706
1. Editorial. Which heart valve prosthesis? Lancet 1985; ii: 756-58. 2. Treasure T. Which heart valves should we use? Lancet 1990; 336: 1115-117. 3. Gersh BJ, Fisher LD, Schaff HV, et al. Issues concerning the clinical evaluation of new prosthetic valves. J Thorac Cardiovasc Surg 1986; 91: 460-66. 4. Bloomfield P, Wheatley DJ, Prescott RJ, Miller HC. Twelve-year comparison of a Björk-Shiley mechanical heart valve with porcine bioprostheses. N Engl J Med 1991; 324: 573-79. 5. Collins JJ. The evolution of artificial heart valves. N Engl J Med 1991; 324: 624-26. 6. Bloomfield P, Kitchin AH, Wheatley DJ, Walbaum PR, Lutz W, Miller HC. A prospective evaluation of the Björk-Shiley, Hancock and Carpentier-Edwards heart valve prostheses. Circulation 1986; 73: 1213-22. 7. Bloomfield P, Wheatley DJ, Prescott RJ, Miller HC. Ten year results of a randomised trial of the Bjork-Shiley, Hancock and CarpentierEdwards heart valve prostheses. Br Heart J 1989; 61: 452 (abstr).
Nuclear medicine and the
hepatobiliary tract Imaging of the hepatobiliary tract with ultrasound and computed tomography (CT) provides doctors with anatomical and morphological information and is popular in hospital practice. By contrast, imaging with radionuclides, which can additionally yield physiological information, tends to be under used. Hepatobiliary scintigraphy (HBS) is often regarded as yet another imaging modality with which to confront and confuse a perplexed clinician. HBS is of greatest use in the recognition of biliary obstruction. Both ultrasound and CT are well suited to the diagnosis of bile duct dilatation and therefore the level of obstruction. Ultrasound scans are cheap, widely available, sensitive, and reliable for detecting stones in the gallbladder. CT will detect the presence, level, and cause of obstruction. Neither system delineates the pathophysiology associated with the flow of bile. HBS is the only imaging technique that reliably shows cystic duct obstruction, which occurs in most patients with acute cholecystitis; visualisation of the gallbladder within an hour virtually excludes this condition. After liver or biliary tract surgery, HBS may be used to monitor the flow of bile and display postoperative biliary reflux, leaks, and the patency of a biliary-enteric anastomosis. It can also be used to evaluate post-cholecystectomy syndromes and afferent loop disorders after gastrectomy. Other indications for HBS include abdominal trauma and the differential diagnosis of neonatal jaundice, in which biliary atresia may be excluded by observing the passage of bile into the gut. For HBS a technetium-99- (99Tc-)-labelled derivative of iminodiacetic acid (IDA) is injected intravenously after a 4-6-hour fast. This tracer is a dimer of high molecular weight and lipid solubility and is taken up by the hepatocytes; it is then preferentially excreted by the biliary tract, because the kidneys are unable to remove substances from the body if their molecular weight exceeds 300 daltons. Sequential images of the liver, intrahepatic ducts, common bileduct, gallbladder, and small intestine are usually seen and monitored with a gamma camera/
computer system within an hour of injection of a radiolabelled tracer. Repeated imaging up to 4 hours, and again at 24 hours, may help to determine biliary
patency. Several 99Tcm-IDA complexes have been used to define hepatobiliary function.1 Of these, 99Tc-EHIDA and 99TcmDISIDA are the preferred tracers because their high extraction rate and fast hepatic transit permit more sensitive imaging. However, in jaundiced patients some 9’YfcmDISIDA is excreted via the kidneys and excretion correlates directly with serum bilirubin concentrations.z Consequently, the biliary excretion rate is reduced, making the differentiation between hepatocellular disease and biliary obstruction or leakage less certain. 99T cm- I OD IDA,3with its lower urinary excretion and higher biliary excretion than other 99Tcm-IDA derivatives, has been advocated for assessment of the biliary tree after liver transplantation,4 but this radionuclide likewise is of only limited use when the serum bilirubin rises to more than 200 unol/1. The general availability of radionuclide imaging facilities in hospitals, the low cost, and the low radiation dose should encourage clinicians to consider HBS as a practical clinicophysiological test that is readily applicable for general use, especially in the early stages of biliary obstruction. 1. Williams
AG, Mettler FA, Christie JH. Hepatobiliary and pancreatic imaging. In: Mettler FA, ed. Radionuclide imaging of the GI tract. London: Churchill-Livingstone, 1986: 183-216. 2. Klingensmith WC, Fritzberg AR, Spitzer VM, Kuni CC, Shanahan WM. Clinical comparison of diisopropyl-IDA-Tc99m and diethylIDA-Tc99m for evaluation of the hepatobiliary system. Radiology 1981; 140: 791-95. R, Kotzerke J, Hundeshagen H, Bocker K, Ringe B
3. Schwarzrock
99Tcm-diethyl-ioda-HIDA (JODIDA): a new hepatobiliary agent in clinical comparison with 99mTc-disopropyl-HIDA (DISIDA) in jaundiced patients. Eur J Nuclear Med 1986, 12: 346-50. 4. Anselmi M, Lancberg S, Deakin M, et al. Assessment of the biliary tract after liver transplantation: T tube cholangiography or IODIDA scanning. Br J Surg 1990; 77: 1233-37.
High-frequency ventilation High-frequency ventilation has been with us now for over twenty years1 and in clinical use for over ten.2,3 Sykes4 referred to it as "a physiological curiosity looking for a clinical application". The term is generally applied when respiratory frequency is more than four times greater than normal-ie, 60-1800 breaths/min (1-30 Hz).5 Tidal volume diminishes as frequency increases,6and dead space increases relative to tidal volume, so high minute volumes are used. Broadly there are three techniques: (a) high-frequency positive-pressure ventilation (1-1-7 Hz) with an endotracheal tube; (b) high-frequency jet ventilation (1-5 Hz) by which pulses of gas are passed through a small bore cannula from a high pressure (1-4 bar) source which may entrain additional gas depending on volume, pressure, compliance, &C;7 and (c) highfrequency oscillation at 3-30 Hz with a sine-wave pump. At lower frequencies (1 -6 Hz) tidal volume still
1. Editorial. Which heart valve prosthesis? Lancet 1985; ii: 756-58. 2. Treasure T. Which heart valves should we use? Lancet 1990; 336: 1115-117. 3. Gersh BJ, Fisher LD, Schaff HV, et al. Issues concerning the clinical evaluation of new prosthetic valves. J Thorac Cardiovasc Surg 1986; 91: 460-66. 4. Bloomfield P, Wheatley DJ, Prescott RJ, Miller HC. Twelve-year comparison of a Björk-Shiley mechanical heart valve with porcine bioprostheses. N Engl J Med 1991; 324: 573-79. 5. Collins JJ. The evolution of artificial heart valves. N Engl J Med 1991; 324: 624-26. 6. Bloomfield P, Kitchin AH, Wheatley DJ, Walbaum PR, Lutz W, Miller HC. A prospective evaluation of the Björk-Shiley, Hancock and Carpentier-Edwards heart valve prostheses. Circulation 1986; 73: 1213-22. 7. Bloomfield P, Wheatley DJ, Prescott RJ, Miller HC. Ten year results of a randomised trial of the Bjork-Shiley, Hancock and CarpentierEdwards heart valve prostheses. Br Heart J 1989; 61: 452 (abstr).
Nuclear medicine and the
hepatobiliary tract Imaging of the hepatobiliary tract with ultrasound and computed tomography (CT) provides doctors with anatomical and morphological information and is popular in hospital practice. By contrast, imaging with radionuclides, which can additionally yield physiological information, tends to be under used. Hepatobiliary scintigraphy (HBS) is often regarded as yet another imaging modality with which to confront and confuse a perplexed clinician. HBS is of greatest use in the recognition of biliary obstruction. Both ultrasound and CT are well suited to the diagnosis of bile duct dilatation and therefore the level of obstruction. Ultrasound scans are cheap, widely available, sensitive, and reliable for detecting stones in the gallbladder. CT will detect the presence, level, and cause of obstruction. Neither system delineates the pathophysiology associated with the flow of bile. HBS is the only imaging technique that reliably shows cystic duct obstruction, which occurs in most patients with acute cholecystitis; visualisation of the gallbladder within an hour virtually excludes this condition. After liver or biliary tract surgery, HBS may be used to monitor the flow of bile and display postoperative biliary reflux, leaks, and the patency of a biliary-enteric anastomosis. It can also be used to evaluate post-cholecystectomy syndromes and afferent loop disorders after gastrectomy. Other indications for HBS include abdominal trauma and the differential diagnosis of neonatal jaundice, in which biliary atresia may be excluded by observing the passage of bile into the gut. For HBS a technetium-99- (99Tc-)-labelled derivative of iminodiacetic acid (IDA) is injected intravenously after a 4-6-hour fast. This tracer is a dimer of high molecular weight and lipid solubility and is taken up by the hepatocytes; it is then preferentially excreted by the biliary tract, because the kidneys are unable to remove substances from the body if their molecular weight exceeds 300 daltons. Sequential images of the liver, intrahepatic ducts, common bileduct, gallbladder, and small intestine are usually seen and monitored with a gamma camera/
computer system within an hour of injection of a radiolabelled tracer. Repeated imaging up to 4 hours, and again at 24 hours, may help to determine biliary
patency. Several 99Tcm-IDA complexes have been used to define hepatobiliary function.1 Of these, 99Tc-EHIDA and 99TcmDISIDA are the preferred tracers because their high extraction rate and fast hepatic transit permit more sensitive imaging. However, in jaundiced patients some 9’YfcmDISIDA is excreted via the kidneys and excretion correlates directly with serum bilirubin concentrations.z Consequently, the biliary excretion rate is reduced, making the differentiation between hepatocellular disease and biliary obstruction or leakage less certain. 99T cm- I OD IDA,3with its lower urinary excretion and higher biliary excretion than other 99Tcm-IDA derivatives, has been advocated for assessment of the biliary tree after liver transplantation,4 but this radionuclide likewise is of only limited use when the serum bilirubin rises to more than 200 unol/1. The general availability of radionuclide imaging facilities in hospitals, the low cost, and the low radiation dose should encourage clinicians to consider HBS as a practical clinicophysiological test that is readily applicable for general use, especially in the early stages of biliary obstruction. 1. Williams
AG, Mettler FA, Christie JH. Hepatobiliary and pancreatic imaging. In: Mettler FA, ed. Radionuclide imaging of the GI tract. London: Churchill-Livingstone, 1986: 183-216. 2. Klingensmith WC, Fritzberg AR, Spitzer VM, Kuni CC, Shanahan WM. Clinical comparison of diisopropyl-IDA-Tc99m and diethylIDA-Tc99m for evaluation of the hepatobiliary system. Radiology 1981; 140: 791-95. R, Kotzerke J, Hundeshagen H, Bocker K, Ringe B
3. Schwarzrock
99Tcm-diethyl-ioda-HIDA (JODIDA): a new hepatobiliary agent in clinical comparison with 99mTc-disopropyl-HIDA (DISIDA) in jaundiced patients. Eur J Nuclear Med 1986, 12: 346-50. 4. Anselmi M, Lancberg S, Deakin M, et al. Assessment of the biliary tract after liver transplantation: T tube cholangiography or IODIDA scanning. Br J Surg 1990; 77: 1233-37.
High-frequency ventilation High-frequency ventilation has been with us now for over twenty years1 and in clinical use for over ten.2,3 Sykes4 referred to it as "a physiological curiosity looking for a clinical application". The term is generally applied when respiratory frequency is more than four times greater than normal-ie, 60-1800 breaths/min (1-30 Hz).5 Tidal volume diminishes as frequency increases,6and dead space increases relative to tidal volume, so high minute volumes are used. Broadly there are three techniques: (a) high-frequency positive-pressure ventilation (1-1-7 Hz) with an endotracheal tube; (b) high-frequency jet ventilation (1-5 Hz) by which pulses of gas are passed through a small bore cannula from a high pressure (1-4 bar) source which may entrain additional gas depending on volume, pressure, compliance, &C;7 and (c) highfrequency oscillation at 3-30 Hz with a sine-wave pump. At lower frequencies (1 -6 Hz) tidal volume still
