Doppler ultrasound estimation of bubble removal by various arterial line filters during extracorporeal circulation

Doppler ultrasound estimation of microbubbles in the arterial line during extracorporeal circulation

A pulsed Doppler ultrasound system was used to analyse microbubble intensity and size in the arterial line during extracorporeal circulation (ECC). Thirty male patients, younger than 70 (range 28-69) years, underwent isolated coronary artery bypass grafting with either a bubble oxygenator (Shiley S-100) without (group 1, n = 10) or with (group 2, n = 10) a depth adsorption arterial line filter (Swank High Flow 6000); or with a membrane oxygenator (Shiley M-2000) without a filter (group 3, n = 10). Mean ECC and aortic crossclamp times were similar in the three groups. Measurements were performed during the initial five minutes of cooling, after 30-40 minutes of ECC and after 10 minutes of rewarming. Microbubble intensity and size did not differ significantly in the three groups at the different intervals. Significantly more and larger bubbles were detected in group 1 (15-150μm) compared to group 2 (< 35μm) (p< 0.001). In group 3 only a minimal number of small bubbles (< 65μm) were observed. An arterial line filter significantly reduced the number and size of microbubbles detected in the arterial line during ECC. A membrane oxygenator was associated with a further reduction of microbubble intensity.

Cardiopulmonary bypass in infants and children

Cardiopulmonary bypass (CPB) systems have evolved from futuristic visions of surgical pioneers to a safe and efficient tool in the therapy of treatment of cardiac disorders. There are many significant differences in the physiology between neonates and adult patients. There are currently very few congenital cardiac malformations that cannot be addressed effectively with surgical therapy. Yet, the necessity of CPB in the repair of these patients can still result in significant morbidity. A clearer understanding of the effects of CPB, hypothermia, and circulatory arrest is evolving and there is a considerable amount of research in these areas. It seems likely that modification of current CPB systems, minimization of exposure, and surgical techniques to avoid or limit the adverse effects may reduce mortality and morbidity in the future. The problems faced in these complex patients and procedures require that infant and neonatal cardiac surgery be performed in specialized centers with a multidisciplinary approach and specialized personnel. Future improvements in technology will likely result in improved long term outcome for children with congenital cardiac disease. Copyright 2000 by W.B. Saunders Company

Changes in jugular bulb oxygenation in patients undergoing warm coronary artery bypass surgery (34-37 degrees C)

BACKGROUND AND OBJECTIVE

Imbalance between cerebral oxygen supply and demand is thought to play an important role in the development of cerebral injury during cardiac surgery with cardiopulmonary bypass.

METHODS

We studied jugular bulb oxygen saturation, jugular bulb oxygen tension, arterial-jugular bulb oxygen content difference and oxygen extraction ratio in 20 patients undergoing warm coronary artery bypass surgery (34-37 degrees C) with pH-stat blood gas management.

RESULTS

Only two patients showed desaturation (jugular bulb oxygen saturation < 50%) at 5 min on bypass, and none from 20 min onwards. Multiple regression models were performed after using bypass temperature, mean arterial pressure, cerebral perfusion pressure, haemoglobin concentration and arterial carbon dioxide tension as independent variables, and arterial-jugular bulb oxygen content difference, jugular bulb oxygen saturation, oxygen extraction ratio and jugular bulb oxygen tension as individual dependent variables.

CONCLUSIONS

We found that jugular bulb oxygen saturation, jugular bulb oxygen tension and oxygen extraction ratio are mainly dependent on arterial carbon dioxide tension, and arterial-jugular bulb oxygen content difference is dependent on arterial carbon dioxide tension and the bypass temperature. Our results suggest jugular bulb oxygenation is mainly dependent on arterial carbon dioxide tension during warm cardiopulmonary bypass.

Assisted venous drainage presents the risk of undetected air microembolism

OBJECTIVES

The proliferation of minimally invasive cardiac surgery has increased dependence on augmented venous return techniques for cardiopulmonary bypass. Such augmented techniques have the potential to introduce venous air emboli, which can pass to the patient. We examined the potential for the transmission of air emboli with different augmented venous return techniques.

METHODS

In vitro bypass systems with augmented venous drainage were created with either kinetically augmented or vacuum-augmented venous return. Roller or centrifugal pumps were used for arterial perfusion in combination with a hollow fiber oxygenator and a 40-micrometer arterial filter. Air was introduced into the venous line via an open 25-gauge needle. Test conditions involved varying the amount of negative venous pressure, the augmented venous return technique, and the arterial pump type. Measurements were recorded at the following sites: pre-arterial pump, post-arterial pump, post-oxygenator, and patient side.

RESULTS

Kinetically augmented venous return quickly filled the centrifugal venous pump with macrobubbles requiring continuous manual clearing; a steady state to test for air embolism could not be achieved. Vacuum-augmented venous return handled the air leakage satisfactorily and microbubbles per minute were measured. Higher vacuum pressures resulted in delivery of significantly more microbubbles to the "patient" (P <.001). The use of an arterial centrifugal pump was associated with fewer microbubbles (P =.02).

CONCLUSIONS

Some augmented venous return configurations permit a significant quantity of microbubbles to reach the patient despite filtration. A centrifugal pump has air-handling disadvantages when used for kinetic venous drainage, but when used as an arterial pump in combination with vacuum-assisted venous drainage it aids in clearing air emboli.

Ex vivo testing of the Quart arterial line filter

Arterial line filters are now routinely used in cardiac surgery in order to decrease the microemboli load to the patient. The Quart filter (Jostra, Hirrlingen, Germany) with a new planar construction design, an easy de-airing system and an integrated bypass, was tested for air filtration capacity and resistance to blood path in a standardized setting with surviving animals. Three calves (mean body weight: 71+/-3.4 kg) were connected to a standard cardiopulmonary bypass (CPB) circuit by jugular venous and carotid arterial cannulation with a mean flow rate of 3.5 l/min. The arterial line filter was challenged with upstream injections of boluses of air of 5, 10 and 15 ml, respectively. A Doppler ultrasound was positioned downstream on the arterial line to measure bubble count and size. The pressure drop through the filter was monitored at flow rates of between 1 and 6 l/min. At the end of the procedure the animals were weaned from the CPB and, thereafter, from the ventilator. After 7 days, the animals were sacrificed electively. This study shows that important quantities of air can be injected into the arterial line upstream of the filter with small volumes of small sized bubbles recorded downstream. With the 5 ml air bolus injection, mean values of 0.3+/-0.6 bubbles of 30 and 40 microm were detected, whereas with the 20 ml bolus, 32.6+/-8.7 bubbles of 10 microm, 3.7+/-1.1 bubbles of 30 microm, 3.3+/-0.6 bubbles of 40 microm and 0.7+/-1.1 bubbles of 50 microm were recorded. The blood path resistance at different blood flow rates was well within the acceptable range with a pressure drop of 20+/-0 and 26.6+/-5.7 mmHg at flow rates of 4 and 5 l/min, respectively. With its planar concept, the Quart filter offers good air filtering capacity both in terms of bubble count and size after injection of large boluses of air, without any increase of resistance to the blood path. Moreover, it offers a venting function and an integrated bypass system.

Determinants of cerebral perfusion during cardiopulmonary bypass

The risk of postoperative neurologic dysfunction in patients undergoing cardiac surgery remains high despite continued improvements in myocardial protective strategies. Part of this neurologic morbidity can be attributed to patients' increased age and underlying pathology, but other factors adversely affecting cerebral blood flow and cerebral metabolism during cardiopulmonary bypass may also contribute. Particulate microembolization during cardiopulmonary bypass appears to be a major cause of postoperative neurologic dysfunction and the pH-stat method of carbon dioxide management during hypothermia may potentiate neurologic damage by allowing a greater embolic load to be delivered to the brain. Echocardiography and transcranial Doppler methods may contribute to reducing the incidence of cerebral embolization by recognizing the timing and number of microemboli. Although hypothermia confers cerebral protection, rewarming may unmask and perhaps potentiate any ischemic damage that occurred with embolization during hypothermia. Both the degree and speed of rewarming may be important factors contributing to the extent of ischemic damage and ultimately neurologic function. In addition, many other factors related to cardiopulmonary bypass can alter cerebral perfusion and metabolism, such as nonpulsatile flow, hemodilution, pressure autoregulation, anesthetic and cerebroprotective drugs, and the neuroimmune response to bypass. In this review, the major factors affecting cerebral blood flow during cardiopulmonary bypass are discussed and their relative importance evaluated with regard to postoperative neurologic function.

Detection and volume estimation of embolic air in the middle cerebral artery using transcranial Doppler sonography

BACKGROUND AND PURPOSE

Cerebral embolism has been implicated in the development of cognitive and neurological deficits following bypass surgery. This study proposes methodology for estimating cerebral air embolus volume using transcranial Doppler sonography.

METHODS

Transcranial Doppler audio signals of air bubbles in the middle cerebral artery obtained from in vivo experiments were subjected to a fast-Fourier transform analysis. Audio segments when no air was present as well as artifact resulting from electrocautery and sensor movement were also subjected to fast-Fourier transform analysis. Spectra were compared, and frequency and power differences were noted and used for development of audio band-pass filters for isolation of frequencies associated with air emboli. In a bench model of the middle cerebral artery circulation, repetitive injections of various air volumes between 0.5 and 500 microL were made. Transcranial Doppler audio output was band-pass filtered, acquired digitally, then subjected to a fast-Fourier transform power spectrum analysis and power spectrum integration. A linear least-squares correlation was performed on the data.

RESULTS

Fast-Fourier transform analysis of audio segments indicated that frequencies between 250 and 500 Hz are consistently dominant in the spectrum when air emboli are present. Background frequencies appear to be below 240 Hz, and artifact resulting from sensor movement and electrocautery appears to be below 300 Hz. Data from the middle cerebral artery model filtered through a 307- to 450-Hz band-pass filter yielded a linear relation between emboli volume and the integrated value of the power spectrum near 40 microL. Detection of emboli less than 0.5 microL was inconsistent, and embolus volumes greater than 40 microL were indistinguishable from one another.

CONCLUSIONS

The preliminary technique described in this study may represent a starting point from which automated detection and volume estimation of cerebral emboli might be approached.

Perioperative stroke. Part II: Cardiac surgery and cardiogenic embolic stroke

The major mechanism of stroke in cardiac surgery is embolization. The risk is higher in intracardiac than in extracardiac surgery. The incidence of stoke associated with CABG is about 5%. The cerebral protective properties of isoflurane and thiopentone, acid-base management, and monitoring of cerebral perfusion during cardiopulmonary bypass are discussed. Prophylactic carotid endarterectomy for patients with asymptomatic carotid disease before cardiac surgery is not necessary. Symptomatic carotid disease increases the risk of stroke, and the management of patients who have both symptomatic coronary and carotid artery diseases is discussed. Cardiogenic embolism is probably responsible for many perioperative strokes. Patients with atrial fibrillation, valvular disease, and prosthetic heart valves are at high risk of cardiogenic embolism. Strokes associated with cardioversion, pacemaker insertion, coronary arteriography and angioplasty are explored.

Comparison of bubble release from various types of oxygenators. An in vivo investigation

The present study compares the creation of free gas bubbles in five different bubble oxygenators and one membrane oxygenator, by use of Doppler ultrasound technique. The study was carried out on groups of male patients undergoing coronary artery bypass surgery. The results show that the bubble oxygenators produce a considerable amount of free gas bubbles, with variances based on type. The membrane oxygenator showed virtually no counts at all.

Comparison of bubble release from various types of oxygenators

A comparative study of microbubble release from various types of oxygenators was performed using ultrasonic Doppler techniques. Bubble count versus amplitude histograms were calculated to derive the statistical distribution of the relative microbubble sizes. To compare the different oxygenators with respect to differences in microbubble releases, several key parameters as, temperature, liquid flow rate, gas to flow relationship, liquid level within the oxygenator, were altered one at a time to indicate different and oxygenator related sensitivities with respect to variations of the key parameters.