Induced hypocapnia is effective in treating pulmonary hypertension following mitral valve replacement

BACKGROUND

Mitral valve stenosis is often associated with increased pulmonary vascular resistance resulting in pulmonary hypertension, which may lead to or exacerbate right heart dysfunction. Hypocapnia is a known pulmonary vasodilator. The purpose of this study was to evaluate whether induced hypocapnia is an effective treatment for pulmonary hypertension following elective mitral valve replacement in adults.

METHODS

In a prospective, crossover controlled trial, 8 adult patients with mitral stenosis were studied in the intensive care unit following elective mitral valve replacement. Hypocapnia was induced by removal of previously added dead space. Normocapnic (baseline), hypocapnic and recovery hemodynamic parameters including cardiac output, pulmonary vascular resistance, pulmonary artery pressure and systemic oxygen delivery and consumption were recorded.

RESULTS

Moderate hypocapnia (an end-tidal carbon dioxide concentration reduced to 28 +/- 5 mmHg) resulted in decreases in pulmonary vascular resistance and mean pulmonary artery pressure of 33% and 25%, respectively. Hypocapnia had no other hemodynamic or respiratory effects. The changes in pulmonary vascular resistance and mean pulmonary artery pressure were reversible.

CONCLUSION

Moderate hypocapnia was effective in decreasing pulmonary vascular tone in adults following mitral valve replacement. The application of this maneuver in the immediate postoperative period may provide a bridge until pulmonary vascular tone begins to normalize following surgery.

Efficacy of preoxygenation using tidal volume and deep breathing techniques with and without prior maximal exhalation

PURPOSE

We evaluated the influence of prior maximal exhalation on preoxygenation in 15 adult volunteers using tidal volume breathing (TVB) for five minutes and deep breathing (DB) for two minutes with and without prior maximal exhalation.

METHODS

Inspired and end-tidal oxygen, nitrogen and carbon dioxide were monitored continuously and recorded during room air breathing and at 30-sec intervals during 100% oxygen TVB or DB (rate of 8 breaths.min(-1)).

RESULTS

Tidal volume breathing with prior maximal exhalation resulted in an end-tidal oxygen concentration (ETO(2)) slightly higher (P = 0.028) at 0.5 and 1.0 min as compared with TVB without prior maximal exhalation at the same time periods. Regardless of whether TVB was preceded by maximal exhalation or not, 2.5 min was required to reach a mean ETO(2) value of 90% or higher. With DB, there were no differences in ETO(2) values at any time period and 1.5 min was required to reach an ETO(2) of 90% or greater, with or without prior maximal exhalation.

CONCLUSIONS

Maximal exhalation prior to TVB slightly steepens the initial rise in ETO(2) during the first minute, but confers no real benefit if maximal preoxygenation is the goal. Maximal exhalation prior to DB has no added value in enhancing preoxygenation.

Isoflurane, but Not Sevoflurane, Increases Transendothelial Albumin Permeability in the Isolated Rat Lung

Background

Caveolae mediated transendothelial transport of albumin has recently been shown to be the primary mechanism regulating microvascular endothelial albumin permeability. The authors investigated the effects of isoflurane and sevoflurane on pulmonary endothelial albumin permeability and assessed the potential role of the caveolae scaffold protein, caveolin-1, in these effects.

Methods

Isolated rat lungs and cultured rat lung microvessel endothelial cells (RLMVECs) were exposed to 1.0 or 2.0 minimum alveolar concentration (MAC) isoflurane or sevoflurane for 30 min. I-albumin permeability-surface area product and capillary filtration coefficient were determined in the isolated lungs. In RLMVECs, uptake and transendothelial transport of I-albumin were measured in the absence and presence of pretreatment with 2 mm methyl-beta-cyclodextrin, a caveolae-disrupting agent. Uptake of fluorescent-labeled albumin, as well as phosphorylation of Src kinase and caveolin-1, was also determined. In Y14F-caveolin-1 mutant (nonphosphorylatable) expressing RLMVECs, uptake of I-albumin and phosphorylation of caveolin-1 were evaluated.

Results

In the isolated lungs, 2.0 MAC isoflurane increased I-albumin permeability-surface area product by 48% without affecting capillary filtration coefficient. In RLMVECs, isoflurane more than doubled the uptake of I-albumin and caused a 54% increase in the transendothelial transport of I-albumin. These effects were blocked by pretreatment with methyl-beta-cyclodextrin. The isoflurane-induced increase in uptake of I-albumin in wild-type RLMVECs was abolished in the Y14F-caveolin-1 mutant expressing cells. Isoflurane also caused a twofold increase in Src and caveolin-1 phosphorylation. Neither 1.0 MAC isoflurane nor 1.0 or 2.0 MAC sevoflurane affected any index of albumin transport or phosphorylation of caveolin-1.

Conclusion

Isoflurane, but not sevoflurane, increased lung transendothelial albumin permeability through enhancement of caveolae-mediated albumin uptake and transport in the isolated lung. This effect may involve an enhanced phosphorylation of caveolin-1.

Isoflurane Prevents Platelets from Enhancing Neutrophil-Induced Coronary Endothelial Dysfunction

We evaluated whether platelets can enhance polymorphonuclear neutrophil-induced coronary endothelial dysfunction, and, after observing this, whether isoflurane can prevent the effect. Neutrophils, coronary artery segments, and platelets were obtained from 25 healthy dogs. Coronary artery rings were exposed to neutrophils activated with platelet-activating factor (1.0 microM), and after washing and preconstriction with U46619, were evaluated for concentration-related responses to acetylcholine, an endothelium-dependent vasorelaxing drug. Superoxide production by activated neutrophils was measured spectrophotometrically. Adherence of the activated neutrophils to the endothelium of coronary segments was assessed by direct counting of neutrophils labeled with fluorescent dye. Measurements were performed in absence and presence of isoflurane (1 minimum alveolar concentration) both with and without platelets. The presence of platelets enhanced the neutrophil-induced rightward shift in the concentration-vasorelaxation response curve to acetylcholine (the concentration of acetylcholine required to elicit 50% of maximal relaxation (-log M) was increased from 6.78 +/- 0.7 to 5.26 +/- 0.6), and it increased superoxide oxide production from 45.0 +/- 4.2 to 54.3 +/- 4.2 nM O2-/5 x 10(6) neutrophils and adherence of activated neutrophils from 204 +/- 10 to 268 +/- 5 neutrophils/mm2. Isoflurane abolished these effects of platelets. In conclusion, platelets enhanced the ability of neutrophils to cause coronary endothelial dysfunction. This effect was prevented by isoflurane. This may be attributable to an inhibitory action on superoxide production by the neutrophils leading to reduced expression of endothelial adhesion molecules and, in turn, reduced neutrophil adherence.

Tracheal intubating conditions and apnoea time after small-dose succinylcholine are not modified by the choice of induction agent

BACKGROUND

In a randomized, double-blind clinical trial, we studied the effect of different i.v. induction drugs on tracheal intubation conditions and apnoea time after small-dose (0.6 mg kg(-1)) succinylcholine used to facilitate orotracheal intubation at an urban, university-affiliated community medical centre.

METHODS

One hundred and seventy-five ASA I and II adult patients scheduled to undergo surgical procedures requiring general anaesthesia and tracheal intubation were allocated to one of five groups according to i.v. anaesthetic induction drug used. General anaesthesia was induced by i.v. administration of lidocaine 30 mg and propofol 2.5 mg kg(-1) (Group 1), thiopental 5 mg kg(-1) (Group 2), lidocaine 30 mg and thiopental 5 mg kg(-1) (Group 3), etomidate 0.3 mg kg(-1) (Group 4), or lidocaine 30 mg and etomidate 0.3 mg kg(-1) (Group 5). After loss of consciousness, succinylcholine 0.6 mg kg(-1) was given i.v. followed by direct laryngoscopy and tracheal intubation after 60 s. Measurements included intubation conditions recorded during laryngoscopy 60 s after succinylcholine administration, and apnoea time.

RESULTS

Overall, clinically acceptable intubation conditions were met in 168 out of the 175 patients studied (96%). They were met in 35/35 patients in Group 1, 33/35 patients in Group 2, 34/35 patients in Group 3, 33/35 patients in Group 4, and 33/35 patients in Group 5. Mean (SD) apnoea time was 4.0 (0.4), 4.2 (0.3), 4.2 (0.6), 4.1 (0.2) and 4.1 (0.2) min respectively in Groups 1-5. There were no differences in the intubation conditions or apnoea times between the groups.

CONCLUSIONS

The use of succinylcholine 0.6 mg kg(-1) produced the same favourable intubation conditions and a short apnoea time regardless of the induction drug used.

Role of Adenosine Receptors in Volatile Anesthetic Preconditioning against Neutrophil-induced Contractile Dysfunction in Isolated Rat Hearts

Background

The authors tested the hypothesis that adenosine receptors in polymorphonuclear neutrophils and the heart mediate the preconditioning effects of volatile anesthetics against neutrophil-induced contractile dysfunction.

Methods

Studies were conducted in buffer-perfused and paced isolated rat hearts. Left ventricular developed pressure served as index of contractility. Neutrophils and platelet-activating factor were added to perfusate for 10 min followed by 30 min of recovery. The effect of selective pretreatment of the neutrophils and the hearts with 1.0 minimum alveolar concentration isoflurane or sevoflurane on the neutrophil-induced contractile dysfunction was assessed. Studies were performed in the absence and presence of the nonselective adenosine receptor antagonist 8-phenyltheophylline (10 microM). Neutrophil retention was determined from difference between those administered and collected in coronary effluent and from myeloperoxidase concentration in myocardial samples. Superoxide production of neutrophils was measured by spectrophotometry.

Results

Under control conditions (no anesthetic pretreatment), activated neutrophils caused marked and persistent reductions in left ventricular developed pressure, associated with increases in neutrophil retention and myeloperoxidase activity. Pretreatment of the neutrophils or the heart with either isoflurane or sevoflurane abolished these effects. Pretreatment of the neutrophils also reduced the platelet-activating factor-induced increase in superoxide production by 29 and 33%, respectively. 8-Phenyltheophylline blunted the effects of anesthetic pretreatment of the neutrophils, whereas it did not alter the effects of anesthetic pretreatment of the heart.

Conclusion

An activation of adenosine receptors in neutrophils, but not in the heart, plays a role in the preconditioning effects of volatile anesthetics against neutrophil-induced contractile dysfunction.

The Neuromuscular Effects and Tracheal Intubation Conditions After Small Doses of Succinylcholine

Succinylcholine 1.0 mg/kg usually produces excellent tracheal intubation conditions in 60 s. Recovery of respiratory muscle function after this dose, however, is not fast enough to forestall oxyhemoglobin desaturation when ventilation cannot be assisted. In this study, we investigated whether smaller doses of succinylcholine can produce satisfactory intubation conditions fast enough to allow rapid sequence induction with a shorter recovery time. Anesthesia was induced with fentanyl/propofol and maintained by propofol infusion and N(2)O in O(2). After the induction, 115 patients were randomly allocated to five groups according to the dose of succinylcholine (0.3 mg/kg, 0.4 mg/kg, 0.5 mg/kg, 0.6 mg/kg, or 1.0 mg/kg). Evoked adductor pollicis responses to continuous 1-Hz supramaximal ulnar nerve stimulation were recorded using acceleromyography. Tracheal intubation conditions were graded 60 s after succinylcholine administration. Onset time, maximal twitch depression, time to initial twitch detection after paralysis, and to 10%, 25%, 50%, and 90% twitch height recovery were recorded. Time to initial diaphragmatic movement as well as time to resumption of regular spontaneous respiratory movements were calculated. Onset times ranged between 82 s and 52 s, decreasing with increasing doses of succinylcholine but not differing between 0.6 and 1 mg/kg. Maximum twitch depression was similar after 0.5, 0.6, and 1 mg/kg (98.2%-100%). Recoveries of twitch height and apnea time were dose-dependent. Intubation conditions were often unacceptable after 0.3- and 0.4-mg/kg doses. Acceptable intubation conditions were achieved in all patients receiving a 0.5, 0.6, and 1 mg/kg dose of succinylcholine. Intubation conditions in patients receiving 0.6 and 1 mg/kg were identical, whereas times to T(1) = 50% and 90% and time to regular spontaneous reservoir bag movements were significantly shorter in the 0.6-mg/kg dose group (5.78, 7.25, and 4.0 min, respectively) versus patients receiving 1 mg/kg (8.55, 10.54, and 6.16 min, respectively). The use of 0.5 to 0.6 mg/kg of succinylcholine can produce acceptable intubation conditions 60 s after administration. The conditions achieved after 0.6 mg/kg are similar to those after 1.0 mg/kg. These smaller doses are associated with faster twitch recovery and shorter apnea time.

IMPLICATIONS

In normal healthy patients, succinylcholine 0.6 mg/kg produces clinical intubation conditions identical to the traditional 1.0-mg/kg dose but is associated with a shorter recovery time.

Isoflurane and Sevoflurane Precondition against Neutrophil-induced Contractile Dysfunction in Isolated Rat Hearts

Background

The authors tested the hypothesis that pretreatment with isoflurane or sevoflurane can protect the heart against neutrophil-induced contractile dysfunction.

Methods

Studies were conducted in buffer-perfused and paced isolated rat hearts. Left ventricular developed pressure served as an index of contractility. Pretreatment consisted of administration of 1.0 minimum alveolar concentration isoflurane or sevoflurane for 15 min followed by a 10-min washout and was performed in the absence and presence of the adenosine triphosphate-sensitive potassium channel inhibitor glibenclamide (10 microM). Polymorphonuclear neutrophils and platelet-activating factor were then added to the perfusate for 10 min, followed by 30 min of recovery. Neutrophil retention was assessed from the difference between those administered and collected in coronary effluent and measurements of myeloperoxidase in myocardial samples. Isolated hearts were also used to assess the effect of volatile anesthetic pretreatment on cardiac dysfunction caused by enzymatically generated superoxide. In additional studies, the authors evaluated the effect of volatile anesthetic pretreatment on the adherence of neutrophils to isolated rat aortic segments.

Results

Platelet-activating factor-stimulated neutrophils caused marked and persistent reductions (> 50%) in left ventricular developed pressure. Pretreatment with either isoflurane or sevoflurane abolished these effects, as well as the associated increases in neutrophil retention. Glibenclamide did not alter these actions of the anesthetics. Pretreatment with either volatile anesthetic attenuated the reductions in left ventricular developed pressure caused by exogenous superoxide and abolished the increases in neutrophil adherence in the aortic segments.

Conclusion

Isoflurane and sevoflurane preconditioned the heart against neutrophil-induced contractile dysfunction. This action was associated with an inhibition to neutrophil adherence and likely involved an increased resistance of the myocardium to oxidant-induced injury; the adenosine triphosphate-sensitive potassium channels played no apparent role.

The relationship of posttetanic count and train-of-four responses during recovery from intense cisatracurium-induced neuromuscular blockade

Posttetanic count (PTC) has been used to quantify intense degrees of nondepolarizing neuromuscular blockade. Our objective in the present investigation was to discern whether PTC correlates with recovery from intense cisatracurium-induced neuromuscular blockade under both inhaled and IV anesthesia. In 60 patients, anesthesia was induced with propofol 2 mg/kg and fentanyl 1.5 micro g/kg IV. Recovery from intense neuromuscular blockade induced by cisatracurium (0.15 mg/kg) was studied in 2 groups. Group 1 (n = 30) had anesthesia maintained with propofol 100-200 micro g x kg(-1) x min(-1) and 60% N(2)O in O(2), whereas Group 2 (n = 30) had anesthesia maintained with isoflurane (end-tidal concentration 0.8%) and 60% N(2)O in O(2). Neuromuscular functions were monitored using acceleromyography. Cycles of posttetanic stimulation were repeated every 6 min with train-of-four (TOF) stimulation in between. Measurement included times to posttetanic responses and to the first response to TOF stimulation (T(1)), as well as the correlation between PTC and T(1). In Group 1, the mean times to PTC(1) and T(1) were 35.6 +/- 7.5 and 46.9 +/- 6.5 min, respectively. Corresponding times in Group 2 were 39.5 +/- 6.8 and 56.7 +/- 5.4 min, respectively. There was a good time correlation, r = 0.919 for propofol (Group 1) and r = 0.779 for isoflurane (Group 2), between PTC and T(1) recovery in both groups. The PTC when T(1) appeared ranged between 8 and 9 in Group 1 and 8 and 14 in Group 2. Conforming to original observations with other neuromuscular blocking drugs, there is a correlation between PTC and TOF recovery from intense cisatracurium-induced neuromuscular blockade allowing better monitoring of this intense degree of blockade during both IV (propofol) and isoflurane anesthesia.

IMPLICATIONS

Monitoring posttetanic count during intense neuromuscular blockade allows the clinician to estimate the intensity of the blockade and estimate recovery time. The relationship between posttetanic count and train-of-four recovery from intense cisatracurium-induced neuromuscular blockade was documented under both IV and inhaled anesthesia.

Neutrophils pretreated with volatile anesthetics lose ability to cause cardiac dysfunction

BACKGROUND

Volatile anesthetics can precondition the myocardium against functional depression and infarction following ischemia-reperfusion. Neutrophil activation, adherence, and release of superoxide play major roles in reperfusion injury. The authors tested the hypothesis that pretreatment of neutrophils with a volatile anesthetic, i.e., simulated preconditioning, can blunt their ability to cause cardiac dysfunction.

METHODS

Studies were performed in 60 buffer-perfused and paced isolated rat hearts. Left ventricular developed pressure served as an index of myocardial contractility. Polymorphonuclear neutrophils and/or drugs were added to coronary perfusate for 10 min, followed by 30 min of recovery. Platelet-activating factor was used to stimulate neutrophils. Pretreatment of neutrophils consisted of incubation with 1.0 minimum alveolar concentration (MAC) isoflurane or sevoflurane for 15 min, followed by washout. Additional studies were performed with 0.25 MAC isoflurane. Effects of superoxide dismutase were compared to those of volatile anesthetics. Superoxide production was measured by spectrophotometry. Neutrophil adherence to coronary vascular endothelium was estimated from the difference between neutrophils administered and recovered in coronary venous effluent.

RESULTS

Activated neutrophils caused marked, persistent reduction (> 50%) in left ventricular developed pressure. Isoflurane and sevoflurane at 1.0 MAC and superoxide dismutase abolished this effect. Isoflurane and sevoflurane reduced superoxide production of activated neutrophils by 29% and 33%, respectively, and completely prevented the platelet-activating factor-induced increases in neutrophil adherence. Isoflurane at 0.25 MAC blunted, but did not abolish, the neutrophil-induced decreases in left ventricular developed pressure.

CONCLUSION

Neutrophils pretreated with 1.0 MAC isoflurane or sevoflurane lost their ability to cause cardiac dysfunction, while those pretreated with a concentration of isoflurane as low as 0.25 MAC were partially inhibited. This action of the volatile anesthetics was associated with reductions in superoxide production and neutrophil adherence to the coronary vascular endothelium. Our findings suggest that inhibitory actions on neutrophil activation and neutrophil-endothelium interaction may contribute to the preconditioning effects of volatile anesthetics observed in vivo during myocardial ischemia-reperfusion.

Does the choice of intravenous induction drug affect intubation conditions after a fast-onset neuromuscular blocker?

STUDY OBJECTIVES

To compare intubation conditions and hemodynamic effects resulting from thiopental-rapacuronium, propofol-rapacuronium, and etomidate-rapacuronium intravenous (IV) induction.

DESIGN

Randomized, blinded study.

SETTING

Operating suites of a large university-affiliated medical center.

PATIENTS

60 ASA physical status I and II adult patients without airway abnormalities, who were scheduled for elective surgery requiring endotracheal intubation. Patients were randomly allocated to receive IV thiopental sodium 5 mg/kg (Group 1), propofol 2 mg/kg (Group 2), or etomidate 0.3 mg/kg (Group 3) followed by rapacuronium 1.5 mg/kg. Fifty seconds later, an anesthesiologist, who had no knowledge of the induction drug used, entered the operating room and attempted laryngoscopy and intubation.

MEASUREMENTS

Intubation conditions were graded as excellent, good, poor, or impossible according to Good Clinical Research Practice criteria. Arterial blood pressure and heart rate changes accompanying both induction techniques were also monitored and recorded.

MAIN RESULTS

All patients were intubated within 55 to 70 seconds. Clinically acceptable intubation conditions were not statistically different among the three groups. Moderate tachycardia after induction was seen in all three groups, and blood pressure was significantly lower in Group 2 than in Groups 1 or 3.

CONCLUSIONS

Clinically acceptable intubation conditions are similar after either thiopental, propofol, or etomidate when a fast-onset neuromuscular blocking drug (rapacuronium 1.5 mg/kg) is used to facilitate tracheal intubation.

Beta-adrenergic stimulation restores oxygen extraction reserve during acute normovolemic hemodilution

Compensatory increases in oxygen extraction (EO(2)) during acute normovolemic hemodilution (ANH) have the effect of decreasing tissue oxygen tension values, thus increasing the threat of tissue hypoxia. We hypothesized that if the beta-adrenergic agonist isoproterenol (ISOP) could augment cardiac output (CO) during ANH, it could reverse the increases in EO(2) and restore the margin of safety for tissue oxygenation. Studies were performed in seven anesthetized (isoflurane) dogs. CO was measured by using thermodilution, and regional blood flow (RBF) was measured by using radioactive microspheres. Systemic oxygen delivery (DO(2)), oxygen consumption (OV0312;O(2)), and EO(2), as well as regional DO(2), were calculated. Measurements were obtained under the following conditions in each dog: 1) baseline-1, 2) ISOP (0.1 micro g. kg(-1). min(-1) IV), 3) baseline-2, 4) ANH, and 5) ISOP during ANH. Hematocrit was 45% +/- 3% under baseline conditions and 18% +/- 3% during ANH. Before ANH, ISOP caused parallel increases in CO and systemic DO(2), which, in the presence of an unchanged OV0312;O(2), reduced EO(2). RBF increased in myocardium and spleen, decreased in pancreas, and did not change in brain, spinal cord, or other tissues. ANH caused increases in CO, which were insufficient to offset the decrease in arterial oxygen content, and thus systemic DO(2) declined; systemic OV0312;O(2) was maintained by an increase in EO(2). ANH-related increases in RBF maintained DO(2) in myocardium, brain, duodenum, and pancreas, whereas DO(2) declined in kidney and spleen. ISOP during ANH increased CO and systemic DO(2), which returned systemic EO(2) to baseline, and it increased RBF in myocardium, kidney, duodenum, and spleen. We conclude that 1) beta-adrenergic stimulation with ISOP restored the systemic EO(2) reserve during ANH, without apparent adverse effects in the individual body tissues, and that 2) the use of inotropic drugs, such as ISOP, may extend the limit to which hematocrit can be reduced safely during ANH.

IMPLICATIONS

By restoring the oxygen extraction reserve, isoproterenol and other inotropic drugs can enhance the margin of safety and extend the limit to which hematocrit can be reduced safely during acute normovolemic hemodilution. The use of this approach will depend on the degree of hemodilution, the extent of mixed venous oxygen desaturation, and whether increases in cardiac output are possible or desirable.