Measuring the efficiency of an artificial lung: 1. Carbon dioxide transfer.

The performance of a hollow fibre artificial lung ('Capiox E') was analysed by measurement of the 'parallel deadspace' of the device under varying conditions in 21 patients. The efficiency with which carbon dioxide was exchanged was determined by the time available for equilibration between the blood and gas phases. When this equilibration coefficient was less than 12 seconds per litre of blood flow per litre of gas flow, there was a marked reduction in the efficiency of gas exchange. Under certain conditions, the 'counter-current' design of the device apparently permitted the clearance of carbon dioxide at a partial pressure greater than that which was found in the mixed venous blood. This anomalous behaviour may represent in vivo confirmation of the Haldane effect.     

Fuzzy logic control of inspired isoflurane and oxygen concentrations using minimal flow anaesthesia

In order to evaluate the performance of feedback fuzzy logic control of inspired oxygen and isoflurane concentrations, we studied 30 patients undergoing discectomy for lumbar (n = 26) or cervical (n = 4) disc herniation. Patients were allocated random to one of two groups: a standard group (n = 15) with low flow anaesthesia (1.2-1.3 litre min-1) and manual control of gas concentrations; and a fuzzy group (n = 15) with minimal flow (0.5 litre min-1) and fuzzy logic feedback control of gas concentrations. Fuzzy logic control achieved and maintained very accurately the desired isoflurane concentration. Oxygen concentration was controlled more precisely than in the standard group. Delivery and costs of oxygen and nitrous oxide were significantly lower in the fuzzy group (P < 0.01). Accumulation of foreign gases was observed in one patient during low flow and in 11 patients during minimal flow anaesthesia. In conclusion, fuzzy logic control of inspired oxygen and isoflurane concentration during minimal flow anaesthesia was reliable and reduced anaesthetic gas delivery and costs.      

Breathing systems: effect of fresh gas flow rate on enflurane consumption

The vaporization rates of enflurane were measured in 412 anaestheticsusing appropriate fresh gas flow rates in Bain (12 litre min^–1Magill (6 litre min^–1) and circle systems (3 litre min^–1,1 litre min^–1 and "closed"). In all patients reducingthe fresh gas flow rate resulted in lower enflurane consumption.The percent savings were 18–86% depending on the initialfresh gas flow rate and the size of the change in fresh gasflow. The reduction in enflurane use was more marked in inpatients(long cases) than in day-case patients (short cases).     

The effects of sevoflurane, halothane, enflurane, and isoflurane on hepatic blood flow and oxygenation in chronically instrumented greyhound dogs.

Inhalational anesthetics produce differential effects on hepatic blood flow and oxygenation that may impact hepatocellular function and drug clearance. In this investigation, the effects of sevoflurane on hepatic blood flow and oxygenation were compared with those of enflurane, halothane, and isoflurane in ten chronically instrumented greyhound dogs. Each dog randomly received enflurane, halothane, isoflurane, and sevoflurane, each at 1.0, 1.5, and 2.0 MAC concentrations. Mean arterial blood pressure and cardiac output decreased in a dose-dependent fashion during all four anesthetics studied. Heart rate increased compared to control during enflurane, isoflurane, and sevoflurane anesthesia and did not change during halothane anesthesia. Hepatic arterial blood flow and portal venous blood flow were measured by chronically implanted electromagnetic flow probes. Hepatic O2 delivery and consumption were calculated after hepatic arterial, portal venous, and hepatic venous blood gas analysis. Hepatic arterial blood flow was maintained with sevoflurane and isoflurane. Halothane and enflurane reduced hepatic arterial blood flow during all anesthetic levels compared to control (P less than 0.05), with marked reductions occurring with 1.5 and 2.0 MAC halothane concomitant with an increase in hepatic arterial vascular resistance. Portal venous blood flow was reduced with isoflurane and sevoflurane at 1.5 and 2.0 MAC. A somewhat greater reduction in portal venous blood flow occurred during 2.0 MAC sevoflurane (P less than 0.05 compared to control and 1.0 MAC values for sevoflurane). Enflurane reduced portal venous blood flow at 1.0, 1.5, and 2.0 MAC compared to control. Halothane produced the greatest reduction in portal venous blood flow (P less than 0.05 compared to sevoflurane).(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of midazolam on cerebral blood flow and oxygen consumption Interaction with nitrous oxide in patients undergoing craniotomy for supratentorial cerebral tumours

Cerebral blood flow and the cerebral metabolic rate of oxygen were measured in 30 patients during craniotomy for supratentorial cerebral tumours by a modification of the Kety-Schmidt technique using Xenon 133 intravenously. Anaesthesia was induced with midazolam 0.3 mg/kg, fentanyl and pancuronium, and maintained with midazolam as a continuous infusion, fentanyl, pancuronium and nitrous oxide in oxygen or oxygen in air. The concentration of midazolam in the blood of 10 patients was about 300 ng/litre during two measurements; the patients' lungs were ventilated with N2O in oxygen. The concentration of midazolam in the blood of another 10 patients was doubled to about 600 ng/litre during the second flow measurement; the patients' lungs were ventilated with N2O/O2. The concentration of midazolam in the blood of the third group of 10 patients was doubled to 600 ng/litre during the second flow measurement; the patients' lungs were ventilated with oxygen in air. No relationship was found between the dose of midazolam and cerebral blood flow or oxygen consumption. Nitrous oxide in combination with midazolam also had no effect on these variables.     

Phenylephrine increases pulmonary blood flow in children with tetralogy of Fallot

PURPOSE: Although it has been reported that the increase in blood pressure improves arterial oxygen saturation (SaO(2)) in children with tetralogy of Fallot, no prospective study has demonstrated that an increase in blood pressure induces an increase in pulmonary blood flow in these patients. The purpose of this study was to see whether a phenylephrine-induced increase in systemic blood pressure increased pulmonary blood flow, resulting in improved arterial oxygenation in tetralogy of Fallot. METHODS: In 14 consecutive children with tetralogy of Fallot (2-32 months old), transesophageal pulsed Doppler signals of left upper pulmonary venous flow (PVF) velocity were recorded before and four minutes after 10 micro g x kg(-1) of phenylephrine i.v. Simultaneously, arterial blood gas analysis and hemodynamic measurements were performed. The minute distance (MD) was calculated as the product of the heart rate and the sum of time-velocity integrals of PVF. RESULTS: Phenylephrine iv increased mean arterial blood pressure from 54 +/- 8 mmHg to 73 +/- 10 mmHg. This phenylephrine-induced hypertension significantly increased SaO(2) and MD (92.0 +/- 7.5 vs 95.0 +/- 5.0% and 1318 +/- 344 vs 1533 +/- 425 cm x min(-1), respectively). There was a significant correlation (r = 0.72) between the change in MD and the change in SaO(2). CONCLUSION: Our results suggest that the phenylephrine-induced increase in systemic blood pressure produces an increase in pulmonary blood flow in tetralogy of Fallot. Our results further suggest that this increase in pulmonary blood flow is involved in the mechanism of phenylephrine-induced improvement of arterial oxygenation in tetralogy of Fallot.     

Tracheal insufflation of oxygen at low flow: capabilities and limitations

Tracheal insufflation of oxygen (TRIO) may provide temporary oxygenation for patients or sustain life in apneic mass casualties when conventional ventilatory techniques are not available or feasible. Logistically, minimum flows of TRIO (Vmin) are desirable for field use and to reduce barotrauma should airway obstruction occur. We carried out a feasibility study to determine the efficacy of Vmin of TRIO delivered within 1 cm of the carina, in nine anesthetized and paralyzed dogs. Minimum flows of TRIO for these dogs of average weight (12 kg) was 91 mL/min. In six of the dogs Vmin TRIO was continued and provided oxygenation for an average of 1.5 h compatible with subsequent resuscitation with conventional ventilation. However, PaCO2 levels increased to mean values of 256 mm Hg in the 90 min. To determine what the effect of increased gas mixing was on gas exchange, we repeated Vmin TRIO for 10 min in six of the dogs with and without high frequency oscillations superimposed on the TRIO flow. The oscillations (60 mL at 16.3 Hz) increased carbon dioxide excretion but significantly impaired oxygenation. In completely apneic animals, TRIO at low flow delivered by cricothyroidotomy may be useful as an emergency procedure when upper airway obstruction limits the use of other airway management techniques. However, enhancement of gas mixing during low-flow TRIO impairs oxygenation, so that higher flows would be required when respiratory efforts occur.     

24-hour extracorporeal membrane oxygenation in the hypoxic dog: hemodynamics and pulmonary gas exchange.

Hemodynamic and pulmonary gas exchange values were investigated during 24-hour extracorporeal membrane oxygenation performed on 7 anesthetized dogs subjected to alveolar hypoxia. The chief effects of extracorporeal membrane oxygenation were demonstrated. The duration of the extracorporeal membrane oxygenation was associated with a progressive decline in the systemic arterial pressure and heart rate and a progressive increase in the pulmonary blood flow rate. Left-ventricular stroke work remained constant. The sum of the pulmonary and extracorporeal oxygen uptakes showed no change in spite of large variations in pulmonary flow rate.     

Description of a flow optimized oxygenator with integrated pulsatile pump

Extracorporeal membrane oxygenation (ECMO) is a well-established therapy for several lung and heart diseases in the field of neonatal and pediatric medicine (e.g., acute respiratory distress syndrome, congenital heart failure, cardiomyopathy). Current ECMO systems are typically composed of an oxygenator and a separate nonpulsatile blood pump. An oxygenator with an integrated pulsatile blood pump for small infant ECMO was developed, and this novel concept was tested regarding functionality and gas exchange rate. Pulsating silicone tubes (STs) were driven by air pressure and placed inside the cylindrical fiber bundle of an oxygenator to be used as a pump module. The findings of this study confirm that pumping blood with STs is a viable option for the future. The maximum gas exchange rate for oxygen is 48mL/min/L(blood) at a medium blood flow rate of about 300mL/min. Future design steps were identified to optimize the flow field through the fiber bundle to achieve a higher gas exchange rate. First, the packing density of the hollow-fiber bundle was lower than commercial oxygenators due to the manual manufacturing. By increasing this packing density, the gas exchange rate would increase accordingly. Second, distribution plates for a more uniform blood flow can be placed at the inlet and outlet of the oxygenator. Third, the hollow-fiber membranes can be individually placed to ensure equal distances between the surrounding hollow fibers.     

Low flow veno-venous ECMO: an experimental study

Clinical use of extracorporeal membrane oxygenation (ECMO) and carbon dioxide removal (ECCO 2R) have become well established techniques for the treatment of severe respiratory failure; however they require full cardiopulmonary bypass, representing major procedures with high morbidity. We theorized the possibility of an efficient low flow veno-venous extracorporeal membrane gas exchange method. Four mongrel 12 kg dogs were submitted to veno-venous extracorporeal membrane gas exchange via a jugular dialysis catheter using a low flow (10 ml/min) roller pump and a membrane oxygenator for a period of four hours. Respiratory rate was set at 4 breaths/min with a FiO 2 of 21% and ventilatory dead space was increased. Adequate gas exchange was obtained (pO 2139, pCO 224, Sat 99.4%), without major hemodynamic changes or hematuria. Our results demonstrate the feasibility of a low flow, less aggressive system. Further research should be considered.     

Interfacing spontaneous breathing and mechanical ventilation. New insights

Mechanical ventilation (MV) with positive pressure insufflations of gas into the lung may be required to ensure sufficient oxygenation of blood and elimination of carbon dioxide in acute respiratory failure. Interfacing spontaneous breathing and mechanical ventilation has been used to improve gas exchange and may offer other advantages regarding integrity of lung tissue. Airway pressure release ventilation (APRV), or bilevel positive airway pressure (BiPAP), is a mechanical ventilatory mode with a low respiratory rate upon which spontaneous breaths can be superimposed during any time of the respiratory cycle. The mechanical pressure variations cause inflation and deflation of the lungs and the spontaneous breaths are added according to the demand of the respiratory center and neuromuscular function. This technique improves oxygenation of blood compared to MV alone. This seems to be caused by recruitment of collapsed lung tissue and increased aeration of the lung. Moreover, ventilation is distributed more to the dependent (dorsal in supine position) regions than with mechanical ventilation alone. Since blood flow goes preferentially to the dependent regions, the altered ventilation distribution results in improved matching of ventilation and perfusion, further enhancing or facilitating gas exchange. Moreover, there is less cyclic collapse, i.e. less re-collapse during expiration and reopening during inspiration than with MV alone. Further development of the interfacing technique can be expected, with synchronization and also dosing of the mechanical support and with triggering of the ventilator that is based on neural recordings rather than mechanical signals as pressure and flow.     

双向腔肺分流术后不同通气程度对脑血流的影响

目的 观察双向腔肺分流术后不同通气程度对脑血流及系统氧饱和度的影响.方法 随机抽取8例功能性单心室行双向腔肺分流术病儿.术后3～5 h血流动力学平稳后进行研究,初始呼吸机参数设定容量控制的SIMV模式.通过调整呼吸机频率获得不同通气状态.更换通气状态后20 min记录数据,记录3种连续的通气状态下动脉血气分析、血流动力学指标、颈内动脉多普勒血流频谱,用近红外组织血氧监测仪持续监测脑组织血氧参数.结果 高通气状态下,动脉血压和上腔静脉压明显下降,平均动脉氧分压和二氧化碳分压明显下降.反映脑血流量的指标颈内动脉血流峰值速度以及脑组织中含氧血红蛋白的含量也明显下降.而在低通气状态下,可以观察到相反的结果.结论 不同通气状态可以明显影响双向腔肺分流术后血流动力学状况、动脉氧饱和度及脑血流量.与高通气相比,低通气状态降低脑血管床阻力,因此可以增加双向腔肺分流术后脑一肺串联的血流量,提高双向腔肺分流术后氧饱和度.     

Extracorporeal membrane oxygenator compatible with centrifugal blood pumps

Coil-type silicone membrane oxygenators can only be used with roller blood pumps due to the resistance from the high blood flow. Therefore, during extracorporeal membrane oxygenation (ECMO) treatment, the combination of a roller pump and an oxygenator with a high blood flow resistance will induce severe hemolysis, which is a serious problem. A silicone rubber, hollow fiber membrane oxygenator that has a low blood flow resistance was developed and evaluated with centrifugal pumps. During in vitro tests, sufficient gas transfer was demonstrated with a blood flow less than 3 L/min. Blood flow resistance was 18 mm Hg at 1 L/min blood flow. This oxygenator module was combined with the Gyro C1E3 (Kyocera, Japan), and veno-arterial ECMO was established on a Dexter strain calf. An ex vivo experiment was performed for 3 days with stable gas performance and low blood flow resistance. The combination of this oxygenator and centrifugal pump may be advantageous to enhance biocompatibility and have less blood trauma characteristics.     

Dobutamine-induced dissociation between changes in splanchnic blood flow and gastric intramucosal pH after cardiac surgery

Gastric intramucosal acidosis, a sign of splanchnic tissue hypoxia,is common after cardiac surgery. We tested the hypothesis thatan increase in splanchnic blood flow induced by dobutamine improvessplanchnic tissue oxygenation after cardiac surgery. We measuredchanges in gastric intramucosal pH, splanchnic blood flow andoxygen transport in response to increased systemic flow inducedby dobutamine (mean 4.4 (range 3.0–7.0) µg kg^–1min^–1) after coronary artery bypass. We studied 22 stablepostoperative patients who were allocated randomly to receivedobutamine (n = 11) or to serve as controls (n = 11). Dobutaminewas given also to a separate group with a low cardiac indexafter operation (n = 6). The end-point was to increase cardiacindex by at least 25% and to exceed 2 litre min^–1 m^–2.Dobutamine consistently increased mean splanchnic blood flow(control 0.6 (SD0.2) vs 0.7 (0.2) litre min^–1 m^–2(P<0.05); normal cardiac output and dobutamine 0.7 (0.2)vs 1.1 (0.4) litre min^–1 m^–2 (P<0.01); low cardiacoutput and dobutamine 0.4 (0.1) vs 0.7 (0.1) litre min^–1m^–2 (P<0.05)) and oxygen delivery (control 102 (29)vs 111 (28) ml min^–1 m^–2 (ns); normal cardiac outputand dobutamine 106 (27) vs 156 (47) ml min^–1 m^–2(P < 0.01); low cardiac output and dobutamine 75 (21) vs110 (26) ml min^–1 m^–2 (P<0.05)) but had no effecton splanchnic oxygen consumption (control 44 (10) vs 49 (10)ml min^–1 m^–2 (ns); normal cardiac output and dobutamine45(12) vs 51 (17) ml min^–1 m^–2 (ns); low cardiacoutput and dobutamine 37 (9) vs 40 (9) ml min^–1 m^–2(ns)). Despite this, dobutamine reduced gastric intramucosalpH in all patients with low cardiac output (7.33 (0.12) vs 7.25(0.06)(P<0.05)) and in 50% of patients with stable haemodynamics(7.37(0.07) vs 7.34(0.06) (ns)). In contrast, gastric intramucosalpH remained stable in the control group (7.34 (0.05) vs 7.34(0.04) (ns)).We conclude that dobutamine resulted in a dissociationbetween splanchnic oxygen delivery and gastric mucosal tissueoxygenation, suggesting inappropriate distribution of bloodflow within the splanchnic region. (Br. J. Anaesth. 1995; 74:277–282)     

Pulmonary blood flow distribution in lobar hypoxia--influence of cardiac output and nitric oxide inhalation.

Inhaled NO is reported to be less effective in patients with ARDS if cardiac output is high (> 10 L/min). It has also been demonstrated that increased blood flow and increased shear stress cause an enhancement of endogenous NO production. In one-lung ventilation and regional hypoxia, nitric oxide (NO) delivered to the ventilated lung may decrease blood flow to the nonventilated lung and improve arterial oxygenation. So far, however, results have been divergent. The present study was performed with the hypothesis that inhaled NO would be less effective if cardiac output was increased. In the anaesthetized pig, hypoxia (5% O2) was induced in the left lower lobe. NO was delivered consecutively to the hypoxic lobe and to the other, oxygenated parts, of the lungs during continuous measurement of lobar blood flow and total lung blood flow. Bleeding and infusion of dextran caused variation in cardiac output. It was found that lobar hypoxia per se reduced lobar blood flow from 22.9+/-3.1% to 4.7+/-0.9% of cardiac output. An increase (3.2+/-0.3 L x min(-1)) and a decrease (2.2+/-0.2 L x min(-1)) in cardiac output did not alter the relative perfusion of the hypoxic lobe from baseline cardiac output (2.6+/-0.2 L x min(-1)) values. When NO was delivered to the hypoxic lobe, there was a marked increase in relative lobar perfusion to 19.0+/-2.9% during low cardiac output and 16.5+/-2.7% during high cardiac output without any significant difference between the two NO-induced increases of lobar perfusion. The increase in lobar perfusion tended to depend inversely on total pulmonary blood flow when cardiac output had been reduced by bleeding but without reaching statistical significance (r = -0.42, p > 0.05). The decrease in mean pulmonary artery pressure and PaO2 seen during NO inhalation to the hypoxic lobe did not correlate with the level of cardiac output. When NO was delivered to the oxygenated parts of the lungs, no significant effect on relative lobar perfusion or arterial oxygenation was observed, either at raised or at lowered cardiac output. The findings give no further evidence to show that variations in cardiac output alter the effect of NO inhalation.     

Blood flow and tissue oxygen pressures of liver and pancreas in rats: effects of volatile anesthetics and of hemorrhage.

The object of this investigation was to compare the effects of volatile anesthetics and of hemorrhage at comparable arterial blood pressures on splanchnic blood flow (radioactive microspheres) and tissue oxygenation of the liver and pancreas (surface PO2 [PSO2] electrodes). In contrast to earlier studies, we did not use identical minimum alveolar anesthetic concentration multiples as a reference to compare volatile anesthetics; rather, we used the splanchnic perfusion pressure. Under general anesthesia (intravenous chloralose) and controlled ventilation, 12 Sprague-Dawley rats underwent laparotomy to allow access to abdominal organs. Mean arterial pressure was decreased from 84 +/- 3 mm Hg (mean +/- SEM) at control to 50 mm Hg by 1.0 +/- 0.1 vol% halothane, 2.2 +/- 0.2 vol% enflurane, and 2.3 +/- 0.1 vol% isoflurane in a randomized sequence. For hemorrhagic hypotension, blood was withdrawn gradually until a mean arterial pressure of 50 mm Hg was attained. Volatile anesthetics and hemorrhage reduced cardiac output, and hepatic arterial, portal venous, and total hepatic blood flows by comparable degrees. Mean hepatic PSO2 decreased significantly from 30.7 +/- 2.6 mm Hg at control to 17.4 +/- 2 and 17.5 +/- 2 mm Hg during enflurane and isoflurane (each P less than 0.05) anesthesia, respectively. The decrease to 11.5 +/- 2.5 mm Hg was more pronounced during halothane anesthesia. Hemorrhagic hypotension was associated with the lowest hepatic PSO2 (3.4 +/- 1.3 mm Hg) and the highest number of hypoxic (0-5 mm Hg 86%) and anoxic PSO2 values (0 mm Hg 46%). Pancreatic blood flow and oxygenation remained unchanged from control during halothane and enflurane administration, whereas isoflurane increased both variables. Hemorrhagic hypotension slightly reduced pancreatic flow (-8%) but significantly decreased PSO2 from 58 +/- 5 mm Hg at control to 36 +/- 3 mm Hg, with 7% of all measured values in the hypoxic range. Thus, volatile anesthetics preserved pancreatic but not hepatic blood flow and tissue oxygenation in this rat model. Despite comparable effects on perfusion, the PSO2 of the liver and pancreas was the least during hemorrhagic hypotension compared to that with the anesthetics. Because the volative anesthetic-induced hypotension has such a different effect on splanchnic tissue oxygenation compared with hemorrhagic-induced hypotension, the authors conclude that the method of inducing hypotension may have different effects on oxygenation of various tissues.     

Ventilation-perfusion distribution related to different inspiratory flow patterns in experimental lung injury

In acute lung injury (ALI), controlled mechanical ventilation with decelerating inspiratory flow (.V(dec)) has been suggested to improve oxygenation when compared with constant flow (.V(con)) by improving the distribution of ventilation and perfusion (.V(A)/.Q). We performed the present study to test this hypothesis in an animal model of ALI. Furthermore, the effects of combined decelerating and constant flow (Vdot;(deco)) were evaluated. Thus, 18 pigs with experimental ALI were randomized to receive mechanical ventilation with either .V(con), .V(dec) or a fixed combination of both flow wave forms (.V(deco)) at the same tidal volume and positive end-expiratory pressure level for 6 h. Hemodynamics, gas exchange, and .V(A)/.Q distribution were determined. The results revealed an improvement of oxygenation resulting from a decrease of pulmonary shunt within each group (P < 0.05). However, blood flow to lung areas with a normal .V(A)/.Q distribution increased only during ventilation with .V(con) (P < 0.05). Accordingly, PaO(2) was higher with .V(con) than with .V(dec) and .V(deco) (P < 0.05). We conclude that contrary to the hypothesis, .V(con)provides a more favorable .V(A)/.Q distribution, and hence better oxygenation, when compared with .V(dec) and .V(deco) in this model of ALI. IMPLICATIONS: In acute lung injury, mechanical ventilation with decelerating flow has been suggested to improve ventilation-perfusion distribution when compared with constant flow. We tested this hypothesis in an animal model. Contrary to the hypothesis, we found a more favorable ventilation-perfusion distribution during constant flow when compared with decelerating flow.     

Uniformity of the fluid flow velocities within hollow fiber membranes of blood oxygenation devices

A finite volume-based computational model was developed to investigate the uniformity of the fluid flow across the hollow fiber membranes in blood oxygenation devices. A two-dimensional annular cross section of a blood oxygenation device including about 3,300 hollow fiber membranes was used in the computation model. The equations governing the steady incompressible laminar flow in the blood oxygenation device were solved numerically and the results were compared with those obtained from the equivalent porous medium approximation. For the porous medium approximation, the Ergun equation was used for evaluating the permeability. The simulation results showed that the fluid molecules spend about six times longer in the fiber bundle region than that in its equivalent porous medium approximation model. The computational model also provides a more detailed fluid flow pattern in the membrane compartment of the blood oxygenator.     

Anaesthesia and the respiratory system.

Pulmonary gas exchange is disturbed during general anaesthesia; both oxygenation and elimination of carbon dioxide are impaired. The shape of the chest wall alters after induction of anaesthesia-paralysis in recumbent subjects, and its motion during inspiration is also altered. The mechanical properties of lung and chest wall are also affected and FRC may be reduced. Inspired gas distribution changes after induction of anaesthesia-paralysis with mechanical ventilation of the lungs. Distribution of pulmonary blood flow is altered in subjects in the sitting and right lateral decubitus positions, but the distribution is not adjusted to the altered distribution of inspired gas. This results in an increased mismatching of ventilation to perfusion, with development of lung regions that have low and high ventilation-to-perfusion ratios. Some lung regions with low ventilation-to-perfusion ratios develop into right-to-left shunt on breathing 100 per cent oxygen. The following sequence of events probably occurs after induction of anaesthesia-paralysis. The initial effect of anaesthesia seems to be on the shape and motion of the chest wall. This may alter the mechanical properties of both the chest wall and the lung. Intrapulmonary gas distribution is altered secondarily. Pulmonary bloodflow distribution, which is primarily determined by gravity, does not seem to adjust to the altered distribution of inspired gas. Hence, an increased mismatching of ventilation to perfusion develops. This includes the development of lung regions with low ventilation-to-perfusion ratios. These regions may progress into right-to-left shung during 100 per cent oxygen breathing. The low ventilation-to-perfusion regions and the shunt may both impair oxygenation. The development of lung regions with high ventilation-to-perfusion ratios after induction of anaesthesia-paralysis contributes to the inefficient elimination of carbon dioxide.     

Effect of propofol on hepatic blood flow and oxygen balance in rabbits

PURPOSE: Propofol has been reported to alter hepatic blood flow and to increase hepatic oxygen consumption. This study was designed to determine the effect of propofol on hepatic blood flow and oxygenation in rabbits, in order to establish its net effect on hepatic oxygen balance. METHODS: Twenty, adult male, New Zealand white rabbits were randomly divided into two groups: Group P (propofol, 0.6 mg.kg(-1).min(-1)) or Group C (10% intralipid, 0.6 mg.kg(-1).min(-1)). An electromagnetic flowmeter was used to measure hepatic blood flow, and blood, from the carotid artery, the portal vein, and the hepatic vein, was used to determine hepatic oxygenation. After we obtained baseline values, we repeated measurements ten, 30, and 60 min after initiating the infusion. RESULTS: Intralipid did not affect systemic hemodynamics, hepatic blood flow, or oxygenation during the 60 min infusion; however, propofol caused a time-dependent decrease in mean arterial blood pressures and an increase in portal venous flow and total hepatic blood flow. In contrast, hepatic arterial blood flow remained unchanged during the propofol infusion. Hepatic oxygen delivery and consumption increased in a time-dependent manner to maximums of 25% and 21.4% (both, P < 0.05) above baseline, respectively. Hepatic venous oxygen saturation and extraction was unchanged throughout the study period. CONCLUSION: Propofol increases total hepatic blood flow, primarily by increasing hepatic portal venous flow. The increase in liver oxygen consumption was fully compensated by an increase in oxygen supply to the liver, resulting in a preserved, hepatic oxygen balance.