONCLUSIONS: Despite success in lowering blood transfusion requirements in liver resection patients, a low CVP should be avoided in patients undergoing liver transplantation.

Intraoperative colloid administration reduces postoperative nausea and vomiting and improves postoperative outcomes compared with crystalloid administration.

The debate over colloid versus crystalloid as the best solution for intraoperative fluid resuscitation is not resolved. Published studies have shown that mortality is not related to the specific fluid used for resuscitation. In addition, the quality of postoperative recovery between colloid and crystalloid has not been well investigated. In a prospective, blinded fashion, we investigated the effects of colloid and crystalloid resuscitation on nausea and vomiting and on the postoperative patient recovery profile. Patients undergoing major elective noncardiac surgery were randomized to receive 6% hetastarch in saline (HS-NS), 6% hetastarch in balanced salt (HS-BS), or lactated Ringer's solution (LR) on the basis of a fluid administration algorithm. The anesthetic was standardized. Hemodynamic targets included maintenance of arterial blood pressure, heart rate, and urine output within a predefined range. A postoperative morbidity survey was performed at baseline and daily after surgery. Ninety patients participated in the study, with 30 patients in each group. The amounts of study fluid (mean +/- SD) administered were 1301 +/- 1079 mL, 1448 +/- 759 mL, and 5946 +/- 1909 mL for the HS-NS, HS-BS, and LR groups, respectively (P < 0.05, HS-NS and HS-BS versus LR). Both the HS-NS and HS-BS (colloid) groups had a significantly less frequent incidence of nausea and vomiting, use of rescue antiemetics, severe pain, periorbital edema, and double vision. We concluded that intraoperative fluid resuscitation with colloid, when compared with crystalloid administration, is associated with an improvement in the quality of postoperative recovery.

Goal-directed intraoperative fluid administration reduces length of hospital stay after major surgery.

BACKGROUND: Intraoperative hypovolemia is common and is a potential cause of organ dysfunction, increased postoperative morbidity, length of hospital stay, and death. The objective of this prospective, randomized study was to assess the effect of goal-directed intraoperative fluid administration on length of postoperative hospital stay. METHODS: One hundred patients who were to undergo major elective surgery with an anticipated blood loss greater than 500 ml were randomly assigned to a control group (n = 50) that received standard intraoperative care or to a protocol group (n = 50) that, in addition, received intraoperative plasma volume expansion guided by the esophageal Doppler monitor to maintain maximal stroke volume. Length of postoperative hospital stay and postoperative surgical morbidity were assessed. RESULTS: Groups were similar with respect to demographics, surgical procedures, and baseline hemodynamic variables. The protocol group had a significantly higher stroke volume and cardiac output at the end of surgery compared with the control group. Patients in the protocol group had a shorter duration of hospital stay compared with the control group: 5 +/- 3 versus 7 +/- 3 days (mean +/- SD), with a median of 6 versus 7 days, respectively ( = 0.03). These patients also tolerated oral intake of solid food earlier than the control group: 3 +/- 0.5 versus 4.7 +/- 0.5 days (mean +/- SD), with a median of 3 versus 5 days, respectively ( = 0.01). CONCLUSIONS: Goal-directed intraoperative fluid administration results in earlier return to bowel function, lower incidence of postoperative nausea and vomiting, and decrease in length of postoperative hospital stay.

Orthotopic liver transplant patients require less postoperative morphine than do patients undergoing hepatic resection.

STUDY OBJECTIVE: To compare postoperative morphine use, analgesic efficacy, and side effect profiles in patients following orthotopic liver transplantation (OLTx) and liver resection (LR). DESIGN: Retrospective study. SETTING: Liver transplant and liver resection surgery at a university hospital. PATIENTS: 25 ASA physical status I, II, III, and IV patients undergoing OLTx or liver resection. MEASUREMENTS AND MAIN RESULTS: Morphine use was significantly decreased in the OLTx patients at 6,12, 24, 48, and 72 hours following commencement of patient-controlled analgesia. After commencement of patient-controlled analgesia, pain scores were significantly reduced in the OLTx group compared with those in the liver resection group at 6 and 12 hours. CONCLUSIONS: Orthotopic liver transplant patients experienced less pain and used less morphine postoperatively than did liver resection patients.

Vascular stenting.

An explosion of technology has occurred in the last 10 years, intended to make treatment of vascular diseases less invasive. Once the exclusive domain of the interventional cardiologist and the coronary circulation, now in 2001 nearly every vascular system has been explored as a site for endovascular treatment of aneurysmal and atherosclerotic disease. This review will focus on endovascular treatment of abdominal aortic aneurysmal disease and carotid artery disease, and relevant issues for the anesthesiologist encountering these patients and procedures.