Expired Minute Volume Measurements During Anaesthesia Using Non-Rebreathing Valves

Non-rebreathing valves may cause leakage of anaesthetic gases from the inspiratory to the expiratory limb, leacling to errors in measurements of exhaled gas volume and gas composition. The fresh gas flow (FGF 1Xmin^-1) leakage (gas-shunt) over a non-rebreathing valve (AMBU Paedi Anaesthesia System) was experimentally investigated in a conscious volunteer. The gas-shunt depended upon the tension within the breathing balloon, which is placed in the inspiratory limb. At low FGF's, i.e. with the breathing balloon close to collapse, no gas-shunting was registered. At higher FGF's, a large gas-shunt was found. Due to the negligible gas-shunt found at low FGF settings, this AMBU system might be used for analyses and measurements of exhaled gases during Anaesthesia and spontaneous ventilation.     

The anaesthetic machine

In 1917, Dr Henry Edmund Gaskin Boyle OBE developed his continuous-flow anaesthesia machine, the design of which is the forerunner of all modern anaesthetic machines. This prototype has undergone significant changes to increase the efficiency of anaesthesia and patient safety. Gases (oxygen, nitrous oxide and air) arrive at the machine via the hospital's piped medical gases and vacuum system via colour-coded tubing. Cylinders attached to the back of the machine provide a back-up supply. Cylinder gases pass through pressure-regulating valves into the ‘back bar’ of the machine, but pipeline gases are supplied at ‘back bar pressure’ of 4 bar. From the back bar, gas flow rate is set using a needle valve that regulates flow into the flowmeter of which Rotameter? is one trademarked example. Rotameters are fixed pressure, variable orifice flowmeters which are accurate to within ±2.5%.Many modern anaesthetic machines have electronic gas mixers rather than conventional rotameters. The gases then pass through a vaporizer where volatile anaesthetic is added to the fresh gas flow. This mixture is delivered, via the common gas outlet, to a patient breathing circuit, nowadays usually a ‘circle system’. This contains a carbon dioxide absorber to stop patients re-breathing and allows higher efficiency than other systems. Waste gases are scavenged from the circle. Monitoring, ventilators and suction apparatus are mounted on the machine. The anaesthetic machine should be thoroughly tested prior to use.     

A coaxial circle circuit: Comparison with conventional circle and bain circuit

A coaxial system to be used for gas delivery to patients in a closed or low fresh gas flow anaesthetic system is described. The resistance to gas flow, humidity of inspired gases, and static compliance of the circuit are provided and compared with the circle tubing customarily employed or the coaxial Mapleson D (“Bain”) circuit, The resistance to gas flow is highest in the coaxial circle and “Bain” circuits; the resistance of the conventional circle is approximately 40 per cent less. Static compliance of this coaxial circle is 50 per cent greater than the conventional circle. During artificial ventilation humidity of inspired gases is maintained at levels recommended in the literature for all circuits, but during spontaneous breathing only the conventional rubber circle maintains appropriate levels. Advantages of this coaxial circle over the conventional circle include light weight and small size. Advantages of this coaxial circle over the “Bain” circuit include lower fresh gas flows and improved humidity during spontaneous breathing. These advantages make this coaxial circle useful for routine use.     

Le circuit-filtre en mode semi-fermé

In a semi-closed circle system, the inspiratory and expiratory limbs are completely separated and part of the patient's expired air recirculates. CO2 rebreathing is prevented by CO2- absorption with soda lime, which is always incorporated in such a circle. The inspiratory and expiratory valves ensure that gas flow is unidirectional and also prevent rebreathing, even at tidal volumes of 10 ml and ventilation frequencies of 60 c . min-1. This circuit can be used as an universal anaesthetic system for all age groups, simply by changing the hoses and connecting pieces. The values of expiratory resistance are within the recommended limits of the ISO; prewarming and humidification of the inspiratory gas mixture are sufficient without additional equipment. Standard monitoring of the circuit such as measurement of inspiratory O2 concentration and ventilation pressure, including a disconnection alarm, can be used for all age groups; spirometry or end-tidal CO2 measurements ensure normoventilation. The fresh gas flow required in a semi-closed circle system is about 2-4 1 . min-1, so that costs and environmental contamination with anaesthetic gases are relatively low in comparison with a semi-open system.     

A New Pulsatile Volumetric Device With Biomorphic Valves for the In Vitro Study of the Cardiovascular System

A pulsatile mock loop system was designed and tested. This prototype represents a versatile, adjustable, and controllable experimental apparatus for in vitro studies of devices meant to interface with the human circulatory system. The pumping system consisted of a ventricular chamber featuring two biomorphic silicone valves as the inlet and outlet valves. The chamber volume is forced by a piston pump moved by a computer‐controlled, low‐inertia motor. Fluid dynamic tests with the device were performed to simulate physiological conditions in terms of cardiac output (mean flow of 5 and 6 L/min, with beat rates from 60 to 80 bpm), of rheological properties of the processed fluid, and of systemic circulation impedance. The pulsating actuator performed a good replication of the physiological ventricular behavior and was able to guarantee easy control of the waveform parameters. Experimental pressure and flow tracings reliably simulated the physiological profiles, and no hemolytic subatmospheric pressures were revealed. The performance of the prototype valves was also studied in terms of dynamic and static backflow, effective orifice area, and pressure loss, resulting in their applicability for this device. Mechanical reliability was also tested over 8 h. The device proved to be a reliable lab apparatus for in vitro tests; the pumping system also represents a first step toward a possible future application of pulsating perfusion in the clinic arena, such as in short‐term cardiac assist and pulsatile cardiopulmonary bypass.     

Development of the Philadelphia Heart System

A new pneumatic artificial heart system has been developed. The design criteria have been to produce an integrated series of blood pumps and drive systems that would reduce blood trauma and reactivity, while incorporating industrial, mass-production techniques. The system attempts to reproduce the natural heart's pressure and flow waveforms and allows the prosthetic valves to be installed in a manner consistent with their design. The system's ventricles are constructed entirely of polyurethane by a combination of vacuum-forming and solution-casting techniques. The atrial cuffs and arterial grafts are permanently attached to the pumps and do not incorporate a quick connect system. The prosthetic valves are sewn into the inflow and outflow tracts using their clinical sewing rings. Besides eliminating the crevices normally found in quick connect systems, this method mounts the valves in an extremely compliant housing to increase shock absorption. The drive system produces a systolic air flow with a variable pressure rise (dP/dt) to reduce mitral valve closing velocity. This system has been implanted into 25 calves to date, of which 17 were chronic experiments. In 14 animals, St. Jude bileaflet valves were used and these animals had a mean survival of 39 days. Six of these animals survived over 30 days, with the longest being 129 days. All of the animals showed the characteristic postoperative drop in red blood cell count and hematocrit that returned to near preoperative values in about 3 weeks. The plasma free hemoglobin values generally remained below 5 mg/dl. The necropsies performed on several of the earlier animals revealed renal infarcts. However, in two of the later experiments, no kidney damage was found. The blood contacting surfaces of the atrial cuffs from the animals surviving over 100 days were covered with a fibroproliferative pseudoneointimal growth that extended from the sewing rings to the natural atrial tissue. Grossly, this appears to be the same type of tissue response seen when only a valve is implanted in a natural calf heart.     

The Application of Bileaflet Mechanical Heart Valves in the Polish Ventricular Assist Device: Physical and Numerical Study and First Clinical Usage

The Polish ventricular assist device (Polvad) has been used successfully in clinical contexts for many years. The device contains two single‐disc valves, one at the inlet and one at the outlet connector of the pneumatic pump. Unfortunately, in recent years, a problem has occurred with the availability of single‐disc valves. This article presents the possibility of using bileaflet mechanical heart valve prostheses in the Polvad to avoid a discontinuity in clinical use. The study is based on experimental and numerical simulations and comparison of the distribution of flow, pressure, and stress (wall, shear, and turbulent) inside the Polvad chamber and the inlet/outlet connectors fitted with Sorin Monodisc and Sorin Bicarbon Fitline valves. The type and orientation of the inlet valve affects valve performance and flow distribution inside the chamber. Near‐wall flow is observed for single‐disc valves. In the case of bileaflet valves, the main jet is directed more centrally, with lower shear stress but higher turbulent stress in comparison with single‐disc valves. For clinical usage, a 45° orientation of the bileaflet inlet valve was chosen, as this achieves good washing of the inlet area near the membrane paste surface. The Polvad with bileaflet valves has now been used successfully in our clinic for over a year and will continue to be used until new assist devices for heart support are developed.     

Digital venous anatomy

The veins of the digits and their valves were studied in nine cadaveric hands by sequential angiography, microdissection, routine histology, cross-sectional microradiography, and corrosion casting. The findings revealed a pattern of dorsal venous arches situated over each digit. Connections between those arches were at the level of the metacarpal heads, the point at which most of the palmar venous blood joins the dorsal system by intercapitular veins. A system of valves, which were arranged so as to direct flow from distal to proximal, from palmar to dorsal, and from radial to ulnar in the hand, was present in all veins as far distal as the pulp.     

Preservation of humidity and heat of respiratory gases during anaesthesia - a laboratory investigation

Humidification and heating of anaesthetic gases are desirable to prevent respiratory tract damage and a fall in body temperature during operative procedures. Numerous studies on the humidity and temperature of inspiratory gases in different breathing systems for anaesthesia have been carried out, but comparisons are difficult since different methods have been used. In this laboratory set-up we studied a non-rebreathing system with and without humidifiers and a circle absorber system with low (0.5 l/min) or medium (5 l/min) fresh gas flows regarding their ability to heat and humidify anaesthetic gases. The humidity of inspired gases was acceptable in the non-rebreathing system using either a Bennett Cascade humidifier or disposable humidifiers and in the circle absorber system using a fresh gas flow of 5 l/min or less. The temperature of the inspired gases was highest with the Bennett Cascade humidifier, followed by the low-flow circle system. The circle absorber system used with low fresh gas flow gave higher inspiratory gas temperature and humidity than the non-rebreathing system with a good disposable humidifier.     

A new valve for draw‐over anaesthesia*

The Diamedica non‐rebreathing valve has been developed for use in draw‐over anaesthesia and can be positioned at the common gas outlet. Its performance was evaluated against the Laerdal, Ruben and Ambu® valves under laboratory conditions. Valve resistance during inspiration and expiration was simulated over a range of constant flow conditions. During flows ranging from 5 to 45 l.min^?1, the Diamedica and Ruben valves exhibited a negligible resistance of < 150 Pa, with the Laerdal and Ambu valves achieving resistance of < 200 Pa. To assess the effects of sterilisation, this procedure was repeated following autoclaving, after which the Diamedica valve exhibited resistance to flow of < 150 Pa at 25 l.min^?1 for inlet resistance and 35 l.min^?1 for outlet resistance. The Diamedica, Ambu and Laerdal valves demonstrated resistance of < 100 Pa when saturated with water, while the Ruben valve exhibited resistance > 500 Pa.     

Opening characteristics of three-cusp tissue heart valves

Since April 1969, frame-mounted three-cusp fascia lata and pericardial valves have been used in over 200 patients for heart valve replacement. Six autologous fascia lata valves have been removed from the mitral position because of regurgitation produced by shrinkage of one or two cusps. To elucidate the cause and mechanism of graft failure the opening characteristics of fascia lata and Silastic valves were studied in a steady state flow rig.     

In Vitro Comparison of Velocity Profiles and Turbulent Shear Distal to Polyurethane Trileaflet and Pericardial Prosthetic Valves

A comparative study of flow dynamics past biomer trileaflet valves and a pericardial bioprosthetic valve under steady and physiological pulsatile flow conditions in vitro is reported in this paper. The velocity profiles and the turbulent shear stresses distal to the valves were measured using laser Doppler anemometry. The authors' results showed that the velocity profiles distal to the trileaflet valves were similar to that measured distal to the pericardial valve. Higher magnitudes of absolute turbulent shear stresses were measured distal to the synthetic valves in comparison to the pericardial valves. However, when the stresses were nondimensionalized with respect to the orifice diameter at the inlet aspect, the stresses were comparable for all of the three valves. With design modifications to increase the orifice diameter at the inlet aspect of the polyurethane valves, the turbulent stresses distal to the valves can be minimized. Such in vitro studies on the flow dynamics past the polyurethane valves can provide information towards design changes to improve the performance characteristics of these valves. Polyurethane valves with flow characteristics comparable to the pericardial valves can be manufactured relatively inexpensively compared to mechanical or tissue valve prosthesis. Hence, the synthetic valves may be a viable alternative for short-term use in total artificial heart devices as a bridge to transplant.     

Design of a new pulsatile bioreactor for tissue engineered aortic heart valve formation

Evidence has been gathered that biomechanical factors have a significant impact on cell differentiation and behavior in in vitro cell cultures. The aim of this bioreactor is to create a physiological environment in which tissue engineered (TE) aortic valves seeded with human cells can be cultivated during a period of several days. The bioreactor consists of 2 major parts: the left ventricle (LV) and the afterload consisting of a compliance, representing the elastic function of the large arteries, and in series a resistance, mimicking the arterioles and capillaries. The TE aortic valve is placed between the LV and the compliance. With controllable resistance, compliance, stroke volume and frequency, and hydrodynamic conditions can be changed over a wide physiological range. This study resulted in a prototype of a compact pulsatile flow system for the creation of TE aortic valves. In addition a biocompatibility study of the used materials is performed.     

Valvular obstruction of blood flow through saphenous veins

To measure effects of vein valves upon blood flow through venous bypass conduits, 15 human saphenous veins (mean length 40.6 cm, 5.3 valves/vein) were perfused with normal saline at constant pressure (100 mm Hg). Flow through vein was measured before and after valve bisection. Vein valves were bisected using Leather's techniques. After valve bisection, flow in antegrade and retrograde directions was measured in seven veins. These data were analyzed using paired t tests. Antegrade flow through seven veins with intact valves averaged 317.1 cc/min. With valve bisection this increased significantly (P < 0.001) to 474.3 cc/min. Retrograde flow through veins with valves bisected increased significantly (P < 0.0001) to 428.3 cc/min. Eleven veins (mean length 42.2 cm, 5.6 valves/vein) perfused simulated capillary beds with banked blood using pulsatile flow (mean pressure 92 mm Hg). Flow, again, was measured before and after valve bisection. Data were analyzed using the paired t test. Antegrade flow increased from 124.4 cc/min in veins with valves intact to 142.5 cc/min once valves were bisected (P = 0.02). These data demonstrate that saphenous vein valves cause significant obstruction to blood flow under conditions similar to those in the arterial system. Bisection of vein valves significantly increases flow through vein. Improved patency of vein grafts using valve bisection techniques may be explained by increased blood flow alone.     

THE NEGATIVE PRESSURE RELIEF VALVE: PRESSURE-FLOW RELATIONSHIPS

The scavenging of gases from anaesthetic circuits may presenthazards to the patient. The negative pressure relief valve preventsthe generation of subatmospheric pressures in the circuit asa result of a discrepancy between the fresh gas flow and thegas evacuation rate. The ideal valve will open at a small negativepressure, and immediately permit a high gas inflow. Leakagewith positive pressure in the circuit and admixture of atmosphericair during spontaneous respiration must not occur. Six differentvalves were studied. Two membrane valves came nearest to fulfillingthe ideal requirements.     

A Newly Designed Flow-Regulating Device in Shunt Therapy of Hydrocephalus

A new flow-regulating device for shunting in hydrocephalus was designed to limit an excess flow by a pressure-head between the cerebral ventricle and the abdominal cavity in a standing position. The device has two diaphragm valves connected to each other by two lines: one is a shunting line to transfer cerebrospinal fluid and the other is a control line filled with a control fluid with a higher density than cerebrospinal fluid. The performance of the model device with a natural rubber sheet diaphragm was tested in a mock system, using glycerol for the control fluid and water for the transfer fluid in vitro. Results show that device decreases the excess flow when the pressure-head between two valves exceeds 35 cm.     

In vitro evaluation of hydrodynamics of prosthetic heart valves

The hydrodynamics of various prosthetic heart valves currently available commercially were studied in our mock circulation system by analysis of flow-pressure gradient and opening angle. Using the circulation system, various prosthetic heart valves were tested in the simulative states of normal hemodynamics, low output, arrhythmia, hypovolemia, and vasoconstriction. The flow-pressure gradient analysis demonstrated the characteristics of each valves clearly. The St. Jude Medical valve showed the superior valve characteristics over other mechanical valves. The opening angles of the tilting disc prosthetic valves were also studied in the same simulative circulation using the photosensor system with real-time monitoring. In contrast to our clinical experiences of poor opening of Omniscience valve in the mitral position, limitation of the opening angle of the valve was not observed in this in vitro study.     

The contribution of valves to saphenous vein graft resistance

Saphenous vein resistance influences graft flow rates and may affect graft patency in lower limb revascularization. To quantitate specifically the contribution of saphenous vein valves to this resistance, 10 human saphenous veins (mean length 68 cm, diameter 0.42 mm, and 5.2 valves per vein) were perfused with water under carefully controlled pressure gradients designed to simulate different peripheral resistances in the outflow bed. The Reynolds number was maintained at 350 to 600, within the physiologic range for in vivo grafts. Veins were perfused under both venous (10 mm Hg) and arterial (100 mm Hg) mean pressures to determine the effects of distension on the overall resistance of the conduit. The valves were bisected according to Leather's techniques and flow was measured in both directions, antegrade (simulating "reversed" grafts) and retrograde (simulating "in situ" grafts). Data (mean +/- standard error) were normalized to the baseline flow for each vein with intact valves and expressed as a percentage change. Data were analyzed by means of Student's t test (p less than 0.05). Baseline antegrade flow with intact valves averaged 71.0 +/- 3.0 ml/min at pressure gradients (delta P) of 10 mm Hg and 95.0 +/- 2.6 ml/min for delta P = 20 mm Hg. After valve incision, antegrade flow (reversed) increased an average of 29% at both pressure gradients. Retrograde flow (in situ) through the bisected valves was only 19% greater than baseline antegrade flow and was significantly less than antegrade flow through bisected valves. The difference is explained by theoretic considerations of stenosis area and orifice shape. The increases in flow did not correlate with vein length or diameter, nor did flow change with different distension pressures.(ABSTRACT TRUNCATED AT 250 WORDS)     

Steady and pulsatile flow studies on a trileaflet heart valve prosthesis

The need for better and longer lasting trileaflet valves has led to the design and development of the ABIOMED polymeric trileaflet valve prosthesis. In vitro fluid dynamic studies in the aortic position indicate that overall it has improved leaflet motion characteristics and pressure drop characteristics compared to the Carpentier-Edwards porcine and Ionescu-Shiley pericardial tissue valves in current clinical use. The ABIOMED valve is, however, more stenotic compared to the St. Jude and Medtronic-Hall low profile mechanical valves, at normal cardiac outputs. Steady and pulsatile flow velocity measurements with a laser-Doppler anemometer system indicate that the flow field downstream of the ABIOMED valve is jet-like and leads to elevated shear stresses. These shear stresses are, however, lower than those observed with the Ionescu-Shiley and Carpentier-Edwards tissue valves. The ABIOMED valves tested had been originally configured for use in valved conduits, and it is therefore our opinion that further improvements can be made to the valve and stent design which would enhance its fluid dynamic performance.     

A proximal system for positive end-expiratory pressure (PEEP) and continuous positive airway pressure (CPAP).

A proximal positive end-expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) system was constructed by the simple addition of a venturi T-piece proximal to the exhalation limb of a breathing circuit. The level of PEEP or CPAP was determined by the amount of flow powering the venturi. This system can provide positive pressure in excess of 40 cm H2O without the need for check valves, dump valves or additional nebuliers and flowmeters.