Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role.

UNLABELLED: We tested dexmedetomidine, an alpha(2) agonist that decreases heart rate, blood pressure, and plasma norepinephrine concentration, for its ability to attenuate stress responses during emergence from anesthesia after major vascular operations. Patients scheduled for vascular surgery received either dexmedetomidine (n = 22) or placebo (n = 19) IV beginning 20 min before the induction of anesthesia and continuing until 48 h after the end of surgery. All patients received standardized anesthesia. Heart rate and arterial blood pressure were kept within predetermined limits by varying anesthetic level and using vasoactive medications. Heart rate, arterial blood pressure, and inhaled anesthetic concentration were monitored continuously; additional measurements included plasma and urine catecholamines. During emergence from anesthesia, heart rate was slower with dexmedetomidine (73 +/- 11 bpm) than placebo (83 +/- 20 bpm) (P = 0.006), and the percentage of time the heart rate was within the predetermined hemodynamic limits was more frequent with dexmedetomidine (P < 0.05). Plasma norepinephrine levels increased only in the placebo group and were significantly lower for the dexmedetomidine group during the immediate postoperative period (P = 0.0002). We conclude that dexmedetomidine attenuates increases in heart rate and plasma norepinephrine concentrations during emergence from anesthesia. IMPLICATIONS: The alpha(2) agonist, dexmedetomidine, attenuates increases in heart rate and plasma norepinephrine concentrations during emergence from anesthesia in vascular surgery patients.

The Hemodynamic and Adrenergic Effects of Perioperative Dexmedetomidine Infusion after Vascular Surgery

Consistent anatomic accessibility, ease of cannulation, and a low rate of complications have made the radial artery the preferred site for arterial cannulation. Radial artery catheterization is a relatively safe procedure with an incidence of permanent ischemic complications of 0.09%. Although its anatomy in the forearm and the hand is variable, adequate collateral flow in the event of radial artery thrombosis is present in most patients. Harvesting of the radial artery as a conduit for coronary artery bypass grafting, advances in plastic and reconstructive surgery of the hand, and its use as an entry site for cardiac catheterization has provided new insight into the collateral blood flow to the hand and the impact of radial arterial instrumentation. The Modified Allen's Test has been the most frequently used method to clinically assess adequacy of ulnar artery collateral flow despite the lack of evidence that it can predict ischemic complications in the setting of radial artery occlusion. Doppler ultrasound can be used to evaluate collateral hand perfusion in an effort to stratify risk of potential ischemic injury from cannulation. Limited research has demonstrated a beneficial effect of heparinized flush solutions on arterial catheter patency but only in patients with prolonged monitoring (>24 h). Conservative management may be equally as effective as surgical intervention in treating ischemic complications resulting from radial artery cannulation. Limited clinical experience with the ultrasound-guided arterial cannulation method suggests that this technique is associated with increased success of cannulation with fewer attempts. Whether use of the latter technique is associated with a decrease in complications has not yet been verified in prospective studies. Research is needed to assess the safety of using the ulnar artery as an alternative to radial artery cannulation because the proximity and attachments of the ulnar artery to the ulnar nerve may potentially expose it to a higher risk of injury.

Radial artery cannulation: a comprehensive review of recent anatomic and physiologic investigations.

SUMMARY The effects of the new, highly selective alpha2-adrenergic agonist, dexmedetomidine, were studied in a randomized, placebo-controlled, double-blind trial in 24 ASA I patients. Dexmedetomidine 0.6 μg kg−1 or saline was given i.v. 10 min before induction of anaesthesia. The required dose of thiopentone was significantly (P

Dexmedetomidine attenuates sympathoadrenal responses to tracheal intubation and reduces the need for thiopentone and peroperative fentanyl

Dexmedetomidine is a selective alpha2-adrenoceptor agonist with centrally mediated sympatholytic, sedative, and analgesic effects. This study evaluated: 1) pharmacokinetics of dexmedetomidine in plasma and cerebrospinal fluid (CSF) in surgical patients; 2) precision of a computer-controlled infusion

Postoperative pharmacokinetics and sympatholytic effects of dexmedetomidine.

The authors examined the effect of clonidine, a preferential alpha 2-adrenergic agonist, upon narcotic requirements, hemodynamics, and adrenergic responses during the perioperative period in patients undergoing CABG surgery. Anesthesia was provided by sufentanil supplemented with isoflurane; sodium nitroprusside was given as needed for hemodynamic control. Ten patients received oral clonidine preoperatively at the time of premedication, and again intraoperatively by nasogastric tube. Another group of ten untreated patients were otherwise managed identically. Intergroup differences in required anesthetic and vasoactive drug doses and recovery times were measured and evaluated, as well as hemodynamics and plasma catecholamines prior to induction, after intubation, and at intervals intra- and postoperatively. Patients who received clonidine required less diazepam prior to induction, and received 40% less sufentanil during the anesthetic period, than did untreated controls. More control patients required the addition of isoflurane to prevent hypertension. Mean blood pressures and heart rates were elevated at many sampling points in patients not treated with clonidine. Four of the clonidine-treated group required atropine for treatment of bradycardia in the pre-incision period. Plasma catecholamines were significantly lower throughout most of the study period in patients treated with clonidine. After cardiopulmonary bypass and postoperatively, cardiac outputs were significantly higher in the treated group. Patients who had received clonidine were extubated significantly earlier, and fewer of them shivered postoperatively. We conclude that perioperative treatment with clonidine reduced narcotic and anesthetic requirements, improved hemodynamics, reduced plasma catecholamines, and shortened the period of postoperative ventilation in patients undergoing coronary artery surgery.

Reduced narcotic requirement by clonidine with improved hemodynamic and adrenergic stability in patients undergoing coronary bypass surgery.

Reduced Narcotic Requirement by Clonidine with Improved Hemodynamic and Adrenergic Stability in Patients Undergoing Coronary Bypass Surgery

Collateral circulation of the hand and digits was evaluated in 20 healthy male volunteers using a modified Allen’s test and photoelectric plethysmography. A borderline (7–14 sec) Allen’s test was present in 12.5% and 2.5% required more than 15 secons for thenar flush. Photoplethysmography was then used to compare the relative contributions of ulnar artery flow to total arterial flow into the hand. This ratio, (ulnar/total) was ternmed the flow index (FI). Average FI for the first digit (thumb) was 0.59, and for the second digit (index) was 0.64. The FI difference between the first digit and that of the third, fourth and fifth digits was statistically significant (p<0.01). The FI difference between the second digit and the third, fourth and fifth digits was also statistically significant (p<0.05). These results imply that Allen’s test may be misleading, indicating adequate collateral circulation when in fact collateral blood flow to the digits may be poor or nonexistent.

Assessment of digital blood flow and palmar collateral circulation. Allen's test vs. photoplethysmography.

Three groups of New Zealand white rabbits were used to study the effects of captopril on the renin-angiotensin system and sympathetic nervous system during sodium nitroprusside (SNP)-induced hypotension and halothane anesthesia. Two groups of rabbits (C and CH) were treated with captopril 2 mg/kg i.v. One captopril-treated group (CH) and the third, untreated group (H) received SNP to induce hypotension. In these two groups, the mean arterial blood pressure (MAP) was reduced by 40% for 150 min. Group C did not undergo SNP-induced hypotension and served to document the effects of captopril alone during the 150-min study period. Arterial blood samples for norepinephrine (NE), epinephrine (EPI), and plasma renin activity (PRA) were drawn prior to, during hypotension, and in the recovery period. The SNP dose required to maintain the hypotension was continuously recorded. NE, EPI, and PRA all increased in group H, indicating activation of both the renin-angiotensin system and the sympathetic system during hypotension. This was accompanied by a dramatic increase in SNP dose requirement. In the CH group, PRA levels rose sharply and remained elevated. Plasma NE levels increased, while EPI levels remained unchanged with a decline in the SNP dose requirement. The C group demonstrated a rise in PRA levels, accompanied by unchanged NE and EPI levels and MAP during the study period. Captopril administration decreased the SNP dose requirement and significantly decreased the sympathetic response (measured by NE and EPI levels) in group CH as compared to the H group.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of captopril on the renin-angiotensin system and the sympathetic nervous system during sodium nitroprusside-induced hypotension in the halothane-anesthetized rabbit.

HE OPHTHALMIC patient is at a particular risk from anesthesia. Patients submitted for ophthalmic surgery represent the extremes of age: the very young and the very old. As such, the underlying medical condition of the patient may be compromised, as well as the patient's response to pharmacological agents may be exaggerated. Frequently, these patients are receiving medications for their ophthalmic needs that may have significant impact on anesthetic management and coexisting disease.

Stanley W. Stead

Papers

Reduced narcotic requirement by clonidine with improved hemodynamic and adrenergic stability in patients undergoing coronary bypass surgery.

Reduced Narcotic Requirement by Clonidine with Improved Hemodynamic and Adrenergic Stability in Patients Undergoing Coronary Bypass Surgery

Assessment of digital blood flow and palmar collateral circulation. Allen's test vs. photoplethysmography.

The effects of captopril on the renin-angiotensin system and the sympathetic nervous system during sodium nitroprusside-induced hypotension in the halothane-anesthetized rabbit.

Complications in Ophthalmic Anesthesiology