Computer Simulation of Hemodynamic Parameter Changes by Mechanical Ventilation and Biventricular Circulatory Support

When a Bi-Ventricular Assist Device (BVAD) is used in conjunction with mechanical ventilation (MV) of the lungs with positive intrathoracic pressure (Pt), the latter influences hemodynamics. The aim of our study was to assess the simultaneous influence of BVAD and MV on hemodynamics. We assumed ventricular pathological conditions as reduced elastances and increased rest volumes. Peripheral systemic arterial resistance was assumed to have different values. Data were obtained by computer simulation. Trends in main hemodynamic variables were compared with clinical data from literature. Simulation showed that systemic venous, pulmonary arterial and left atrial pressures are very sensitive to Pt (-2 to 5 mmHg).

The Impact of Rotary Blood Pump in Conjunction with Mechanical Ventilation on Ventricular Energetic Parameters

Objectives: Aim of this work is to study the impact of left ventricular rotary blood pump assistance, on energetic variables, when mechanical ventilation (MV) of the lungs is applied.

Methods: Computer simulation was used to perform this study. Lumped parameter models reproduce the circulatory system. Variable elastance models reproduce the Starling’s law of the heart for each ventricle. After the reproduction of ischemic heart disease left ventricular assistance was applied using a model of rotary blood pump. The pump speed was changed in steps and was assumed to be constant during each step. The influence of mechanical ventilation was introduced by different values of positive mean thoracic pressure.

Results: The increase of the rotational speed has a significant influence on some ventricular energetic variables. In fact it decreased left ventricular external work, left and right ventricular pressure-volume area and the left ventricular efficiency. Finally, it increased the right ventricular efficiency but had no influence on the right ventricular external work. The increase of thoracic pressure from –2 to +5 mmHg caused a significant decrease of external work, pressure-volume area (right ventricular pressure-volume area dropped up to 50%) and an increase of right ventricular efficiency (by 40%) while left ventricular efficiency remained almost stable.

Conclusions: Numerical simulation is a very suitable tool to predict changes of not easily measurable parameters such as energetic ventricular variables when mechanical assistance of heart and/or lungs is applied independently or simultaneously.

In Vivo and Simulation Study of Artificial Ventilation Effects on Energetic Variables in Cardiosurgical Patients

Objectives: The analysis of energetic ventricular variable changes during artificial ventilation, obtained by numerical simulation was done. Twenty-one sets of hemodynamic parameters for eight cardiosurgical patients were used to estimate left and right stroke work. The data were collected for three methods of ventilation: conventional, lung-protective (with minute ventilation diminished by half) and high frequency ventilation (with frequency 5, 10, or 15 Hz).

Methods: The computer simulator (CARDIOSIM©) of the cardiovascular system, was used as a tool to calculate values of energetic ventricular variables for conditions that corresponded to these during in vivo measurements. Different methods of ventilation caused differences of intrathoracic pressure, haemodynamic and finally energetic ventricular variables. The trends of these variable changes were the same in in vivo and simulation studies, in the whole range of intrathoracic pressure changes (Pt = 1.5-3.5 mmHg).

Results: As values of main hemodynamic variables like cardiac output or arterial, systemic and pulmonary pressures were very close in both studies. Cardiac index and left ventricular stroke work also differed less than 10% for all examined patients and computer simulation. In a case of right ventricular stroke work the difference between in vivo data and simulation was a bit greater than 10% for two of eight patients under study.

Conclusions: Our comparative analysis proved that numerical simulation is a very useful tool to predict changes of main hemodynamic and energy-related ventricular variables caused by different levels of positive Pt. It means that it can help an anesthesiologist to choose an appropriate method of artificial ventilation for cardiosurgical patients.

Application of a user-friendly comprehensive circulatory model for estimation of hemodynamic and ventricular variables

Purpose

Application of a comprehensive, user-friendly, digital computer circulatory model to estimate hemodynamic and ventricular variables.

Methods

The closed-loop lumped parameter circulatory model represents the circulation at the level of large vessels. A variable elastance model reproduces ventricular ejection. The circulatory model has been modified embedding an algorithm able to adjust the model parameters reproducing specific circulatory conditions. The algorithm reads input variables: heart rate, aortic pressure, cardiac output, and left atrial pressure. After a preliminary estimate of circulatory parameters and ventricular elastance, it adjusts the amount of circulating blood, the value of the systemic peripheral resistance, left ventricular elastance, and ventricular rest volume. Input variables and the corresponding calculated variables are recursively compared: the procedure is stopped if the difference between input and calculated variables is within the set tolerance. At the procedure end, the model produces an estimate of ventricular volumes and Emaxl along with systemic and pulmonary pressures (output variables). The procedure has been tested using 4 sets of experimental data including left ventricular assist device assistance.

Results

The algorithm allows the reproduction of the circulatory conditions defined by all input variable sets, giving as well an estimate of output variables.

Conclusions

The algorithm permits application of the model in environments where the simplicity of use and velocity of execution are of primary importance. Due to its modular structure, the model can be modified adding new circulatory districts or changing the existing ones. The model could also be applied in educational applications.

Modelling of Cardiovascular System: Development of a Hybrid (Numerical-Physical) Model

Physical models of the circulation are used for research, training and for testing of implantable active and passive circulatory prosthetic and assistance devices. However, in comparison with numerical models, they are rigid and expensive. To overcome these limitations, we have developed a model of the circulation based on the merging of a lumped parameter physical model into a numerical one (producing therefore a hybrid). The physical model is limited to the barest essentials and, in this application, developed to test the principle, it is a windkessel representing the systemic arterial tree. The lumped parameters numerical model was developed in LabVIEW environment and represents pulmonary and systemic circulation (except the systemic arterial tree). Based on the equivalence between hydraulic and electrical circuits, this prototype was developed connecting the numerical model to an electrical circuit--the physical model. This specific solution is valid mainly educationally but permits the development of software and the verification of preliminary results without using cumbersome hydraulic circuits. The interfaces between numerical and electrical circuits are set up by a voltage controlled current generator and a voltage controlled voltage generator. The behavior of the model is analyzed based on the ventricular pressure-volume loops and on the time course of arterial and ventricular pressures and flow in different circulatory conditions. The model can represent hemodynamic relationships in different ventricular and circulatory conditions.

Virtual respiratory system for education and research: simulation of expiratory flow limitation for spirometry

BACKGROUND

Due to economic and ethical problems, virtual organs may appear more convenient than experiments on animals or limited investigations on patients. In particular, a virtual respiratory system (VRS) may be useful for tasks such as respirators and support methods testing, education, staff (medical and technical) training, (initial) testing of scientific hypotheses.

METHODS

A comparative study of simulated and real spirometric results for different patient states (healthy lungs, restrictive lung disease, and obstructive lung disease of different localization and degree) was performed. The volume-flow curve and such standard parameters as FEV1, FEV1%VC, MEF75 etc. were analyzed.

RESULTS

A mathematical description of collapsing bronchi was proposed. All fundamental phenomena present during spirometry also appeared in VRS, especially characteristic dependence between lung volume and air flow for forced expiration. In particular, both airway resistance and the flow limitation were described with one formula derived from commonly known dependence of the resistance on lung volume. Generally there were no significant differences between simulated results and those seen in clinical practice. Only simulation of obstruction in upper airways gave incorrect results, which suggested a different flow limitation mechanism (perhaps wave-speed limitation).

CONCLUSIONS

Our VRS can already be used in medical education, e.g. courses of spirometry, and in some other applications. It seems that the significance of the wave-speed criterion has been overestimated.

The impact of rotary blood pump in conjunction with mechanical ventilation on ventricular energetic parameters - numerical simulation

OBJECTIVES

Aim of this work is to study the impact of left ventricular rotary blood pump assistance, on energetic variables, when mechanical ventilation (MV) of the lungs is applied.

METHODS

Computer simulation was used to perform this study. Lumped parameter models reproduce the circulatory system. Variable elastance models reproduce the Starling's law of the heart for each ventricle. After the reproduction of ischemic heart disease left ventricular assistance was applied using a model of rotary blood pump. The pump speed was changed in steps and was assumed to be constant during each step. The influence of mechanical ventilation was introduced by different values of positive mean thoracic pressure.

RESULTS

The increase of the rotational speed has a significant influence on some ventricular energetic variables. In fact it decreased left ventricular external work, left and right ventricular pressure-volume area and the left ventricular efficiency. Finally, it increased the right ventricular efficiency but had no influence on the right ventricular external work. The increase of thoracic pressure from -2 to +5 mmHg caused a significant decrease of external work, pressure-volume area (right ventricular pressure-volume area dropped up to 50%) and an increase of right ventricular efficiency (by 40%) while left ventricular efficiency remained almost stable.

CONCLUSIONS

Numerical simulation is a very suitable tool to predict changes of not easily measurable parameters such as energetic ventricular variables when mechanical assistance of heart and/or lungs is applied independently or simultaneously.

Development of a hybrid (numerical-hydraulic) circulatory model: prototype testing and its response to IABP assistance

Merging numerical and physical models of the circulation makes it possible to develop a new class of circulatory models defined as hybrid. This solution reduces the costs, enhances the flexibility and opens the way to many applications ranging from research to education and heart assist devices testing. In the prototype described in this paper, a hydraulic model of systemic arterial tree is connected to a lumped parameters numerical model including pulmonary circulation and the remaining parts of systemic circulation. The hydraulic model consists of a characteristic resistance, of a silicon rubber tube to allow the insertion of an Intra-Aortic Balloon Pump (IABP) and of a lumped parameters compliance. Two electro-hydraulic interfaces, realized by means of gear pumps driven by DC motors, connect the numerical section with both terminals of the hydraulic section. The lumped parameters numerical model and the control system (including analog to digital and digital to analog converters)are developed in LabVIEW environment. The behavior of the model is analyzed by means of the ventricular pressure-volume loops and the time courses of arterial and ventricular pressures and flows in different circulatory conditions. A simulated pathological condition was set to test the IABP and verify the response of the system to this type of mechanical circulatory assistance. The results show that the model can represent hemodynamic relationships in different ventricular and circulatory conditions and is able to react to the IABP assistance.

Modelling in the study of interaction of Hemopump device and artificial ventilation

The aim of this work is to evaluate in different ventricular conditions the influence of joint mechanical ventilation (MV) and Hemopump assistance. To perform this study, we used a computer simulator of human cardiovascular system where the influence of MV was introduced changing thoracic pressure to positive values. The simulation confirmed that haemodynamic variables are highly sensitive to thoracic pressure changes. On the other hand, Hemopump assistance raises, among the others, mean aortic pressure, total cardiac output (left ventricular output flow plus Hemopump flow) and coronary flow. The simulation showed that the joint action of Hemopump and positive thoracic pressure diminishes these effects.

In vivo and simulation study of artificial ventilation effects on energetic variables in cardiosurgical patients

OBJECTIVES

The analysis of energetic ventricular variable changes during artificial ventilation, obtained by numerical simulation was done. Twenty-one sets of hemodynamic parameters for eight cardiosurgical patients were used to estimate left and right stroke work. The data were collected for three methods of ventilation: conventional, lung-protective (with minute ventilation diminished by half) and high frequency ventilation (with frequency 5, 10, or 15 Hz).

METHODS

The computer simulator (CARDIOSIM) of the cardiovascular system, was used as a tool to calculate values of energetic ventricular variables for conditions that corresponded to these during in vivo measurements. Different methods of ventilation caused differences of intrathoracic pressure, haemodynamic and finally energetic ventricular variables. The trends of these variable changes were the same in in vivo and simulation studies, in the whole range of intrathoracic pressure changes (Pt = 1.5-3.5 mmHg).

RESULTS

As values of main hemodynamic variables like cardiac output or arterial, systemic and pulmonary pressures were very close in both studies. Cardiac index and left ventricular stroke work also differed less than 10% for all examined patients and computer simulation. In a case of right ventricular stroke work the difference between in vivo data and simulation was a bit greater than 10% for two of eight patients under study.

CONCLUSIONS

Our comparative analysis proved that numerical simulation is a very useful tool to predict changes of main hemodynamic and energy-related ventricular variables caused by different levels of positive Pt. It means that it can help an anesthesiologist to choose an appropriate method of artificial ventilation for cardiosurgical patients.

A hybrid mock circulatory system: development and testing of an electro-hydraulic impedance simulator

Mock circulatory systems are used to test mechanical assist devices and for training and research purposes; when compared to numerical models, however, they are not flexible enough and rather expensive. The concept of merging numerical and physical models, resulting in a hybrid one, is applied here to represent the input impedance of the systemic arterial tree, by a conventional windkessel model built out of an electro-hydraulic (E-H) impedance simulator added to a hydraulic section. This model is inserted into an open loop circuit, completed by another hybrid model representing the ventricular function. The E-H impedance simulator is essentially an electrically controlled flow source (a gear pump). Referring to the windkessel model, it is used to simulate the peripheral resistance and the hydraulic compliance, creating the desired input impedance. The data reported describe the characterisation of the E-H impedance simulator and demonstrate its behaviour when it is connected to a hybrid ventricular model. Experiments were performed under different hemodynamic conditions, including the presence of a left ventricular assist device (LVAD).

Study of systolic pressure variation (SPV) in presence of mechanical ventilation

Systolic pressure variation (SPV) and its components (dUp and dDown) have been demonstrated to be of interest in assessing preload in mechanically ventilated patients. The aim of this paper is to analyse the sensitivity of these variables to preload and volemic changes during mechanical ventilation in different conditions of the cardiovascular system. Computer simulation experiments have been done using a modular lumped parameter model of the cardiovascular system. The effect of mechanical ventilation has been reproduced operating on intrathoracic pressure. Experiments have been performed varying preload through filling pressure. Sensitivity of SVP dUp and dDown is described varying separately left ventricular elastance (Ev), systemic arterial resistance (Ras) and systemic arterial compliance (Cas). The sensitivity of SPV and dDown to preload and filling pressure is appreciable for high values of Ev and for a wide variation of Ras. Preliminary clinical data concerning the three parameters show good correlation with simulation results.

Energetic parameter changes with mechanical ventilation in conjunction with BVAD assistance

The aim was to assess the influence of a biventricular assist device (BVAD) on ventricular energetics parameters (external work, oxygen consumption, cardiac mechanical efficiency) for both ventricles, when mechanical ventilation was applied. The experiments were performed using a computer simulator of cardiovascular system (CARDIOSIM) after modelling a pathological state of the left ventricle (E(v)Left = 0. 9 mmHg cm(-3) and increasing pulmonary resistance (Rap = 0.3 mmHg cm(-3 s). The effect of mechanical ventilation was mean intrathoracic pressure changes from 0 to +5 mmHg. This simulation showed that application of BVAD for both ventricles reduces external work and that this effect is stressed by positive intrathoracic pressure, reduces cardiac mechanical efficiency that is quite insensitive to intrathoracic pressure and increases oxygen consumption, which is reduced by positive intrathoracic pressure. The increase of potential energy at the onset of BVAD evidences a rightwards shift of ventricular work cycle (unloading of the ventricles). In general, positive intrathoracic pressure during BVAD assistance adversely affects ventricular energetics.

Ventricular energetics during mechanical ventilation and intraaortic balloon pumping--computer simulation

Computer simulation of a cardiovascular system enabled us to predict the effects of simultaneous application of mechanical ventilation (MV) and intraaortic ballon pumping (IABP) on ventricular energetics. External work (EW), pressure-volume area (PVA), potential energy (PE) and cardiac mechanical efficiency (CME) were calculated. Nummerical simulation showed that changes of positive intrathoracic pressure have a considerable effect on left and right ventricular EW, PE, PVA and CME, whether IABP is used or not. The right ventricular energetics was much less influenced by systemic resistance (Ras) changes than the left ventricular one. Simultaneous application of IABP and MV showed a remarkable effect on left ventricular EW. The net result was reversed sensitivity to pulmonary resistance (Rap) and reduced sensitivity to Ras. PVA was generally reduced, while CME is increased by simultaneous presence of IABP and MV. The sensitivity of CME to Rap and Ras variation was diminished in this situation.

Heart and lung support interaction--modeling and simulation

Mechanical support of the lungs used to preserve life or during any kind of surgery may have an adverse effect on the cardiovascular system. Usually, positive pressure in alveoli diminishes lung perfusion, venous return and cardiac output. Positive pressure during the respiratory cycle is transfered into the thoracic space. The aim of this study was to assess how synchronization of the respirator with spontaneous breathing influences the distribution of pressure and ventilation in nonhomogeneous lungs and how it should influence hemodynamics. For this purpose a multicompartmental model of respiratory system mechanics was used in the electrical analog of a respirator-lung circuit, which enabled us to simultaneously simulate ventilatory support and spontaneous breathing. Mechanical properties of the respiratory system were modeled by lumped parameters: resistances and capacitances of constant values, independent of lung volume or inspiratory flow changes. A multicompartmental model of the respiratory system enabled us to simulate lung pathology characterized by non-homogeneity of the mechanical properties of the different parts of the lungs. The results of simulations presented in the paper enable us to conclude that lung volume increase, independent of the respirator-patient breathing synchronization, may be modeled as the increase in pulmonary vascular resistance and alveolar pressure increase, dependent on respirator-patient breathing synchronization, may be averaged by esophageous balloon measurements which show intrathoracic pressure changes.

Artificial ventilation of the lungs for emergencies

The necessity for extraordinary ventilatory support may appear in different places all over the world in cases of a massive disaster (industrial or natural), connected with gas poisoning on a huge scale. Hospitals equipped with limited number of respirators, adequate for peacetime activity, are not able to meet suddenly multiplied requirements for ventilatory support. This paper describes a preliminary study to develop a convenient, reliable method of performing artificial ventilation of at least two patient by means of only one ventilator. We developed a unique, new control system (patent pending) which, when placed between a respirator and endotracheal tubes of the patients, divides the total tidal volume between the patients' lungs and controls pressures at their airways. A special arrangement of valves in the control system enables us to separate inspiratory and expiratory paths for each patient and to avoid cross-infection. The model study performed, according to ISO standards, on mechanical test lungs has shown that the proposed control system enables us to adjust ventilatory parameters at desired values, when lung compliance or respiratory airway resistance differ. The proposed one-source artificial ventilation is a simple solution to provide ventilatory support when the number of patients is greater than the number of respirators that are available.

Cerebral function monitoring in paediatric intensive care: useful features for predicting outcome

Neurological integrity in sick children is difficult to assess clinically. The aim of this study was to determine the predictive value of EEG activity recorded with a bedside EEG analysing monitor in an intensive care unit. EEG activity was monitored in 108 children (age range 2 weeks to 16 years, median 1.7 years) considered at risk for cerebral abnormalities with a cerebral function analysing monitor (CFAM). Recordings were evaluated for features of background EEG activity including mean amplitude, frequencies, and symmetry. Electrical seizure activity was quantified if present. Predictive value of the EEG features was evaluated relative to the clinical neurological outcome after one year. Asymmetrical recordings were not seen in any child with a normal outcome. Suppression of background activity was seen in 75% of the children who died. Seizures were present in 68% of children with a poor outcome. Seventeen of the 32 children (65%) who died had prolonged seizures. Absence of seizures and the presence of superimposed fast EEG activity in response to benzodiazepine infusions correlated with good outcome. A combination of two or more predictive EEG features demonstrated >90% specificity and positive predictive likelihood of poor outcome. EEG features provide information about the functional cerebral integrity of sick children. Changes in cerebral activity detected by the CFAM aid decision making by providing such information readily at the bedside in intensive care.

Computer simulation of hemodynamic parameter changes by mechanical ventilation and biventricular circulatory support

When a Bi-Ventricular Assist Device (BVAD) is used in conjunction with mechanical ventilation (MV) of the lungs with positive intrathoracic pressure (Pt), the latter influences hemodynamics. The aim of our study was to assess the simultaneous influence of BVAD and MV on hemodynamics. We assumed ventricular pathological conditions as reduced elastances and increased rest volumes. Peripheral systemic arterial resistance was assumed to have different values. Data were obtained by computer simulation. Trends in main hemodynamic variables were compared with clinical data from literature. Simulation showed that systemic venous, pulmonary arterial and left atrial pressures are very sensitive to Pt (-2 to 5 mmHg).

Computer simulation of haemodynamic parameters changes with left ventricle assist device and mechanical ventilation

Left Ventricular Assist Device is used for recovery in patients with heart failure and is supposed to increase total cardiac output, systemic arterial pressure and to decrease left atrial pressure. Aim of our computer simulation was to assess the influence of Left Ventricular Assist Device (LVAD) on chosen haemodynamic parameters in the presence of ventilatory support. The software package used for this simulation reproduces, in stationary conditions, the heart and the circulatory system in terms of pressure and volume relationships. Different circulatory sections (left and right heart, systemic and pulmonary arterial circulation, systemic and pulmonary venous circulation) are described by lumped parameter models. Mechanical properties of each section are modelled by RLC elements. The model chosen for the representation of the Starling's law of the heart for each ventricle is based on the variable elastance model. The LVAD model is inserted between the left atrium and the aorta. The contractility of the heart and systemic arterial resistance were adjusted to model pathological states. Our simulation showed that positive thoracic pressure generated by mechanical ventilation of the lungs dramatically changes left atrial and pulmonary arterial pressures and should be considered when assessing LVAD effectiveness. Pathological changes of systemic arterial resistance may have a considerable effect on these parameters, especially when LVAD is applied simultaneously with mechanical ventilation. Cardiac output, systemic arterial and right atrial pressures are less affected by changes of thoracic pressure in cases of heart pathology.

Abnormal surfactant composition and activity in severe bronchiolitis

A prospective study of infants under 1 y of age, ventilated for severe viral bronchiolitis, was carried out in four paediatric intensive care units in order to study surfactant activity and composition in this condition. Lung lavage fluid from 24 infants with bronchiolitis, 19 with bronchiolitis and sepsis or cardiac failure and 12 controls were analysed by the "click test" for surfactant activity and for phospholipids. Surfactant activity was present in all controls, but in only 2 of the 24 infants with bronchiolitis alone. The presence of phosphatidylglycerol correlated perfectly with the click test, suggesting that reduced activity is due to changes in surfactant lipid composition. In those with bronchiolitis plus coexisting disease, surfactant activity and phosphatidylglycerol were absent in only half. Surfactant activity and phosphatidylglycerol re-appeared by extubation. Severe viral bronchiolitis is associated with an absence of surfactant activity and PG, which resolves by clinical recovery. Infants with coexisting conditions are not always surfactant deficient. Surfactant administration is likely to be beneficial, but requires a selective approach.

The influence of simultaneous intra-aortic balloon pumping and mechanical ventilation on hemodynamic parameters--numerical simulation

Intra-aortic balloon counterpulsation (IABC) is one of the methods to assist circulation. Its application is supposed to increase coronary blood flow (Qcor) and myocardial oxygen supply. Mechanical ventilation of the lungs causes some side effects that influence mainly hemodynamics, i.e. it decreases cardiac output (CO) and systemic aortic pressure (Pas) but increases systemic venous pressure (Pvs). The aim of this study was to assess the influence of IABC on hemodynamic parameters when mechanical ventilation of the lungs is used as well. We assumed pathological conditions of the heart as reduced left ventricle elastance (Ev = 0.77 mmHg/cm3) and increased left ventricle rest volume (V0 = 10 cm3). Also peripheral systemic arterial resistance (Ras) took three different values. CARDIOSIM, a computer simulator of the human cardiovascular system, was used to assess the changes of hemodynamic parameters [CO, Pas, Pvs, and pulmonary arterial pressure (Pap)] when IABC and mechanical ventilation of the lungs were applied simultaneously. Computer simulation showed that application of IABC increased such variables as CO (by approximately equal to 17%), maximum value of Pas (by approximately equal to 20%), while left atrial pressure (Pla) was diminished (by approximately equal to 30%). On the other hand, Pvs and Pap were not strongly affected by IABC, but they were dramatically dependent on intrathoracic pressure. This means that Pvs, Pap, and Pla should be carefully monitored during IABC and artificial ventilation.

Complications of percutaneous dilatational tracheostomy in children

Experience with percutaneous dilatational tracheostomy in children is limited. This report discusses two significant complications which occurred following the use of this technique.

The influence of left ventricle assist device and ventilatory support on energy-related cardiovascular variables

One of the main purposes in using Left Ventricle Assist Devices (LVAD) to assist recovery in patients with heart failure, is to reduce the external work (EW) of the left natural ventricle. The simultaneous presence of mechanical ventilatory support can affect the value of this variable. The aim of our computer simulation was to trace the influence of LVAD on EW, cardiac mechanical efficiency (CME) and pressure volume area (PVA) in the presence of ventilatory support. Pathological conditions of the heart were reproduced. Peripheral systemic arterial resistance (Ras) was also changed to model physiological and pathological states. The influence of mechanical ventilation was introduced by changing levels of mean thoracic pressure. In this way we were able to predict changes of EW, CME and PVA in both ventricles, during ventilatory (mechanical) and cardiovascular (LVAD) support. Our simulation showed that positive thoracic pressure seems to affect the energy-related cardiovascular variables and should be taken into account during the assessment of LVAD effectiveness. Pathological changes of systemic peripheral resistance have a considerable effect on EW, CME and PVA of left ventricle. On the other hand energy-related parameters of the right ventricle are not especially affected by changes in systemic peripheral resistance.

Assessment of respiratory system compliance by a flow recording method

Standard methods for the assessment of the compliance of the respiratory system require recordings of both airway pressure and ventilatory volume. A method based on the recording of respiratory flow only, with no need of measuring airway pressure was reported some years ago. The purpose of the present study was to test a modification of this flow-recording technique. The technique is based on the measurement of inspiratory flow with and without the addition of a source of external compliance added to the inspiratory circuit. The difference in inspiratory flow between these two situations can be used to derive the compliance of the respiratory system. The method was and in patients with acute respiratory failure, and compared with a standard method based on airway pressure and inspiratory gas flow recording. Model experiments showed strong correlation between the new flow-recording method and the standard method (r = 0.99, P < 0.001) with a coefficient of variation of less than 2% with both methods. In patients with acute respiratory failure, the flow and standard methods gave similar results when no end-inspiratory pause was applied (mean difference 5 +/- 40 ml/kPa (x +/- s.d.)). The standard method gave significantly higher compliance values, different from the flow method, when an end-inspiratory pause of 10% was applied (mean difference 53 +/- 2 ml/kPa, P < 0.01). The flow method was not influenced by the use of an end-inspiratory pause.(ABSTRACT TRUNCATED AT 250 WORDS)

Anaesthesia and mucopolysaccharidoses. A review of airway problems in children

The airway problems encountered during anaesthesia in all children with mucopolysaccharidoses presenting for a surgical procedure from 1988 to September 1991 are reviewed. Thirty-four patients underwent 89 anaesthetics for 110 procedures. The results reveal a high incidence of airway problems. The overall incidence of difficult intubation was 25% and failed intubation 8%. In those children with Hurler's syndrome, the difficult intubation incidence was 54% and failed intubation incidence 23%. Other potential anaesthetic problems such as cardiac anomalies and obstructive sleep apnoea are also reviewed.

Pain relief after tonsillectomy in adults: intramuscular diclofenac and papaveretum compared

A wide variety of surgical and pharmacological methods have been described in an attempt to reduce pain after tonsillectomy, with conflicting results. Opiates are still widely used, despite unwanted side-effects. Recently the non-steroidal anti-inflammatory drugs have been shown to be effective against a variety of post-operative pains. Diclofenac is effective in both children and adults in the relief of pain after tonsillectomy when administered rectally, but absorption is variable, and suppositories are not widely accepted in the UK. The present double-blind study compared a single intramuscular dose of diclofenac with papaveretum in adults undergoing tonsillectomy. Post-operatively, the patients who received diclofenac had less pain and started drinking significantly sooner than the control group. There were no undesirable side-effects. Intramuscular diclofenac is superior to papaveretum in the relief of pain in adults undergoing tonsillectomy.

A new method for calculating total respiratory system compliance. Theory and model experiments

A new method for calculating total respiratory system compliance is described, based on simple modelling of a ventilator-respiratory system circuit that assumes linear characteristics of the circuit parameters compliances and resistances. The method requires only that flow measurement be conducted continuously to obtain compliance, if the internal compliance of the circuit is known beforehand. Model experiments showed that the compliance of a child test lung, calculated from the flow recording, differed at most by 10% from the compliance obtained by separate measurements of pressure and volume under static conditions, over a wide range of respiratory flows and airway resistances.

Development and evaluation of a flow-dividing unit for differential ventilation and selective PEEP

Differential ventilation with selective positive end-expiratory pressure (PEEP) was studied in a two-compartment lung model, using one ventilator and a flow-dividing unit consisting of inspiratory flow resistors and an inspiratory threshold valve. The compliance of each lung compartment was varied between 0.15 and 0.23 1 X kPa-1 and the resistance was varied from 0 to 3.5 kPa X 1(-1) X s. The minute volume was 12 1 and the respiratory frequency 12/min, with an inspiratory:expiratory ratio of 1:2. An even distribution of ventilation to the two lung compartments was obtained with the inspiratory flow resistors or the threshold valve under all conditions studied. However, a stepwise increase in the inspiratory resistance of one lung compartment from 1.0 to 2.5 or from 2.5 to 3.5 kPa X 1(-1) X s required readjustment of the inspiratory flow resistor to achieve an even distribution of ventilation, whereas the inspiratory threshold valve needed no readjustment. Large differences in the inspiratory impedance of the two lung compartments caused asynchronous gas delivery when the ventilation distribution was adjusted by means of the flow resistors. Use of the threshold valve resulted in synchronous gas delivery. The flow-dividing unit consists of non-active elements and can thus be connected to any ventilator.

Practical aspects of differential ventilation with selective peep in acute respiratory failure

Hypoxaemia in association with acute respiratory failure continues to be a severe problem in some intensive care patients. Among strategies proposed, we want to focus attention on differential ventilation with selective PEEP, administered in the lateral position. This ventilation technique has proved successful in the treatment of refractory hypoxaemia due to severe bilateral lung disease. The rationale of this concept is briefly presented in this paper, where the main emphasis is laid on the practical aspects of its clinical application. Two case reports are included as examples of our experiences.