Maurice Baroody

Continuous venous oximetry during cardiopulmonary bypass: influence of temperature changes, perfusion flow, and hematocrit levels

This study was performed in 11 patients undergoing cardiac surgery during cardiopulmonary bypass (CPB). A Bentley-10 oxygenator (American Bentley, Irvine, CA) was used during bypass, and 1,500 mL of Ringers solution was used to prime the oxygenator. A perfusion flow of 2.4 L/min/m2 was used, and an equivalent flow of 100% oxygen was added to the oxygenator. The mixed venous oxygen saturation (SvO2) was monitored by continuous in-line venous oximetry using the Bentley Oxy-Stat Meter. Body temperature and SvO2 measurements were made at the venous blood port of the oxygenator. The authors investigated the correlation between SvO2 and changes of body temperature, perfusion flow, and hematocrit values. SvO2 correlated inversely, in a linear fashion, with the body temperature. The SvO2 was markedly decreased after rewarming, and was significantly affected during normothermia by changes in perfusion flow rates and hematocrit levels. An adequate SvO2 was found when the flow was greater than 2.4 L/min/m2 and the hematocrit was greater than 20%. In-line oximetry can help to optimize perfusion during CPB and to detect episodes of desaturation.

Effect of alpha-stat versus pH-stat strategy on oxyhemoglobin dissociation and whole-body oxygen consumption during hypothermic cardiopulmonary bypass.

To determine whether alpha-stat or pH-stat strategy should be used, 20 patients undergoing coronary artery bypass grafting during moderate hypothermic hemodilutional cardiopulmonary bypass were studied. The carbon dioxide management during bypass was randomly done according to alpha-stat strategy in 10 patients (i.e., temperature-uncorrected PaCO2 was kept near 40 mm Hg and uncorrected pHa was kept at about 7.4) and according to pH-stat strategy in the other 10 patients (i.e., temperature-corrected PaCO2 was kept near 40 mm Hg and uncorrected pHa was kept at about 7.4). In both groups, when the central venous temperature was stable at 26.5 +/- 2.5 degrees C, the perfusion flow was altered sequentially from 2.4 to 1.8 and 1.2 L.min-1.m-2. The mixed venous oxyhemoglobin saturation at the different perfusion flows was monitored by the Oxy-Stat meter and was correlated with the corresponding mixed venous oxygen tension to construct an oxyhemoglobin dissociation curve. Also, the whole-body oxygen consumption at the different perfusion flows was computed. The whole-body oxygen consumption and the oxyhemoglobin dissociation were not significantly different between the alpha-stat and the pH-stat groups. In both groups, the dissociation curve is shifted to the left, but the oxygen consumption per unit time does not significantly change despite decreasing the perfusion flow from 2.4 to 1.2 L.min-1.m-2. The results suggest that oxygen delivery is not impaired during moderate hypothermic cardiopulmonary bypass independent of whether alpha-stat or pH-stat strategy is used.

Action of adrenergic agonists on resistance ν capacitance vessels during cardiopulmonary bypass

The peripheral vascular effects of three different adrenergic agonists were investigated in 13 patients undergoing valve replacement during cardiopulmonary bypass (CPB). The venous reservoir (RV) and mean arterial pressure (MAP) were used as indices of the changes in venous capacitance and arterial resistance, respectively, produced by the adrenergic agonists. Isoproterenol, a pure beta-adrenergic agonist, decreased both MAP and RV. Norepinephrine (NE), which activates both alpha 1- and alpha 2-receptors in humans, increased both MAP and RV, while phenylephrine (PH) a selective alpha 1-adrenergic agonist, increased only MAP with no significant change in RV. It is concluded that in humans during hypothermic CPB, beta-agonists dilate both the resistance and capacitance vessels, selective alpha 1-adrenergic agonists preferentially constrict the resistance vessels, and non-selective alpha 1- and alpha 2-adrenergic agonists constrict both the resistance and capacitance vessels.

Effects of metaraminol on the resistance and capacitance vessels in humans.

During cardiopulmonary bypass (CPB), we have previously shown in patients undergoing open heart surgery that injection of ephedrine 30 mg into the venous reservoir of the Bentley oxygenator increased the mean perfusion pressure (MAP) by about 30%, but decreased rather than increased the reservoir volume (RV) by about 30%, suggesting a moderate increase of the peripheral resistance associated with venodilation (1). The present report describes the peripheral vascular effects of metaraminol during CPB in 10 patients undergoing aortic and/or mitral valve replacement. Anesthesia was induced with diazepam 0.1 mgkg, a mixture of alcuronium 0.25 mg/kg and pancuronium 0.1 mg/kg, and fentanyl 20 &kg. After orotracheal intubation, controlled ventilation was performed using 100% 0, and anesthesia was maintained with incremental doses of morphine amounting to 0.5 mg/kg. The pump oxygenator was primed with 1.5 liter of lactated ringers solution. The patients were perfused with a constant pump flow 2.3 L/m2/min. After 10-20 minutes of CPB, a steady state of perfusion was reached. Metaraminol2 mg was then injected into the venous line of the Bentley oxygenator and its effect on the MAP and RV monitored. Changes in MAP and RV were considered to reflect changes of the peripheral vascular resistance and venous capacitance respectively. The injection of metaraminol2 mg was rapidly followed by a significant increase in the MAP to 125.5 * 39.5 mmHg from 64.5 5 13.2 mmHg, associated with an increase of the RV to 2950 * 1066 ml from 2690 * 1097 ml. The results of the present report shows that metaramino1 produces a marked increase of MAP associated with a moderate increase of RV. In contrast, ephedrine has been previously shown to produce in man a moderate increase of MAP associated with a decrease of RV (1). Metaraminol has some direct effect on vascular smooth muscle, but its primary effect is due to release of endogenous norepinephrine, which exerts p-adrenergic effects on the heart and a-adrenergic action on the blood vessels (2,3). Thus, metaraminol acts on the peripheral vessels, similar to norepinephrine, as a selective a-adrenergic agonists which can constrict both the resistance and capacitance vessels, while ephedrine acts as a mixed aand padrenergic agonist (2,3) which constricts the resistance vessels but dilates the capacitance vessels (1). Anis Baraka, MD Sania Haroun, MD Maurice Baroody, MD Maud Nawfal, MD Abla Sibaii, MS Department of Anesthesiology and Department of Epidemiology G. Biostatistics American University of Beirut Beiru t-Labanon

Verapamil pretreatment before aortic cross-clamping in patients undergoing coronary artery bypass graft

In 12 of 22 patients with coronary artery disease undergoing elective coronary artery bypass graft (CABG), verapamil (0.075 mg/kg body weight) pretreatment was given on initiation of cardiopulmonary bypass (CPB) before aortic cross-clamping (ACC), whereas no verapamil was used in 10 control patients. The volume of cardioplegia solution required to achieve and maintain cardiac asystole during ACC while on CPB was not significantly different in the two groups. After the release of ACC, the energy of direct current countershock required for defibrillation, the incidence of heart block, and the need for pacemaker were not significantly different. However, defibrillation was more readily achieved by a single countershock in the verapamil group than in the control group. Also, the incidence of ST segment changes after defibrillation was significantly lower in the verapamil group, suggesting that verapamil pretreatment before ACC may potentiate the myocardial preservation achieved by the cardioplegia, and hence may decrease the incidence of ischemic changes during the critical reperfusion period.

Effects of preoperative calcium channel and beta-adrenergic blockade on the vascular response to adrenergic agonists during cardiopulmonary bypass

The present report compares the effects of isoproterenol (ISO), norepinephrine (NE), and phenylephrine (PH) on the mean arterial pressure (MAP) and reservoir volume (RV) during cardiopulmonary (CPB) in 16 patients undergoing coronary artery bypass grafting (CABG) who were treated preoperatively with oral nifedipine (10 to 40 mg, three times a day) and propranolol (40 to 60 mg, three times a day). The changes of RV and MAP were used as indices of the changes in venous capacitance and arterial resistance, respectively, produced by the adrenergic agonists. ISO, a beta-adrenergic agonist, decreased both MAP and RV. NE, which activates both alpha 1- and alpha 2- adrenoceptors, increased both MAP and RV, while PH, a selective alpha 1-adrenergic agonist, increased only MAP with no significant change in RV. The changes are qualitatively similar to those previously achieved in patients undergoing valve replacement who did not receive any blocker preoperatively. However, the decrease of MAP by ISO and its increase by NE were significantly less in the CABG group. It is concluded that preoperative preparation of patients undergoing CABG with beta-adrenergic blockers and calcium channel blockers can modify the effect of ISO and NE on the peripheral resistance, but they have no significant effect on the action of PH at the doses selected in this study.

Dependent PEEP during two-lung ventilation in the lateral decubitus position.

In anesthetized patients in the lateral decubitus position, the dependent lung is less ventilated and more perfused than the nondependent lung, resulting in ventilation-perfusion mismatching (1-3). A physiologic solution to the adverse effects of anesthesia and surgery in the lateral decubitus position on the distribution of ventilation and perfusion would be the application of selective positive end expiratory pressure (PEEP) to the dependent lung (2,3). Selective PEEP to the lower lung may increase ventilation of this lung by moving it up to a steeper, more favorable portion of the lung pressure-volume curve; it may also increase vascular resistance in the lower lung and shift more blood flow to the upper lung. The present report investigates the effect of selective application of dependent PEEP in patients undergoing radical nephrectomy in the lateral decubitus position. The investigation compares the differential distribution of ventilation between the dependent and the nondependent lungs, as well as the arterial PO, and PCO, levels before and after application of the dependent PEEP.