Effect of Body Repositioning after Venous Air Embolism: An Echocardiographic Study

Background: Current therapy for massive venous air embolism (VAE) may include the use of the left lateral recumbent (LLR) position, although its effectiveness has been questioned. This study used transesophageal echocardiography to evaluate the effect of body repositioning on intracardiac air and acute cardiac dimension changes.

Methods: Eighteen anesthetized dogs in the supine position received a venous air injection of 2.5 ml/kg at a rate of 5 ml/s. After 1 min the dogs were repositioned into either the LLR, LLR 10 degrees head down (LLR-10 degrees), right lateral recumbence, or remained in the supine position.

Results: Repositioning after VAE resulted in relocation of intracardiac air to nondependent areas of the right heart. Peak right ventricular (RV) diameter increase and mean arterial pressure decrease were greater in the repositioned animals compared with those in the supine position (P < 0.05). Right ventricular diameter and mean arterial pressure showed an inverse correlation (r = 0.81). Peak left atrial diameter decrease was greater in the LLR and LLR-10 degrees positions compared with the supine position (P < 0.05). Repositioning did not influence peak pulmonary artery pressure increase, and no correlation was found between RV diameter and pulmonary artery pressure. All animals showed electrocardiogram and echocardiographic changes reconcilable with myocardial ischemia.     

AN EXPERIMENTAL COMPARISON IN DOGS OF EXPIRED AIR AND OXYGEN VENTILATION DURING EXTERNAL CARDIAC MASSAGE

A comparison has been made between the effects of ventilationwith oxygen and expired air via a non-rebreathing circuit duringmanual external cardiac massage for 40–60 minutes in twogroups of dogs. Ten of the eighteen dogs receiving expired airsurvived, as did six of the twenty which received oxygen. ThePa_co2 remained below 10 mm Hg in the oxygen group and was about28 mm Hg in the expired air group. In the expired air groupthe Pa_o2 remained above 63 mm Hg. The calculated venous arterialadmixtures in eight of the dogs were between 2.1 and 14.7 percent. The importance of pupillary changes and the complicationsoccurring during and after external cardiac massage are discussed.It is suggested that, when oxygen is not available, expiredair ventilation combined with external cardiac massage may safelybe encouraged during resuscitation in previously fit humans. ^*Chester Beatty Research Institute, Royal Cancer Hospital, PollardsWood Research Station, Chalfont St Giles, Buckinghamshire. Department of Industrial Medicine, National Research Centre,Dokki, Cairo, U.A.R.     

Detection of air embolism by transesophageal echocardiography

In this study transesophageal echocardiography was utilized for detecting air embolism in dogs in the supine position and in patients undergoing neurosurgery in the sitting position. In dogs, the threshold dose of venous air for detection was determined using either a bolus injection or continuous infusion of air via the jugular vein for up to three minutes. The ability to detect air in the aorta also was determined by a bolus injection into the left ventricular via an arterial catheter. For venous injection of air, the threshold dose by bolus was 0.02 ml/kg. When given by infusion, air could be detected in all cases by both contrast echocardiogram and Doppler sound changes at the rate of 0.05 ml . kg-1 . min-1. When air was injected into the left ventricle, the threshold dose was 0.001 ml/kg using contrast echocardiogram. In the clinical evaluation, air was clearly demonstrated in five of six patients by transesophageal echocardiogram along with appropriate changes in Doppler sounds, pulmonary artery pressure, and end-tidal carbon dioxide concentration. Our results suggest that transesophageal echocardiography may be a more sensitive and accurate method for detecting venous air embolism than other commonly used monitors for patients undergoing neurosurgical procedures in the sitting position. This device may also be able to detect air in the aorta in patients experiencing paradoxical air embolism during surgery due to intracardiac or pulmonary shunts.     

Modification of a new catheter for air retrieval and resuscitation from lethal venous air embolism: effect of nitrous oxide on air retrieval.

A modification of a new Arrow prototype catheter was evaluated for its ability to retrieve venous air emboli and for its effect on the success rate of resuscitation from venous air emboli in dogs anesthetized with isoflurane and nitrous oxide (66%) in oxygen. In an additional group of dogs, nitrogen was substituted for nitrous oxide in the inspired gases to determine whether the presence of nitrous oxide (as traditionally used in this model of lethal venous air emboli) increased the apparent amount of gas retrieval or altered the success rate of resuscitation. Dogs were placed in the seated position with the head 90 degrees to the horizontal. The modified Arrow prototype catheter was placed with the proximal orifice just above the superior vena cava-right atrial junction and the distal orifice near the mid-right atrium. Dogs were then given a predetermined lethal dose of air (5 mL/kg) over 30 s through the jugular vein. Attempts to aspirate venous air emboli were begun with the first decrease in expired CO2. The amounts of gas retrieved, expressed as a percent of the injected air, and the incidence of successful resuscitation were compared between groups. In both treatment groups (nitrous oxide and nitrogen), the percent of injected venous air retrieved (73% +/- 13% and 65% +/- 21%, mean value +/- SD, respectively) and success rate of resuscitation (four of six dogs in each group) were significantly increased compared with the control group in which no attempt was made to retrieve injected venous air, and none of the six dogs survived.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bunegin-Albin catheter improves air retrieval and resuscitation from lethal venous air embolism in upright dogs

Three types of catheters, the Arrow multi-orifice catheter, the American Edwards 7 Fr Swan-Ganz catheter and the Cook Bunegin-Albin multi-orifice CVP catheter were evaluated for their ability to retrieve venous air emboli and effect on the success rate of resuscitation from venous air emboli. The catheters were inserted in dogs anesthetized with isoflurane (1.7%, inspired) and N2O (66%) in O2 and placed in the sitting position with the head 90 degrees to the horizontal. Swan-Ganz catheters were positioned with the right atrial (RA) port just above the junction of the superior vena cava (SVC) and the RA and the pulmonary artery (PA) port in the pulmonary artery. The Arrow and Bunegin-Albin multi-orifice catheters were placed with the proximal orifice just above the SVC-RA junction and the distal orifice near the mid-RA. Dogs were then given a predetermined lethal dose of air (5 ml.kg-1) over 30 sec via the jugular vein. Attempts to aspirate venous air emboli were begun with the first decrease in expired CO2. Both RA and PA ports of the Swan-Ganz catheter were used for aspiration. The amounts of gas retrieved expressed as a percent of the injected air and the incidence of successful resuscitation were compared. Significantly greater percentages of injected venous air were retrieved with the Bunegin-Albin catheter (63 +/- 14%, mean +/- SEM) than with the Arrow multi-orifice catheter (6 +/- 2%) or the Swan-Ganz catheter (14 +/- 5%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of catheter placement in the treatment of venous air embolism.

Venous air embolism is a potential complication of many surgical, therapeutic, and diagnostic procedures. Aspiration of air via a catheter placed in the superior vena cava or right atrium or placed in the pulmonary outflow tract and pulled through the right heart chambers had been advocated for the treatment of venous air embolism. In this study, three catheter positions were analyzed to determine which was best for removal of gas after induction of massive venous air embolism in dogs. In 18 dogs, 9 of which were suspended by their forelegs to simulate the sitting position used in posterior fossa exploration and 9 of which were supine, a Swan-Ganz catheter was placed in the right atrium, right ventricle, or pulmonary artery. A measured amount of air was injected into the left jugular vein and syringe aspiration of the air was attempted through the catheter. In the group with the catheter in the pulmonary artery, aspiration was continuous while the catheter was withdrawn through the right heart chambers. The amount of air aspirated varied widely among the three catheter positions, and no one catheter position proved superior to the other two.     

Gas embolism during hysteroscopy

PURPOSE: Gas embolism during hysteroscopy is rare but sometimes fatal. A fatal case of gas embolism during diagnostic hysteroscopy using carbon dioxide (CO2) is presented. CLINICAL FEATURES: A 68 yr old woman was admitted for treatment of myoma and cancer of the uterus. Hysteroscopy using CO2 was performed without monitoring or anesthesia on the ward. At the end of the examination, just after the hysteroscope was removed, she developed tonic convulsions, lost consciousness, and her pulse was impalpable. Cardiac massage was started, anesthesiologists were called and the trachea was intubated. She was transferred to the intensive care unit with continuous cardiac massage. Cardiac resuscitation was successful. A central venous line was inserted into the right ventricle under echocardiography in an attempt to aspirate gas with the patient in the Trendelenberg position, but the aspiration failed. Positive end expiratory pressure and heparin for emboli, midazolam for brain protection, and catecholamines were administered. Fifteen hours after resuscitation, the pupils were enlarged and she died 25 hr after resuscitation. CONCLUSION: Gas embolism is a rare complication of hysteroscopy. The procedure should be performed with monitoring of blood pressure, heart rate, oxygen saturation and end-tidal CO2 concentration.     

Pulmonary Blood Flow Does Not Redistribute in Dogs with Reposition from Supine to Left Lateral Position

Background: Recent studies have questioned the classical gravitational model of pulmonary perfusion. Because the lateral position is commonly used during surgery, the authors studied the redistribution of pulmonary blood flow in the left lateral decubitus position using a high spatial resolution technique.

Methods: Distributions of pulmonary blood flow were measured using intravenously injected 15-[micro sign]m diameter radioactive-labeled microspheres in eight halothane-anesthetized dogs, which were studied in the supine and left lateral decubitus positions in random order. Lungs flushed free of blood were air-dried at total lung capacity and sectioned into 1,498-2,396 (1.7 cm3) pieces per animal. Radioactivity was measured by a gamma counter, and signals were corrected for piece weight and normalized to mean flow.

Results: Blood flow to the dependent left lung did not increase, and blood flow to the nondependent right lung did not decrease in the lateral position. The left lung received 39.3 +/- 7.0% and 39.2 +/- 8.8% (mean +/- SD) of perfusion in the supine and left lateral positions, respectively. Detailed assessment of the spatial distributions of pulmonary blood flow revealed the lack of a gravitational gradient of blood flow in the lateral position. The distributions of blood flow did not differ in the supine and left lateral decubitus positions.     

Lipid emulsion infusion rescues dogs from bupivacaine-induced cardiac toxicity

BACKGROUND AND OBJECTIVES: We previously demonstrated in rats that intravenous infusion of a lipid emulsion increases survival in resuscitation from severe bupivacaine cardiac toxicity. The present studies were undertaken to determine if this method is similarly effective in a non-rodent model using a larger animal. METHODS: Bupivacaine, 10 mg/kg, was administered intravenously over 10 seconds to fasted dogs under isoflurane general anesthesia. Resuscitation included 10 minutes of internal cardiac massage followed with either saline or 20% lipid infusion, administered as a 4-mL/kg bolus followed by continuous infusion at 0.5 mL/kg/min for 10 minutes. Electrocardiogram (EKG), arterial blood pressure (BP), and myocardial pH (pHm) and pO2 (pmO2) were continuously measured. RESULTS: Survival after 10 minutes of unsuccessful cardiac massage was successful for all lipid-treated dogs (n = 6), but with no survivors in the saline controls (n = 6) (P <.01). Hemodynamics, PmO2, and pHm were improved during resuscitation with lipid compared with saline treatment in which dogs did not recover. CONCLUSIONS: We found that infusing a lipid emulsion during resuscitation from bupivacaine-induced cardiac toxicity substantially improved hemodynamics, pmO2, and pHm and increased survival in dogs.     

Transfemoral balloon aortic occlusion during open cardiopulmonary resuscitation improves myocardial and cerebral blood flow

These experiments were designed to determine whether the limited cardiac output during open cardiac massage could be preferentially directed to the coronary and cerebral vessels by balloon occlusion of the descending thoracic aorta. Sixteen dogs were instrumented to monitor cardiac output and left atrial, right atrial, right ventricular, left ventricular, and arterial blood pressures. Measurements of myocardial and cerebral blood flow distribution during massage were made using the radioactive microsphere technique. Each animal underwent two episodes of fibrillation and resuscitation. In one episode the arrest was managed by open massage alone, and in the other, open massage was accompanied by balloon occlusion, with the order randomized. When compared to control, open cardiac massage was associated with a significant decrease in mean arterial pressure; however, the addition of balloon occlusion produced a 130% increase in the mean arterial pressure that was obtained during open CPR (control, 93 +/- 5 mm Hg; massage alone, 35 +/- 2 mm Hg; massage + balloon, 76 +/- 2 mm Hg, P less than 0.01). In a similar fashion, although the absolute blood flow was reduced by 50% when compared to control, the blood flow (ml/min/g) to the brain and heart during massage was 100% better when balloon occlusion was employed (brain: control, 0.41 +/- 0.03; massage only, 0.05 +/- 0.01; massage + balloon, 0.25 +/- 0.02, P less than 0.01; heart: control, 1.46 +/- 0.11; massage alone, 0.35 +/- 0.05; massage + balloon, 0.71 +/- 0.05, P less than 0.01). These results suggest that aortic occlusion significantly increased myocardial and cerebral perfusion patterns during ventricular fibrillation and open cardiac massage.(ABSTRACT TRUNCATED AT 250 WORDS)     

Venous air embolism: a warning not to be complacent--we should listen to the drumbeat of history

Clinical Considerations Concerning Detection of Venous Air Embolism. By Maurice S. Albin, Robert G. Carroll, Joseph C. Maroon. Neurosurgery 1978; 3:380-84. Abstract used with permission from the Congress of Neurological Surgeons, copyright 1978. Venous air embolism during neurosurgical procedures (detected by Doppler ultrasound and aspiration via a right atrial catheter) was noted in 100 of 400 patients in the sitting position, 5 of 60 patients in the lateral position, 7 of 48 patients in the supine position, and 1 of 10 patients in the monitored prone position. We confirmed venous air embolism in many of these patients by using serial technetium-microaggregated albumin lung scans. Gravitational gradients from the venous portal of entrance to the right side of the heart were as small as 5.0 cm, with aspiration of 200 ml of air occurring. Doppler ultrasonic air bubble detection and aspiration through a previously inserted right atrial catheter are critical factors in the diagnosis and treatment of this condition.     

Evaluation of transesophageal Doppler detection of air embolism in dogs

The authors assessed the use of a cylindrical sensor which transceives (transmits and receives) ultrasound in a 360 degree arc mounted on a standard esophageal stethoscope catheter to detect air embolism in dogs. They used electronic circuitry developed specifically to provide a continuous analog recording of high frequency Doppler energy as well as an audible signal. They found that the esophageal Doppler sensor was easy to position. In 25 of 30 dogs, the system distinguished Doppler sounds of venous air emboli that were present either in the superior vena cava, right atrium, right ventricle, or pulmonary artery from normal cardiovascular sounds. In the remaining five animals, Doppler heart and air emboli sounds were initially of poor quality, but improved with aspiration of air from the esophagus. In another five dogs, arterial air emboli produced by left ventricular injections also were detected. The esophageal sensor was sensitive to both venous and arterial air emboli ranging from 0.05--0.2 ml of air, and the authors consistently detected repeated intravenous injections of air throughout a 5-h time period. Optimal position of the sensor for detection of venous air emboli was at the level of the superior vena cava above its junction with the right atrium. Optimal position to detect arterial air emboli introduced via a left ventricular catheter was at the level of the aortic arch. Tissue analysis of the esophagus revealed no morphologic damage due to the sensor or transmitted energy.     

Thermal filament continuous thermodilution cardiac output delayed response limits its value during acute hemodynamic instability.

BACKGROUND: It has been suggested that measurement of continuous cardiac output (CCO) is an advancement in the management of critically ill patients. Our objective was to determine the accuracy of CCO during the rapid hemodynamic changes induced by hemorrhage and resuscitation. METHODS: In 12 anesthetized dogs (20.2+/-0.9 kg), pulmonary artery blood flow, our "gold standard" cardiac output, was measured with an sonographic flowprobe, whereas CCO, intermittent bolus cardiac output (ICO), and mixed venous oxygen saturation were measured with a thermodilution fiberoptic pulmonary artery catheter with a thermal filament. A graded hemorrhage (20 mL/min) was produced to a mean arterial pressure of 40 mm Hg, which was maintained at this level for 30 minutes. Total shed blood volume (701+/-53 mL) was retransfused at a rate of 40 mL/min, over 30 minutes, after which a massive hemorrhage (100 mL/min) was produced over 10 minutes. RESULTS: Hemorrhage induced significant decreases in mean arterial pressure, mixed venous oxygen saturation, and oxygen delivery, which were all restored during early resuscitation. However, CCO showed a delayed response after hemorrhage and resuscitation, compared with pulmonary blood flow, throughout the study (r = 0.549), matching only at baseline and at the end of both graded hemorrhage and resuscitation periods. There was a good correlation between ICO and pulmonary artery blood flow (r = 0.964) and no significant differences between them throughout the study. CONCLUSION: CCO has a delayed response during acute hemodynamic changes induced by hemorrhage and resuscitation. When sudden changes in mean arterial pressure or in mixed venous oxygen saturation are detected, cardiac output must be estimated by the standard bolus thermodilution technique, not by CCO.     

An experimental study on CPR and lung metabolism]

We investigated the influence of cardiac arrest upon the metabolic function of the lungs, especially on prostaglandins and angiotensin in pulmonary arterial and venous blood (PA, PV) in dogs resuscitated after cardiac arrest. Experiments were performed on 36 dogs. Cardiac arrest was produced with the electric ventricular fibrillation. The resuscitation was carried out with 100% oxygen using a ventilator. Open chest cardiac massage was performed and defibrillation was done with DC of 25-75 W.sec. Epinephrine and 7% sodium carbonate were administered. Experimental groups were as follow; Group 1 (G I): CPR 5 minutes after the cardiac arrest. Group 2 (G II): CPR 10 minutes after the cardiac arrest. There were no significant differences between G I and G II in hemodynamics and blood gas. AT I was higher in PA than in PV, and AT II showed no significant difference between PA and PV. PGE2 and PGF2 alpha showed no significant changes in G I. And in G II they reached the peak in PA after resuscitation, PGE2 increased in PV, and PGF2 alpha showed no significant changes in PV. TXB2 and 6-keto-PGF1 alpha showed no differences between PA and PV, and they reached a peak after resuscitation in G II.     

In vitro comparison of central venous catheters for aspiration of venous air embolism: effect of catheter type, catheter tip position, and cardiac inclination

STUDY OBJECTIVE: To test the relative efficiency of balloon-tipped and plain catheters for aspiration of venous air embolism. DESIGN: The following four single-lumen central venous catheters were studied in a silastic model of the right atrium, tricuspid valve, and vena cavae: (1) the 16-gauge single-orifice catheter; (2) the 14-gauge Bunegin-Albin multiorifice catheter; (3) the 7-Fr pulmonary angiography catheter with balloon distal to orifices; (4) the 7-Fr pulmonary angiography catheter with balloon proximal to orifices. A 10% glycerol-water solution was circulated at 3.7 to 4.0 L/min. Catheter tips were positioned at 1 cm increments from -3 to +3 cm around the superior vena cava-right atrial junction with cardiac inclinations of 65 degrees and 80 degrees. Air (10 ml) was infused over 30 seconds; aspiration from the test catheter began 5 seconds later at 40 ml/min for 75 seconds. The balloon catheters were evaluated with the balloons inflated and deflated. SETTING: Experimental laboratory of a university-affiliated hospital. MEASUREMENTS AND MAIN RESULTS: Amounts of air aspirated were compared by analysis of variance and Tukey's multiple comparison, p less than 0.05, for all combinations. The 16-gauge single-orifice catheter tested best at a 65 degrees cardiac inclination with 86% of the venous air embolism recovered, while the 14-gauge Bunegin-Albin multiorifice catheter tested best at an 80 degrees cardiac inclination with 62% of the venous air embolism recovered. Both catheters functioned most efficiently at or above the superior vena cava-right atrial junction. CONCLUSIONS: This study demonstrated that efficacy of air recovery depends on catheter type, catheter tip position, and cardiac inclination. No benefit was derived from positioning the catheter tip inside the atrium or from using balloon-tipped catheters.     

Transcranial Doppler for the monitoring of gas embolism in neurosurgical operations in the sitting position]

The risk of venous air embolism is significant in neurosurgical procedures performed in the sitting position. Monitoring for venous air embolism, therefore, is crucial and can be approached from several aspects. The most sensitive generally applicable clinical method for the detection of intracardiac gas is based on an application of the Doppler principle. The Authors describe the technique used to adapt a transcranial Doppler (TC 2-64 B, EME, Germany), and a probe designed to record the Doppler signal from intracranial arteries (Transcran FP 2, EME, Germany), as precordial Doppler in order to monitor venous air embolism in neurosurgical procedures performed in the sitting position.     

Mechanically Induced Ventricular Systoles During External Cardiac Massage and Thumping of the Chest

The correlation between cardiac compression, ECG potentials and the rise of intracardiac and arterial pressure was studied in dogs during external cardiac massage and thumping of the chest. The direct mechanical effect of the compressions was unsatisfactory. The compressions often elicited QRS complexes, particularly with thumping of the chest, which were frequently followed by a secondary rise of pressure equal to that associated with spontaneous heart beats. The time analysis indicated that the artificially elicited QRS complexes had evoked effective heart contractions. This indirect mechanical effect of compressions of the chest was much superior to the direct effect and is probably of major importance for resuscitation.     

132. Therapie der massiven Luftembolie

Zusammenfassung Äußere Herzmassage allein ist nicht effektiv, wenn bei massiver venöser Luftembolie bereits ein Herzstillstand vorliegt. Nur die sofortige Evakuation der Luft aus dem rechten Herzen kann Erfolg bringen. Tritt das Ereignis im Operationssaal ein, ist die sofortige Thorakotomie indiziert: sie ermöglicht die optimale Luftevakuation aus dem rechten Ventrikel ohne Zeitverlust durch ungenügende Punktionsversuche oder ungenügende äußere Herzmassage. Luftblasen in den Coronararterien können erkannt und evakuiert werden.Wir haben bei einem Patienten mit Asystolie durch sofortige Thorakotomie die Luft aus dem eindrücklich überblähten rechten Ventrikel durch Punktion entfernt. Bei einer Luftembolie mit Kreislaufzusammenbruch während dem Einlegen eines Subclaviakatheters konnte durch Vorschieben des Katheters ins rechte Herz die Luft evakuiert werden. Beide Patienten überlebten ohne Nachwirkungen.

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Treatment of massive air embolism

Summary In massive venous air embolism producing cardiac arrest external cardiac massage alone cannot be a successful therapy. Immediate evacuation of the air from the right ventricle is essential. If air embolism occurs in the operating theater, emergency thoracotomy with needle aspiration of the right ventricle should be performed; only by this procedure is complete and prompt evacuation of air possible. Air embolism into the coronary arteries is recognized and can be treated.In a patient with cardiac arrest, immediate thoracotomy was performed and the air was evacuated from the extended right ventricle by needle aspiration. In another patient, massive air embolism occurred during introduction of a subclavian catheter: immediate advancing of the catheter made aspiration of air from the right heart possible. Both patients survived without sequelae.

     

External cardiac massage using a hand-powered chest compressor on dogs with ventricular fibrillation

We devised a hand-powered portable chest compressor for external cardiac massage. The purpose of this study was to assess the efficacy and safety of this device in comparison to manual chest compression in dogs with ventricular fibrillation. Five out of 7 dogs that received manual chest compression during cardiopulmonary resuscitation (CPR) were successfully resuscitated. Seven out of 8 dogs that received mechanical chest compression with this device during CPR were successfully resuscitated. There were no differences between the two methods in maximum arterial pressure at 1 and 10min after the initiation of CPR. There was also no difference between the two methods in pulmonary arterial pressure or arterial and mixed venous blood gases during CPR. Minimum arterial pressure during CPR was higher in dogs receiving mechanical chest compression than those receiving manual chest compression. This study reveals that the hand-powered chest compressor is equally efficient for external cardiac massage as manual cardiac massage. Moreover, this device can be useful in a situation where manual compression has to be interrupted, such as during litter transport of the patient, and so on.(Kishi H, Okamoto K, Goto K, et al.: External cardiac massage using a hand-powered chest compressor on dogs with Ventricular Fibrillation. J Anesth 5: 352–358, 1991)     

The effect of position on autonomic nervous activity in late pregnancy

We investigated the effect of three different recumbent positions on autonomic nervous activity in late pregnancy. Thirty pregnant and 24 nonpregnant aged-matched women were studied, and measures of heart rate variability in both time and frequency domains were compared using supine, right and left lateral decubitus positions. In the nonpregnant women, the normalised high-frequency power was greatest in the right lateral decubitus position. In the pregnant women, the normalised high-frequency power was lowest and the low/high-frequency power ratio was greatest in the supine position. Both the percentage decrease of normalised high-frequency power and the percentage increase of low/high-frequency power ratio in the supine and right lateral positions were greater than those in the left lateral position. For women in late pregnancy, the left lateral decubitus position may be beneficial because cardiac vagal activity is least suppressed and cardiac sympathetic activity is least enhanced. Aortocaval compression might be the mechanism underlying the change in cardiac autonomic nervous activity when supine and right lateral decubitus positions are assumed in late pregnancy.