Feasibility of a photosynthetic artificial lung.

The success of extracorporeal membrane oxygenation (ECMO) for the treatment of acute respiratory failure has led to consideration of the development of a more portable, and perhaps even implantable, artificial lung. The authors suggest a bioregenerative life support system that includes a photo-synthetic organism that can remove CO2 and produce O2 in the presence of an energy source. To build a model of such a photosynthetic artificial lung, the photosynthetic capability of a high temperature strain of the algae Chlorella pyrenoidosa was maximized at a cell density of 25 million cells/ml to serve as the O2 producer and CO2 remover. The "patient" in this model was comprised of 1 L of medium or 350 ml of blood, interfaced with the photosynthetic system across a gas transfer membrane. The experiments demonstrated the ability of the plant cells to supply O2 and remove CO2 from the "patient" with a maximum rate of 0.55 mmoles/L/hr under the most favorable measured operating conditions. The projected rate of 1.0 mmoles/L/hr required for physiologic applications is not totally ab absurd idea, with a slightly modified set-up. Modifications may be in the form of regulating the photosynthetic pathway or genetically engineering a hybrid strain with enhanced O2 producing and suppressed photoinhibition capacity.

Evaluation of contrast media for bronchography.

BACKGROUND: Bronchography is occasionally needed for the evaluation and management of some congenital pulmonary anomalies as well as some acquired diseases, usually of the tracheo- bronchial tree. There is currently no effective, approved contrast agent for this imaging techniq ue. OBJECTIVE: We evaluated five agents (barium sulfate, iohexol, propyliodone oily, propyliodone aqueous, and perflubron) in terms of image quality, histologic changes, and effects on hemodynamics, blood gases, and standard laboratory tests in New Zealand White rabbits. MATERIALS AND METHODS: Animals were anesthetized and intubated. Each contrast agent (0.25 ml/kg) was administered intratracheally. Three animals in each group had intravenous lines placed for blood sampling and blood pressure monitoring and were sacrificed at 1 h. An additional three animals for each agent were sacrificed at 24 h and 1 week after imaging. Blood samples were taken immediately before contrast instillation and at 1 h postbronchography. Fluoroscopic images were recorded on standard VHS video tape and evaluated in blind fashion. Segments of lung tissue and bronchi were obtained for histologic examination. RESULTS: Necrosis and/or inflammatory infiltrates were noted in 78 % of the bronchograms performed with propyliodone aqueous, 67 % with propyliodone oily, 55 % with perflubron, and 33 % with iohexol 120, 240 and 350. No histologic damage was observed with barium. The propyliodones gave the best-quality imaging results and the most histologic changes. Iohexol, in any concentration, gave the least acceptable images and a moderate number of histologic changes. Barium sulfate demonstrated acceptable images with virtually no histologic changes. CONCLUSION: From the histologic and imaging results, barium is the best available contrast material for bronchography.

Carotid artery and jugular vein ligation with and without hypoxia in the rat.

A continuing concern about the use of extracorporeal membrane oxygenation (ECMO) is the cannulation of the common carotid artery or the internal jugular vein. The authors investigated the changes that might occur in the brain with neck vessel ligation in the normal and the hypoxic rat. Two groups of 60 rats each were studied. The first group was divided into three subgroups of 20 animals each. Subgroup 1 (HH) was hypoxic both 24 hours before and 24 hours after operation. Subgroup 2 (HN) (the ECMO model) was hypoxic before operation and recovered for 24 hours in room air. Subgroup 3 (NN) underwent the entire procedure in room air. For each oxygen environment, four different operations were performed: carotid artery ligation, jugular vein ligation, carotid artery and jugular vein ligation, and dissection of the vessels without ligation (sham). Thus each subgroup was further divided into four sub-subgroups based on the operation performed. Rats were again anesthetized after a 24-hour recovery period and killed using low, blunt cervical dislocation. In the first group of 60 rats, the skull was opened and the brain was carefully removed from the cranial vault and placed in a fixative. The brains were placed in a small magnetic resonance imaging (MRI) head coil in groups of five and scans were obtained to provide T1 and T2 images that correlated with histological sections. MRI scans were reviewed in random, blinded fashion by an imager unaware of how these animals had been treated. The brains were then sectioned coronally at six corresponding levels: frontal, mid and posterior cerebrum, midbrain, pons, and medulla. Histological examination was performed in blinded fashion. The number of lesions (usually ischemic as noted by a decrease in the number of neurons) was totaled for each area of the brain. There were no differences that were consistent or statistically significant in the MR images of brains removed from the head, although it would appear that rats with jugular vein and carotid artery ligation were relatively protected. In the HN group jugular vein ligation was worst, and adding carotid artery ligation was best. In the histological studies the NN group had significantly more lesions than the HH group (P < .01). The second group of 60 rats was divided and treated as the first group in all respects except that MRI was conducted immediately after death on intact heads, and no histological studies were performed. This was done to control for lesions that might have been produced by removal of the brains from the skulls. In this group all findings were right sided. One animal in the HN group showed midcerebral white matter edema after jugular and carotid ligation. Focal anterior cerebral edema was seen in another animal (HH) after isolated carotid ligation. An occipital infarct was found in one animal (HH) after both carotid and jugular ligation. The authors conclude that neck vessel ligation in the hypoxic or normoxic rat causes only occasional and sporadic brain injury much as is seen clinically in newborn ECMO patients.

Pediatric cardiac surgical ECMO: multivariate analysis of risk factors for hospital death.

BACKGROUND: Extracorporeal membrane oxygenation (ECMO) has emerged as an effective technique for the mechanical support of many pediatric postcardiotomy patients with medically refractory cardiac failure. METHODS: We retrospectively reviewed the records of 73 pediatric patients with congenital heart disease who were placed on ECMO support between August 1984 and February 1994. The patients were divided into groups defined by the timing of ECMO cannulation relative to the time of operation. Group 1 patients (n = 7, 9.6%) were placed on ECMO preoperatively. Group 2 patients (n = 66, 90.4%) were a heterogeneous population placed on ECMO at any interval after cardiac repair. Subgroup 2A consisted of patients (n = 17, 25.8%) who could not be weaned from cardiopulmonary bypass and were converted directly to ECMO support immediately after repair. Subgroup 2B patients (n = 49, 74.2%) were cannulated postoperatively after an initial period of clinical stability. RESULTS: Hospital survival for all study patients (42/73) and for group 2 patients (38/66) was 58%. Only 4 group 2A patients (23.5%) survived their hospitalization compared with 34 group 2B patients (69.4%) (p = 0.001). Multivariate analysis identified elevated right atrial pressure after ECMO decannulation (p = 0.049) and, possibly, membership in group 2A (p = 0.061) as independent risk factors for hospital death. CONCLUSIONS: Extracorporeal membrane oxygenation is most effective in salvaging pediatric cardiac surgical patients who demonstrate medically refractory hemodynamic deterioration at some interval after being successfully weaned from cardiopulmonary bypass. The right atrial pressure after extracorporeal membrane oxygenation decannulation is an independent predictor of hospital death.

Congenital diaphragmatic hernia with or without extracorporeal membrane oxygenation: are we making progress?

BACKGROUND: Congenital diaphragmatic hernia (CDH) continues to have a high mortality rate (24 to 57 percent) despite changing management schemes, which include extracorporeal membrane oxygenation (ECMO) for treatment of associated persistent pulmonary hypertension of the newborn. STUDY DESIGN: The medical records of 123 acutely symptomatic newborns with CDH treated from 1972 to 1994 were retrospectively reviewed. Patients were divided into three groups to compare historical treatment modalities: group 1, no ECMO available; group 2, postoperative ECMO if necessary; and group 3, delayed repair with preoperative ECMO if necessary. The blood gas values, alveolar-arterial oxygen gradient (A-aDO2), mean airway pressure (MAP), and oxygenation (OI) and ventilation indices (VI) prior to treatment were compared between survivors and nonsurvivors. Chi-square and Student's t tests were used to determine statistical significance. RESULTS: The overall survival rate was 41 percent: 27 percent in group 1, 45 percent in group 2, and 39 percent in groups 3. If those who were not candidates for ECMO were excluded from analysis, the survival rate improved to 35 percent in group 1, 51 percent in group 2, and 50 percent in group 3. No published prognostic scoring system, such as arterial blood gas values, A-aDO2 gradient, MAP, OI, or VI consistently distinguished survivors from nonsurvivors. Extracorporeal membrane oxygenation decreased the mortality rate of patients having large defects. CONCLUSIONS: Prognostic scoring systems do not predict which patients with CDH should be treated. Extracorporeal membrane oxygenation has improved survival in newborns with CDH who present in early respiratory distress. There is no advantage or disadvantage to using ECMO prior to repair of CDH.

Extracorporeal CO2 removal in a lung lavage model of respiratory failure.

Extracorporeal life support (ECLS) is a recognized treatment for neonatal respiratory distress unresponsive to other forms of therapy. Variations of this technique are being developed in an effort to extend its applicability and safety. Extracorporeal CO2 removal (ECCO2R) is one such modification that requires blood flows of 20% to 50% of cardiac output and therefore lends itself to percutaneous venous cannulation. The authors evaluated ECCO2R in conjunction with low-frequency ventilation, using a lung lavage-induced model of respiratory failure in rabbits. Six rabbits were lavaged an average of 9 times with 15 mL/kg Plasma-Lyte A at 37 degrees C via an endotracheal tube. Incremental ventilatory changes were made during lavage, to an FIO2 of 1.0, rate of 80, peak inspiratory pressure (PIP) of 37 cm H2O, and positive end-expiratory pressure (PEEP) of 4 cm H2O. Arterial blood gas values of PaO2 < 40 mm Hg and PaCO2 > 60 mm Hg resulted, meeting our criteria for respiratory failure. The rabbits were placed on veno-venous ECCO2R using a 0.8-m2 hollow fiber oxygenator and a commercially available double-lumen dialysis catheter. Blood flows of 10 to 20 mL/kg/min were used to manage CO2 removal. A low-frequency ventilation technique was employed using an FIO2 of 1.0 and a rate of 5 breaths per minute. PEEP was increased incrementally to maintain the PaO2 above 80 mm Hg. After initiation of ECCO2R, the arterial PaO2 increased to 165 +/- 109 mm Hg, with PEEP above 15 cm H2O, and PaCO2 decreased to 37 +/- 5 mm Hg, with a bypass flow rate of 15 mL/kg/min.(ABSTRACT TRUNCATED AT 250 WORDS)

Feasibility studies for a photosynthetic artificial lung. Optimization of parameters affecting photosynthesis.

As an alternative to conventional extracorporeal membrane oxygenation (ECMO) hardware, preliminary studies were conducted toward the development of a life support system based on biologic processes for the direct generation of O2 and removal of CO2 by the action of a photosynthetic organism. A high temperature strain of Chlorella pyrenoidosa, which functions optimally at 37 degrees C and pH 7.4, was cultured in a 3 I fermenter with artificial lighting provided with Hg-metal halide lights. The pH, total CO2 and partial pressure of oxygen (PO2) of the system were monitored at regular intervals using flow-through microelectrodes. The degree of importance of light intensity, substrate concentration, and cell density in the photosynthetic ability of Chlorella in batch culture were assessed over a 2 hour period. At high light intensities, the O2 production, CO2 removal, and pH changes were significantly greater than those at low irradiance levels. Over the range of HCO3- concentrations used in these experiments, the initial HCO3- levels did not appear to have a significant effect on the rates of O2 production/CO2 removal. The total amount of O2 produced/CO2 removed and pH changes were found to be greater with higher cell densities. Under more optimal culture conditions, it may be feasible to eventually interface this photobioreactor with blood across a semipermeable membrane and catalytically convert blood CO2 to O2 directly, needing only an adequate light source.

Extracorporeal membrane oxygenation.

Extracorporeal membrane oxygenation has now evolved into standard therapy for patients unresponsive to conventional ventilatory and pharmacological support. This article presents a clinical review of extracorporeal life support and its application to neonatal and pediatric patients as well as children requiring circulatory support after open heart surgery.

Thermal, operational, and storage stability of immobilized carbonic anhydrase in membrane lungs.

The significant improvement in CO2 removal rates from blood through membrane lungs with EMCO and ECCO2R is achievable by means of the immobilization of carbonic anhydrase (CA) onto the membrane surface. The practical application of this technology requires that the enzyme be maintained for long periods without loss of activity. Thus, studies were performed to evaluate the thermal, operational, and storage stability of CA in a cellulose nitrate-encapsulated, silicone rubber membrane-immobilized form. Cellulose nitrate microcapsules containing 1000 micrograms/ml CA were prepared using a modified version of the method of Chang, and immobilized onto a 0.06 m2 section of commercial silicone rubber membrane material. The extent of enzyme activity in free solution and in the encapsulated form was determined after long-term storage at 4 degrees C, 37 degrees C, and 50 degrees C. Likewise, in an in vitro test circuit, CO2 removal efficiency in both CA treated and untreated membrane lungs was measured for extended time periods of 10 hr over a 10 day period. The thermal stability tests showed a significantly greater degree of retained enzyme activity in the encapsulated form, over the free enzyme in solution, at all temperatures. This was especially evident at higher temperatures and when the enzyme was stored for extended periods. In the operational stability tests, the CO2 removal efficiency of the treated membrane was not degraded, and stayed significantly higher than the untreated membrane for extended time periods. This further illustrates the potential for the use of the immobilized enzyme, carbonic anhydrase, for improved CO2 removal efficiency.

ECMO without heparin: laboratory and clinical experience.

To evaluate the feasibility of long-term extracorporeal membrane oxygenation (ECMO) without heparin, we placed six lambs on standard venoarterial ECMO for 71 to 96 hours. Group 1 (3 animals) was given doses of heparin to maintain activated clotting times (ACT) greater than 400 seconds. No form of anticoagulant was used for the three animals in group 2. Blood flow was maintained at 60 mL/kg/min. No histological evidence of thrombosis was noted at necropsy. ACT, prothrombin time, and partial thromboplastin time were higher in group 1, and much lower, although still above normal in group 2. Fibrinogen was significantly lower in group 2 (75 +/- 35 v 219 +/- 64 mg/dL group 1), and, although the platelet count was lower in group 2 (142 +/- 76 x 10(3)/mm3 v 225 +/- 167 x 10(3)/mm3), it was clinically acceptable. These results encouraged us to discontinue heparin when faced with severe hemorrhage in four patients on ECMO, rather than withdraw support at a time when there was little chance of survival. Heparin was discontinued for 10.5 +/- 6 hours. The mean ACT was reduced from 220 +/- 23 seconds to 144 +/- 22 seconds. One patient, who required repair of gastric necrosis while on ECMO following repair of a congenital diaphragmatic hernia, survived and had a decrease in blood loss from 2 to 0 mL/kg/h after the heparin was discontinued. One of the three patients who died had an autopsy with no evidence of thrombosis. We conclude that it may be reasonable to discontinue heparin in the face of life-threatening hemorrhage while on ECMO.

Extracorporeal membrane oxygenation for the circulatory support of children after repair of congenital heart disease.

We have treated 39 infants and children with congenital heart disease with extracorporeal membrane oxygenation during the past 5 years. Thirty-six were treated for low cardiac output or pulmonary vasoreactive crisis after repair of congenital heart defects. Twenty-two (61%) survived. Most patients were cannulated from the neck via the right internal jugular vein and the right common carotid artery. Six patients were cannulated from the chest, including three who had separate drainage of the left side of the heart with a left atrial cannula. Two of these patients survived and were the only survivors of the nine patients cannulated in the operating room because they could not be weaned from cardiopulmonary bypass after open cardiac operations. We also reviewed 312 patients (the predictor study series) having open cardiac operations before the availability of extracorporeal membrane oxygenation; 27 of these patients died. Data were collected at 1 and 8 hours postoperatively to determine if any parameters might predict early mortality. With these parameters used as criteria, patients who went on extracorporeal membrane oxygenation were as sick as those who died before extracorporeal membrane oxygenation was available. The most common complication was bleeding related to heparinization. The mean transfusion requirement in survivors was 1.50 +/- 1.13 ml/kg/hr, 5.63 +/- 7.0 ml/kg/hr in the nonsurvivors, and 7.46 +/- 8.29 ml/kg/hr in those cannulated in the operating room because they could not be weaned from bypass. Four children had intracranial hemorrhage, and two of them died. There was one late death. Nine of the 22 survivors are entirely normal. All survivors who do not have Down's syndrome are considered to have normal central nervous system function. We conclude that extracorporeal membrane oxygenation can improve survival in patients with both pulmonary artery hypertension and low cardiac output after operations for congenital heart disease.

Effect of extracorporeal membrane oxygenation on body water content and distribution in lambs.

Water content of the various body compartments were estimated immediately after beginning extracorporeal membrane oxygenation (ECMO), and daily thereafter for 3 days, in seven healthy 2-month-old lambs. Total body water, extracellular water, and plasma volume were estimated simultaneously by 18O, bromide, and T-1824 dilution, respectively. Volumes of intracellular water, interstitial water, blood, and red cells were calculated from the experimental estimates. No statistically significant changes occurred in the water content of the various body compartments in relation to duration of ECMO. The data suggest that water retention, clinically noted in human neonates treated with ECMO for persistent pulmonary hypertension, may be related to the primary disease process and/or its medical management, rather than to ECMO.

Sensitivity of the Azure heparin assay and comparisons with activated clotting time and whole blood partial thromboplastin time.

In order to compare the Azure A Assay to other commonly accepted measures of heparinization, samples of fresh donor sheep blood were prepared with known levels of heparin between 0 and 9 U/ml. For each sample, two determinations of the Activated Clotting Time (ACT) and Whole Blood Partial Thromboplastin Time (WBPTT) were performed. Similarly, an Azure A Assay standard curve of absorbance versus heparin concentration was constructed for the same blood. Three replicates of the procedure were performed, and the average relative error in each assay was computed as the 95% confidence interval on the regression line divided by the slope of the regression curve. Parallel studies were also performed to assess the effect of various plasma constituents on the Azure Assay. Results indicate that all of the assays are equally accurate for heparin levels at or below 2 U/ml, while the ACT and Azure assays are equivalent at heparin levels near 4 U/ml, but the WBPTT is not usable at concentrations above 2 U/ml. The Azure Assay is not sensitive enough for very low heparin levels (less than 1 U/ml), but it is quite accurate and, indeed, is the only rapid assay for heparin concentrations higher than 4 U/ml. In the sensitivity studies, variations in albumin levels and plasma platelet count were shown to have a significant effect on the accuracy of the Azure assay, whereas calcium levels had no effect.

Immobilized carbonic anhydrase in a membrane lung for enhanced CO2 removal.

The potential use of carbonic anhydrase (CA), immobilized within membrane lungs, to accelerate the removal of CO2 from blood was investigated. Using a variation on the technique of Chang, 500 micrograms/mL of CA was encapsulated in 5-20 microns cellulose nitrate microcapsules which were then immobilized onto a 0.1 m2 silicone rubber membrane. Using an in vitro test circuit, 0.68 mmol/m2/min of CO2 was removed from venous blood through an untreated membrane at steady state. With the CA-immobilized membrane, CO2 removal was enhanced to 1.09 mmol/m2/min. This technique has the potential of greatly improving the CO2 efficiency of commercial membrane lungs for ECCOR.

CO2 removal for ventilatory support: a comparison of dialysis with and without carbonic anhydrase to a hollow fiber lung.

All CO2 in blood is in equilibrium, catalyzed by carbonic anhydrase (CA). This has prompted some investigators to consider treating ventilatory failure by using dialysis to remove HCO3-. Since there is at least 18 times more CO2 in the form of HCO3- than dissolved CO2, theoretically, lower blood flows could be used than with current extracorporeal support with artificial membrane lungs. HCO3- removal for ventilatory support has required alkalinization to compensate for the resulting acidosis and has been capable of removing 26 to 38 ml CO2/100 ml blood flow, compared to 14 ml CO2/100 ml for clinically employed silicone membrane lungs. We designed a HCO3- removal system using recirculation of dialysate through a membrane lung to remove CO2, rather than alkalinization of blood, and removed 8.8 ml CO2/100 ml. Adding CA improved this to 12.2 ml CO2/100 ml, but a conventional hollow fiber lung removed 30 ml CO2/100 ml. We conclude that the complexities of an HCO3- removal system may not be necessary with the advent of more efficient hollow fiber lungs.

Regional blood flow distribution during extracorporeal membrane oxygenation in rabbits.

To study regional blood distribution during extracorporeal membrane oxygenation, we stabilized three groups of five rabbits each (3 to 5 kg) on venoarterial bypass at a flow rate of 30 ml/kg/min. Albumin aggregates (15 to 30 microns) labeled with technetium 99m were injected into the left ventricle during bypass (ventricle), the perfusion cannula during bypass (cannula), and the left ventricle with no bypass (control). Animals were put to death, organs were removed, and the percent distribution was determined with a gamma camera. The Student Newman-Keuls test was used for statistical comparisons. Distribution to both the heart and brain in the cannula group were decreased from control by 55% and 35%, respectively. Distribution to the brain in the ventricle group was also decreased from control by 39%. Intestinal distribution was elevated above control in the ventricle group by 37%, whereas musculoskeletal distribution was elevated 33% above control in the cannula group. No significant changes were noted for the kidneys, stomach, or liver. These data suggest that overall perfusion of some vital organs may be significantly reduced during low-flow extracorporeal membrane oxygenation, specifically in the case of the heart and brain, which may be deprived of oxygenated blood.

Extracorporeal membrane oxygenation without anticoagulation: a study using quantitative scanning electron microscopy.

In current clinical applications of extracorporeal circulation (ECC) of blood, heparin is administered to prevent thrombosis in the circuit and to eliminate thromboembolism in the patient. Systemic heparinization, however, causes impairment of normal coagulation properties and significantly increases the risks of major bleeding complications. It has been observed in the past that ECC may be performed in the absence of any systemic anticoagulants or material surface treatments. In this study, short-term extracorporeal membrane oxygenation (ECMO) was performed on rabbits in three groups. The animals in the first group received standard clinical dosages of heparin. No form of anticoagulation was used in the second group. The ECMO circuits in the third group were treated with an albumin-heparin complex surface-coating. Thrombosis in the extracorporeal circuit was evaluated by using quantitative scanning electron microscopy. The results indicated very few statistically significant differences between the three experimental systems in the amounts of blood components adhering to the surface. Considerably more platelet activation and attachment was noted in the systemic heparin group. These experiments failed to demonstrate any benefit of systemic heparinization in short-term ECMO. The findings from these experiments suggest that the current levels of systemic heparinization may be unwarranted, and they indicate that more controlled studies must be performed to determine optimal levels of anticoagulation for different applications of ECC.

Regional blood flow during extracorporeal membrane oxygenation in lambs.

To determine changes in blood flow to different organs during extracorporeal membrane oxygenation (ECMO), the authors performed venoarterial ECMO in four young lambs for 71-96 hr (Group 1). Macroaggregated albumin microspheres labeled with technetium 99m were injected through the perfusion cannula before termination of ECMO to determine percent of blood flow by measuring radioactivity from the microspheres lodged in specific organs. The control group (Group 2) consisted of three animals not on bypass; injections were made through a catheter placed in the left ventricle. Relative coronary blood flow from the perfusion cannula was significantly less than relative coronary blood flow in the control group, possibly because of cannula location. Renal flow from the perfusion cannula also was decreased. Contrary to observations in rabbits, cerebral perfusion did not decrease in the bypass group despite ligation of the carotid artery and the external jugular vein. There were no statistically significant differences between the two groups in the relative blood flow to other organs. The authors conclude that ECMO may significantly alter myocardial and renal perfusion, with minimal effects to other organs.

Quantitative scanning electron microscopy for the evaluation of thrombosis in extracorporeal circuits.

Methods devised in the past to evaluate blood-material interactions in extracorporeal devices assess small segments of test materials and do not analyze the entire circuit. In order to develop an effective method for the control of thrombosis in extracorporeal circulation, it is important to perform a quantitative evaluation of random samples of all the circuit elements. In an effort to quantify thrombosis in experimental extracorporeal membrane oxygenation circuits, random samples of silicone rubber membrane and polyvinyl chloride tubing were examined using scanning electron microscopy (SEM) and quantitative morphometry. The results indicate no statistically significant differences in the amounts of various adhered blood components between the tubing and the membrane. Greater than 60% of the examined surface had no blood components present. Platelet adhesion on the surface was predominant. Evidence of fibrin, gross thrombi, red cells, and white cells in small quantities were noted. It is concluded that quantitative scanning electron microscopy (QSEM) not only provides accurate information about the morphologic changes of blood cells due to surface contact, but also quantifies the individual elements and therefore assesses their contributions to thrombosis. This quantitative description will allow more meaningful comparison of blood-compatible materials and surfaces.