Effect of positive end-expiratory pressure on the incidence of venous air embolism and on the cardiovascular response to the sitting position during neurosurgery

We have studied prospectively the effect of 10 cm H2O of PEEP on the incidence of venous air embolism and on the cardiovascular response to change from the supine to the seated position in a large neurosurgical population. Patients were allocated randomly to receive either PEEP (10 cm H2O, n = 45) or conventional (control, n = 44) ventilation. Cardiovascular and respiratory variables were measured in the supine and sitting positions, and monitoring included precordial Doppler probe, pulmonary artery pressure and expiratory carbon dioxide concentration. Venous air embolism was assumed if changes in precordial Doppler sounds occurred, end-tidal carbon dioxide concentration decreased or air could be retrieved from a central venous multi-orifice catheter. The incidence of venous air embolism (26%) did not differ between patients undergoing conventional ventilation and those undergoing ventilation with 10 cm H2O of PEEP. Venous air embolism was always detected first by alterations in Doppler sounds. Cardiac output was significantly higher in patients undergoing conventional ventilation than in those undergoing ventilation with PEEP in the supine but not in the sitting position. Furthermore, pulmonary vascular resistance increased significantly only in the upright position in those undergoing ventilation with PEEP. The pulmonary artery wedge pressure to central venous pressure gradient did not attain negative values with PEEP or with upright positioning. We conclude that the use of PEEP during neurosurgical procedures performed in the sitting position should be abandoned as it does not decrease the incidence of venous air embolism but is associated with significant adverse cardiovascular effects.      

Prevention of air embolism with positive end expiratory pressure

Pulmonary air embolism is recognized as a possible complication of neurosurgical procedures performed with the patient in the sitting position. A variety of preventive and therapeutic modalities have been proposed in the literature. We have used a consistent regimen consisting of precordial Doppler monitoring, measurement of end expiratory CO2, the semireclining position, and positive end expiratory pressure (PEEP). A right atrial catheter was not used. This approach has given good results in 81 patients; there was significant air embolism in only 1 case (1.2%). We believe that PEEP is as important in the prevention as it is in the treatment of pulmonary air embolism. By flexibly adjusting the level of PEEP, one may recreate the hemodynamic equivalent of the prone position, thereby eliminating the risk of venous air embolism and simultaneously the need for right heart catheterization.     

Intravenous fluid loading as prophylaxis for paradoxical air embolism

Paradoxical cerebral air embolism has been described in neurosurgical operations performed on patients in the seated position. This problem is thought to result most often from a probe-patent foramen ovale. It has been postulated that right atrial pressure exceeds left atrial pressure when paradoxical air embolism occurs. A study is described in which intravenous fluid loading is compared with routine fluid management in 20 patients undergoing neurosurgical operations in the seated position. In order to investigate if intravenous fluid loading would decrease the risk of paradoxical air embolism during neurosurgical operations on seated patients, 20 patients were assigned randomly to two groups: 10 patients received normal intravenous fluid replacement (1220 +/- 102 ml), and 10 received augmented fluid replacement (2800 +/- 400 ml). Right atrial and pulmonary capillary pressures were monitored for evidence of an interatrial pressure gradient that would force air emboli from the right atrium into the left atrium via a probe-patent foramen ovale. Four of 10 patients receiving routine fluid administration developed right atrial pressure greater than pulmonary capillary wedge pressure (and hence, indirectly, greater than left atrial pressure), whereas none of the 10 patients with augmented fluid loading developed this condition (p = 0.04). The authors conclude that augmented intravenous fluid loading may be effective in preventing systemic air embolism during neurosurgical operations performed on patients in the seated position.     

The incidence of venous air embolism during total hip arthroplasty

Venous air embolism has been reported to occur during total hip arthroplasty. The incidence of venous air embolism, however, has not been previously studied in a large series using Doppler ultrasound and mass spectrometry. Seventy patients undergoing total hip arthroplasty were monitored for venous air embolism with precordial Doppler ultrasound, central venous catheter, end-tidal N2 and CO2 (mass spectrometry), and arterial blood gases (ABG). Changes in the monitored variables consistent with venous air embolism were noted in 57% by Doppler ultrasound, 9% by mass spectrometry, 4% by central venous catheter and 3% of the cases by ABG. A total of 77 Doppler ultrasound events were detected in 40 of the 70 patients studied. Hemodynamic changes consisting of either hypotension, defined as a greater than or equal to 20% decrease in mean arterial pressure (MAP), or cardiac dysrhythmia occurred during 43% of these events. The Doppler ultrasound was the only monitor that detected all cases of venous air embolism with concomitant hemodynamic changes. Air was aspirated from the central venous catheter during 10% of the detections of venous air embolism by Doppler ultrasound. Venous air embolism in total hip arthroplasty is a common event and may be responsible for hemodynamic changes previously ascribed to the use of methylmethacrylate cement. Routine monitoring with Doppler ultrasound appears warranted. The routine use of central venous catheterization may also be warranted.     

Venous air embolism prophylaxis with a surface-active agent.

The protective effect of a nonionic surface-active polyol agent (Pluronic F-68) against bolus injection and constant-rate IV infusion of air was studied in 21 dogs anesthetized with pentobarbital. Aortic, pulmonary artery and right ventricular pressures, cardiac output, end-tidal CO2 concentration, wasted ventilation, and blood surface tensions were measured before and after the IV administration of this surfactant. The magnitudes of change in the cardiorespiratory responses measured after venous air embolism were significant (p less than 0.05) reduced in the treated animals. This agent may be advantageous for surgical patients when an increased risk of venous air embolism exists.     

Precordial Doppler probe placement for optimal detection of venous air embolism during craniotomy

Verification of appropriate precordial Doppler probe position over the anterior chest wall is crucial for early detection of venous air embolism. We studied responses to normal saline (NS) and carbon dioxide (CO2) test injections at various probe locations during elective craniotomy. All patients received four IV injections (10 mL of NS and 1 mL of CO2 via central and peripheral venous catheters). Doppler sounds were simultaneously recorded with two separate probes. In Group A, probes were placed in left and right parasternal positions. In Group B, the left probe was intentionally malpositioned as far laterally over the left precordium as was compatible with an audible signal. In Group A (n = 23), a left parasternal Doppler signal was easily obtainable in 23 of 23 patients, versus 18 of 23 patients for the right parasternal probe (P < 0.05). In Group B (n = 17), central CO2 injection yielded a positive right parasternal response rate of 88% compared with 29% over the far left precordium (P < 0.015), where central NS injections yielded a 76% response rate (P < 0.015 versus central CO2 injection). Left parasternal placement is at least as sensitive to clinical venous air embolism events as right parasternal placement. Peripheral saline injection represents a viable alternative (83% response rate). Vigorous central injection of 10 mL of NS however, risks false positive verification of left lateral precordial probe placement.     

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)     

Effects of Venous Air Embolism on the Cardiovascular System and Acid Base Balance in the Presence and Absence of Nitrous Oxide

Cardiovascular responses and acid-base changes with graded volumes of intravenously injected air were measured in dogs anesthesized with pentobarbital and either 100% oxygen or 50% oxygen and nitrous oxide. Mean arterial blood pressure decreased significantly with 2.5 ml of air/kg in the oxygen group and at all volumes in the nitrous oxide group. The volume of air embolus appeared to increase more than twice in the presence of 50% nitrous oxide. Pulmonary artery wedge pressure increased significantly in both groups, while end-tidal carbon dioxide decreased significantly in both groups. The changes in pulmonary artery wedge pressure and end-tidal carbon dioxide were simultaneous, suggesting that the less invasive monitoring technique may be utilized in diagnosing significant air embolism. Right atrial pressure increased while left atrial pressure decreased significantly in both groups. With injected volumes of 2.0 and 2.5 ml or air/kg, right atrial pressure exceeded left atrial pressure creating the potential of paradoxical air embolism. The pH and PaO2 decreased while PaCO2 increased significantly during air embolization.     

Transesophageal echocardiography and transcutaneous O2 and CO2 monitoring for detection of venous air embolism

The sensitivities of current monitoring methods for detection of air embolism were compared in eight anesthetized dogs. Air was infused at controlled rates of 0.001 and 0.005 ml X kg-1 X min-1 for 1 min; 0.01, 0.05, 0.1, 0.2, and 0.4 ml X kg-1 X min-1 for 6 min; and 5 ml X kg-1 bolus injection. Based on the mean quantity of air infused to elicit a positive response, the monitors could be placed into three significantly different sensitivity groups. Transesophageal echocardiography (TEE) and precordial Doppler ultrasound were the most sensitive monitoring methods detecting 0.19 and 0.24 ml X kg-1 of air, respectively. TEE detected air during six infusions in which the Doppler failed to do so. The next most sensitive group of monitoring methods included pulmonary artery pressure (PAP), end-tidal CO2 (PETCO2), arterial oxygen tension (PaO2), and transcutaneous oxygen tension (PtcO2). The mean quantity of air infused to elicit a positive response in this group of monitors ranged from 0.61 to 0.76 ml X kg-1. The response of PtcO2, PaO2, PETCO2, and PAP equally reflected the quantity of air infused. The least-sensitive group of methods included arterial and transcutaneous carbon dioxide tension and systemic arterial blood pressure. These data indicate that TEE is more sensitive than Doppler ultrasound and that PAP, PETCO2, and PtcO2 are equally sensitive in detecting venous air embolism in the dog.     

Incidence of venous air embolism during craniectomy for craniosynostosis repair

BACKGROUND: Investigations to determine the incidence of venous air embolism in children undergoing craniectomy for craniosynostosis repair have been limited, although venous air embolism has been suspected as the cause of hemodynamic instability and sometimes death. A precordial Doppler ultrasonic probe is an accepted method for detection of venous air embolism and is readily available at most institutions. METHODS: A prospective study was conducted using a precordial Doppler ultrasonic probe in children undergoing craniectomy for craniosynostosis repair. The Doppler signal was continuously monitored intraoperatively for characteristic changes of venous air embolism. A recording was made of the precordial Doppler probe pulses, which was later reviewed by a neuroanesthesiologist, blinded to the intraoperative events. This information was correlated with the intraoperative events and episodes of venous air embolism were graded. RESULTS: Twenty-three patients were enrolled in the study during the 2-yr study period. Nineteen patients (82.6%) demonstrated 64 episodes of venous air embolism; six patients (31.6%) had hypotension associated with venous air embolism. Thirty-two episodes of hypotension were demonstrated in eight patients (34.7%). None of the patients developed cardiovascular collapse. CONCLUSION: The incidence of venous air embolism in our study of 23 children undergoing craniectomy for craniosynostosis was 82.6%. Though most episodes of venous air embolism during craniosynostosis repair are without hemodynamic consequences, the preemptive placement of a precordial Doppler ultrasonic probe is a noninvasive, economic, and safe method for the detection of venous air embolism. Prompt recognition may allow for the early initiation of therapy, thereby decreasing morbidity and mortality rates related to venous air embolism.     

Anaesthesia for neurosurgery in the sitting position: a practical approach

Neurosurgery in the sitting position offers advantages for certain operations. However, the approach is associated with potential complications, in particular venous air embolism. As the venous pressure at wound level is usually negative, air can be entrained. This air may follow any of four pathways. Most commonly it passes through the right heart into the pulmonary circulation, diffuses through the alveolar-capillary membrane and appears in expelled gas. It may pass through a pulmonary-systemic shunt such as a probe patent foramen ovale (paradoxical air embolism); it may collect at the superior vena cava-right atrial junction. Rarely it may traverse through lung capillaries into the systemic circulation. Many monitors, such as the precordial Doppler; capnography, pulmonary artery catheter; transoesophageal echocardiography are useful for venous air embolism detection, with transoesophageal echocardiography being today's gold standard. Various manoeuvres, including neck compression and volume loading, are also useful in reducing the incidence of venous air embolism. Volume loading, in particular; is very helpful as it reduces the risk of hypotension. Other particular concerns to the anaesthetist are airway management, avoidance of pressure injuries, and the risk of pneumocephalus, oral trauma, and quadriplegia. Newer anaesthetic agents have made the choice of anaesthetic technique easier. An appreciation of the implications of neurosurgery in the sitting position can make the procedure safer     

Fluid Replacement Monitoring: Effect of Dextran Overload, Norepinephrine Drip, and Positive Pressure Ventilation on Systemic Arterial, Right Atrial Pulmonary Wedge, and Left Atrial Pressures in Dogs

The effects of dextran overload, norepinephrine drip and positive pressure ventilation upon right atrial, pulmonary wedge, left atrial and systemic arterial pressures were studied in 15 dogs. Rapid intravenous infusion of Dextran 70 invariably produced a marked and statistically significant (p < .001) rise in right atrial, pulmonary wedge and left atrial pressures. The rise in left atrial pressure invariably exceeded the rise in right atrial pressure, and the difference in maximum pressures averaged 10.8 mm Hg (p < .001). Thus acute fluid overload and pulmonary edema can be produced by rapid infusion of colloid solution in the absence of a marked rise in right atrial pressure, a point of considerable clinical importance. The rapid infusion of dextran produced a rise in systemic arterial blood pressure in all dogs so studied, though this rise was mild in some animals. This finding may explain in part the hypertension exhibited by patients in the recovery room who may have been overtransfused. A norepinephrine drip usually produced an increase in right atrial, wedge, left atrial and systemic arterial blood pressure (p < .01). When there was a significant rise in right atrial pressure and left atrial pressure, the maximum increase in left atrial pressure was always greater than the maximum increase in right atrial pressure (p < .005). This finding again emphasizes the fact that blood transfusion requirements cannot always be accurately assessed on the basis of right and left atrial pressure measurements when a vasopressor agent is being administered. Positive pressure ventilation increased both right and left atrial pressures, as expected. It was again confirmed that pulmonary wedge pressure, as measured with the Swan-Ganz catheter, is approximately equal to left atrial pressure over a wide range of induced variations. The Swan-Ganz catheter, introduced at the bedside in the intensive care unit when necessary, can provide highly useful information regarding left atrial pressure and left ventricular end-diastolic pressure.     

Incidence of Venous Air Embolism during Craniectomy for Craniosynostosis Repair

Background: Investigations to determine the incidence of venous air embolism in children undergoing craniectomy for craniosynostosis repair have been limited, although venous air embolism has been suspected as the cause of hemodynamic instability and sometimes death. A precordial Doppler ultrasonic probe is an accepted method for detection of venous air embolism and is readily available at most institutions.

Methods: A prospective study was conducted using a precordial Doppler ultrasonic probe in children undergoing craniectomy for craniosynostosis repair. The Doppler signal was continuously monitored intraoperatively for characteristic changes of venous air embolism. A recording was made of the precordial Doppler probe pulses, which was later reviewed by a neuroanesthesiologist, blinded to the intraoperative events. This information was correlated with the intraoperative events and episodes of venous air embolism were graded.

Results: Twenty-three patients were enrolled in the study during the 2-yr study period. Nineteen patients (82.6%) demonstrated 64 episodes of venous air embolism; six patients (31.6%) had hypotension associated with venous air embolism. Thirty-two episodes of hypotension were demonstrated in eight patients (34.7%). None of the patients developed cardiovascular collapse.     

Venous air embolism following repositioning from sitting to supine

We have experienced a patient in whom venous air embolism reoccurred, when the patient's position was changed from sitting to supine. A 40 year old male with Arnold-Chiari malformation underwent suboccipital decompression and cervical laminectomy under the sitting position. During surgery, three episodes of venous air embolism were detected by high pitched sound through precordial ultrasound Doppler stethoscope, an abrupt increase in pulmonary arterial pressure, a decrease in end-tidal carbon dioxide concentration; and a small amount of bubbled air was removed from the central venous catheter. At the end of surgery when the patient was turned to supine position, the signs of venous air embolism reappeared and 3 ml of bubbled air was also removed. This case suggests that there is some remaining air in the large veins of the upper part of the body once the air embolism has occurred during sitting position and thus we need to confirm that no air is left in the large veins before repositioning. We should be cautious of reoccurrence of venous air embolism whenever patient's position is changed.     

Capnography in the early diagnosis of carbon dioxide embolism during laparoscopy

Venous embolism of carbon dioxide occurred during elective diagnostic laparoscopy in a healthy adult female. The diagnosis of gas embolism was made on the basis of the sudden abrupt onset of systolic and diastolic murmurs. The continuously recorded end-tidal carbon dioxide concentration (FETCO2) increased abruptly from 3.8 to 4.2 per cent and then slowly decreased to 4.0 per cent over the subsequent 30 seconds. CO2 insufflation was terminated immediately following the establishment of the diagnosis. The patient recovered uneventfully. A transient but rapid rise in FETCO2 is suggested as a useful early sign of venous CO2 embolism during laparoscopy.     

Venous air embolism, hypotension, and end-tidal nitrogen

The most sensitive method of venous air embolism (VAE) detection in clinical use is the precordial Doppler detector. Because the Doppler detector may provide false-positive and false-negative data, many clinicians rely on end-tidal gas measurements for verification of VAE in the operating room. End-tidal nitrogen (ETN2) increases soon after experimental VAE are small enough to cause minimal changes in blood pressure; however, decreases in end-tidal CO2 (ETCO2) are more sensitive. A large VAE causes hemodynamic instability, and the effect of low cardiac output on ETN2 has not been evaluated. This study was done to compare the changes in ETN2 and ETCO2 during large bolus and infusion VAE. Five mongrel dogs were anesthetized, intubated, and ventilated (FIO2 1.0, PaCO2 38 +/- 4 (SD) mm Hg). The animals were studied in the supine position; anesthesia and paralysis were maintained with a constant infusion of thiamylal and pancuronium. Maintenance fluids were administered at 5 ml kg-1 h-1. Mean arterial and pulmonary arterial pressures (PAP) and ETN2 and ETCO2 were displayed on a strip chart recorder. The dogs underwent both bolus and infusion VAE in separate experiments 10 to 14 days apart. The air emboli were given in random order by automated syringe over 1 minute (infusion) (1 to 2.5 ml kg-1 min-1) or by hand injection over 5 seconds (bolus) (1 to 2.5 ml kg-1). Changes in precordial Doppler sounds occurred in all animals at all doses. The peak increase in PAP and decrease in ETCO2 were significant after all air doses. ETN2 changes were biphasic. The peak increase was significant after all air doses; the peak decrease was significant in 37 of 40 bolus and infusion VAE episodes, occurred within 1 to 3 minutes, and lasted 20 to 30 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hemodynamic consequences of PEEP in seated neurological patients--implications for paradoxical air embolism

In order to better understand the hemodynamic consequences of the use of positive end-expiratory pressure (PEEP) in patients in the seated position, 11 patients undergoing neurosurgical operations were monitored with radial arterial and thermistor-tipped Swan-Ganz catheters both before and during 10-cm H2O PEEP. Significant (P less than 0.05) reductions in cardiac output (15%), stroke volume (15%), and mean arterial pressure (14%) occurred with the introduction of PEEP, while pulmonary vascular resistance increased 47% and right atrial pressure (RAP) increased from 3.6 +/- 0.7 SEM mm Hg to 8.9 +/- 0.9 SEM mm Hg (P less than 0.05). Pulmonary capillary wedge pressure (PCWP) did not increase significantly during PEEP. RAP exceeded PCWP in only two patients before PEEP, but RAP exceeded PCWP in seven patients during PEEP. We conclude that PEEP is potentially detrimental during operations in the seated position because it not only impairs hemodynamic performance, but might predispose patients with a probe-patent foramen ovale to the risk of paradoxical air embolism.     

Complications caused by a Swan-Ganz catheter. An inadvertent entrapment in the right atrium during cardiac surgery

Insertion of a Swan-Ganz catheter for measurement of pulmonary artery and pulmonary capillary wedge pressures is common for operative management of patients with borderline left ventricular function. However, use of the catheter has associated risks. This case report illustrates the inadvertent snaring, by a suture, of a Swan-Ganz line in the right atrium and its subsequent removal transvenously during rethoracotomy.     

Surveillance hémodynamique au cours de l''exérèse micro-neurochirurgicale par voie sous- et rétrosinusienne en position assise du neurinome de l''acoustique

Haemodynamic data (thermodilution Swan-Ganz catheter and radial artery cannula) were collected in 17 patients (52.4 +/- 8 yr) during retrosigmoid approach for removal of an acoustic tumour in the seated position. Measurements were made before stimulation of posterior fossa structures (period 1) and during tumour dissection along the brain stem (period 2). Significant increases in systolic, diastolic and mean blood pressures, in pulmonary capillary wedge pressure, in cardiac index and in stroke index were observed during period 2, whereas heart rate, right atrial pressure and systemic vascular resistances were unaffected. The greater the size of the tumour and the difficulties in dissection, the greater were these intraoperative haemodynamic changes. In addition, the pulmonary arterial blood temperature and the noradrenaline plasma concentrations (double isotope enzymatic assay) increased significantly during period 2. In conclusion, the prolonged microsurgical technique of acoustic tumour dissection through the retrosigmoid approach may modify left ventricular loading conditions and may lead to pulmonary oedema, even if intravascular volume expansion was minimal and ventricular function was near normal.     

Detection of arterial gas embolism by increased end-tidal nitrogen in dogs

Increased end-tidal (ET) nitrogen in a patient being ventilated with a nitrogen-free gas mixture through a leak-free circuit has been considered a specific sign of venous air embolism. We hypothesized that increased ETN2 would occur after arterial air emboli, just as following venous air emboli, and that clinically relevant arterial air emboli could be detected with respiratory gas monitoring by mass spectrometry. After approval from the institutional Animal Utilization Committee, eight mongrel dogs were studied. All were anesthetized with pentobarbital and ventilated with room air by a volume ventilator. Each animal was monitored by a femoral artery and a pulmonary artery catheter for systemic and pulmonary blood pressures, respectively, an electrocardiograph, pulse oximetry, and inspired and expired respiratory gas measurements by mass spectrometry. Arterial blood gas analysis was undertaken after one series of air emboli. Air boluses (containing the nonradioactive nitrogen isotope N2) of 50, 100, 200, and 500 mul/kg were injected slowly into the distal aorta through a second arterial catheter advanced 35 cm above the inguinal ligament. All emboli >/=100 mul/kg and 60% of the 50 mul/kg emboli were detected by increased ETN2 within 30 s, reaching peak levels in <2.75 min. The washout time for the N2 was longer for larger emboli, ranging from 2.9 +/- 2.8 min for 50 mul/kg emboli to 17.3 +/- 3.2 min for the 500 mug/kg emboli. There were no significant changes in end-tidal carbon dioxide, pulmonary or systemic blood pressures, or arterial blood gases. Increased ETN2 can no longer be considered pathognomonic for venous air embolism; arterial air embolism may have occurred.