Performance of the right ventricle: a pressure plane analysis.

A load invariant presentation of the heart's ventricular performance has been investigated. Flow from the right ventricle of isolated cat hearts was passed through a controlled, variable load. Ventricular pressure and flow were recorded and ventricular volume computed by flow integration. The three-dimensional trajectories of ventricular pressure, ventricular volume and time after onset of contraction all closely followed a single three-dimensional surface, which was denoted as the PVt surface. The PVt surface depends on the inotropic condition of the heart, but is load invariant. It is therefore a measure of the intrinsic ventricular performance. Given the arterial load on the ventricle, it can be applied for predictions of pressure and flow profiles, as well as any parameter related to them.

Right ventricular mechanics: a comparison of models.

Using data from an isolated supported cat right ventricle preparation, we investigated the following models or methods for characterising the mechanical function of the ventricle: (1) a pressure generator in series with an internal impedance; (2) a variable elastance in series with an internal, pressure dependent flow resistance; (3) pulse response analyses, theoretically based on the pressure response to a small volume step of short duration; and (4) geometric mapping of the variables pressure and volume as functions of time. We tested the reproductive and predictive strengths of the four models, and found that all methods could reproduce 50% of the observed pressure curves with an RMS error less than 0.2 kPa, and in most cases also gave a close prediction of pressure curves which were not used to establish the respective model parameters. We see this as one reason for the fact that no single ventricular model has yet been universally accepted.

Reduction of pulsatile hydraulic power in the pulmonary circultation caused by moderate vasoconstriction.

Vascular input impedance and associated hydraulic power was measured in rabbit isolated lungs. The study was focused on changes in impedance and in pulsatile hydraulic power during relaxation and contraction of vascular smooth muscle. Pulsatile power was found to be at a minimum when smooth muscle tone was such that the pulmonary arterial pressure was in the physiological range, and increased both when the vessels were relaxed and further constricted. Input impedance was found to be determined mainly by the large, proximal ('extra-alveolar') arteries.

Haemodynamic effects of arterial compliance, total peripheral resistance, and glyceryl trinitrate on regurgitant volume in aortic regurgitation.

STUDY OBJECTIVE: Afterload reduction is known to reduce regurgitant flow in patients with aortic regurgitation. Both arterial compliance and total peripheral resistance are determinants of afterload. The aim of this study was to evaluate the influence of arterial compliance and total peripheral resistance on the regurgitant volume. DESIGN: The values of arterial compliance and total peripheral resistance were assessed during aortic regurgitation at different regurgitant orifice areas in eight pigs before and after a bolus of glyceryl trinitrate. In a computer model the importance of arterial compliance and total peripheral resistance on the regurgitant volume was assessed by keeping each of them constant while the other variable was changed. MEASUREMENTS AND MAIN RESULTS: In both the experimental and computer models a very strong correlation was found between decreased total peripheral resistance and decreased regurgitant volume. Arterial compliance was of hardly any importance. A bolus of glyceryl trinitrate reduced regurgitant volumes and regurgitant fractions significantly. CONCLUSIONS: Total peripheral resistance is an important factor in influencing the regurgitant volumes at a given regurgitant orifice area in aortic regurgitation, while arterial compliance is of less importance. Glyceryl trinitrate effectively reduces the regurgitant volumes by its effect on peripheral resistance.

Regurgitant volume in aortic regurgitation from a parameter estimation procedure.

The regurgitant volume and regurgitant orifice area as well as total peripheral resistance and arterial compliance were estimated in a cardiovascular hydromechanical simulator and in 10 patients with aortic regurgitation. A parameter estimation procedure based on a simple model of the cardiovascular system, Doppler measurements of the regurgitant jet, aortic systolic flow, and systolic and diastolic blood pressures was used. In the cardiovascular simulator the estimated regurgitant orifice area was compared with the size of a hole in the disk of a mechanical aortic valve. In the patients the regurgitant fraction was compared with semiquantitative grading from echocardiography routinely performed in our laboratory. In the hydromechanical simulator, the estimated regurgitant orifice area of 26.5 +/- 3.5 (SD) mm2 (n = 9) was not different from the true value of 24 mm2. In the patients there was a fair relationship between the estimated regurgitant fraction and the semiquantitative grading. The estimated regurgitant orifice areas varied between 1.6 and 31.2 mm2. The estimated mean values of total peripheral resistance and arterial compliance were 1.67 +/- 0.55 mmHg.s.ml-1 and 1.30 +/- 0.42 ml/mmHg, respectively.

Pressure half-time in aortic regurgitation: evaluation with Doppler in a cardiovascular hydromechanical simulator and in a computer model.

Doppler echocardiographic determination of pressure half-time has been proposed as a method of assessing the severity of aortic regurgitation. To evaluate this method, we assessed the relation between pressure half-time and simulated aortic regurgitant flow under various conditions in two models of the cardiovascular system. In a hydromechanical model we assessed the influence of total peripheral resistance and arterial compliance on the pressure half-time as measured by continuous wave Doppler echocardiography. In a computer model that used the half-time of the pressure gradient between the aorta and the left ventricle as an expression of pressure half-time, we assessed the influence of total peripheral resistance and arterial compliance and also the influence of left ventricular compliance on pressure half-time. In both models, although we found an inverse relation between regurgitant orifice area and pressure half-time, changing total peripheral resistance and arterial compliance (but not left ventricular compliance) within the physiologic range significantly altered the pressure half-times. We concluded that the influence of total peripheral resistance and arterial compliance limits the usefulness of Doppler echocardiographic determination of pressure half-time as a method of assessing the severity of aortic regurgitation.

Instantaneous cross-sectional flow velocity profiles: a comparative study of two ultrasound Doppler methods applied to an in vitro pulsatile flow model.

Two methods based on different techniques for construction of cross-sectional flow velocity profiles from Doppler ultrasound signals were compared: an intraluminal method using pulsed-wave Doppler echocardiography and an extraluminal method using two-dimensional (color) Doppler ultrasound. The methods were applied to an in vitro pulsatile flow model. With the intraluminal method, pulsed Doppler recordings obtained throughout several flow pulses at different positions across a tube were digitized, and cross-sectional flow velocity profiles were obtained by matching the onset of flow velocity at the various positions. With the extraluminal method, cross-sectional flow velocity profiles were obtained by time interpolation between the digital flow velocity data obtained from several flow velocity maps. The first flow velocity map was recorded at onset of flow and the following maps were incrementally delayed with 20 msec from one flow pulse to the next. The time lag caused by the time needed to update each of the flow velocity maps was compensated for by time interpolation between the sequentially recorded flow velocity maps. The cross-sectional flow velocity profiles obtained with the two methods were compared at identical positions within the tube model at equal flow settings and throughout the pulsatile flow periods. At three different flow settings with peak flow velocity of 0.3, 0.5, and 0.7 m/sec, the difference (mean +/- SD) between the obtained velocities were 0.01 +/- 0.04, -0.01 +/- 0.05, and -0.03 +/- 0.07 m/sec, respectively. The findings suggest that cross-sectional flow velocity profiles from pulsatile flow velocity recordings can be obtained equally well with both methods.

Estimation of regurgitant volume and orifice in aortic regurgitation combining CW Doppler and parameter estimation in a Windkessel-like model.

A method for noninvasive estimation of regurgitant orifice and volume in aortic regurgitation is proposed and tested in anesthetized open chested pigs. The method can be used with noninvasive measurement of regurgitant jet velocity with continuous wave ultrasound Doppler measurements together with cuff measurements of systolic and diastolic systemic pressure in the arm. These measurements are then used for parameter estimation in a Windkessel-like model which include the regurgitant orifice as a parameter. The aortic volume compliance and the peripheral resistance are also included as parameters measurements in the open chest pigs are used. Electromagnetic flow measurements in the ascending aorta and pulmonary artery are used for control, and a correlation between regurgitant volume obtained from parameter estimation and electromagnetic flow measurements of 0.95 over a range from 2.1 to 17.8 mL is obtained.

Estimation of arterial compliance in aortic regurgitation: three methods evaluated in pigs.

Three methods for measuring arterial compliance when aortic regurgitation is present are examined. The first two methods are based on a Windkessel model composed of two elements, compliance C and resistance R. Arterial compliance was estimated from diastolic pressure waveforms and diastolic regurgitant flow for one method, and from systolic aortic pressure waveforms and systolic flow for the other method. The third method was based on a three-element Windkessel model, composed of characteristic resistance r, compliance C and resistance R. In this method arterial compliance was calculated by adjusting the model to the modulus and phase of the first harmonic term of the aortic input impedance. The three methods were compared and validated in six anaesthetised pigs over a broad range of aortic pressures. The three methods were found to give quantitatively similar estimates of arterial compliance at mean aortic pressures above 60 mm Hg. Below 60 mm Hg, estimates of arterial compliance varied widely, probably because of poor validity of the Windkessel models in the low pressure range.

Quantification of aortic regurgitation by Doppler echocardiography: a new method evaluated in pigs.

We have developed a method to quantify aortic regurgitant orifice and volume, based on measurements of the velocity of the regurgitant jet, aortic systolic flow, the systolic and diastolic arterial pressures, a Windkessel arterial model, and a parameter estimation technique. In six pigs we produced aortic regurgitant flows between 2.1 and 17.8 ml per beat, i.e. regurgitant fractions from 0.06 to 0.58. Pulmonary and aortic flows were measured with electromagnetic flow probes, aortic pressure was measured invasively, and the regurgitant jet velocity was obtained with continuous-wave Doppler. The parameter estimation procedure was based on the Kalman filter principle, resulting primarily in an estimate of the regurgitant orifice area. The area was multiplied by the velocity integral of the regurgitant jet to estimate regurgitant volume. A strong correlation was found between the regurgitant volumes obtained by parameter estimation and the electromagnetic flow measurement. These results from our study in pigs suggest that it may be possible to quantify regurgitant orifice and volume in patients completely noninvasively from Doppler and blood pressure measurements.

Radiology in the Nordic Countries.

NEMT, Nordic Evaluation of Medical Technology, conducted a study in 1988-89 on the use and diffusion of diagnostic radiology technologies in Denmark, Finland, Iceland, Norway and Sweden, i.e. the Nordic Countries. The study analysed the responses to a questionnaire sent to all Nordic radiology departments. Our findings show a variation from about 500 to nearly 900 radiology examinations per 1000 inhabitants among the Nordic Countries. Some of the differences are explained by unique structural factors of the health care system in each country, even if they all provide comparable public health services. Other differences are explained by variations in medical practice, accessibility to new imaging modalities, and replacement policies. This paper summarizes the results of the study.

Impedance matching between ventricle and load.

Impedance matching in the cardiovascular system is discussed in light of two models of ventricle and load: a Thevenin equivalent consisting of a hydromotive pressure source and an internal, source resistance and compliance in parallel; and a time-varying compliance filled from a constant pressure source and ejecting into a load of three components, a central resistor, a compliance, and a peripheral resistance. According to the Thevenin analog, the energy source and the load are matched when the load resistance is T/t times the internal source resistance (T is total cycle length, t is systolic time interval). Both from this model and from the variable compliance model it appears that optimum matching between source and load depends on the compliance of the Windkessel, as low compliance shifts the matching load resistance to a low value. Animal experiments (isolated cat hearts) indicated that both left and right ventricles at normal loads work close to their maxima of output hydraulic power, and, according to experiments in the right ventricle, maximum power output is related to load resistance and compliance as predicted by the above models. From an experimentally determined relationship among instantaneous ventricular pressure and volume (right ventricle of isolated cat hearts), an optimum load impedance was calculated on the basis of the assumption that the ratio between stroke work and static, potential energy developed in the ventricular cavity is maximum. The optimum load impedance found by this procedure closely resembles the normal input impedance of the cat lung vessel bed.

Interaction between the rigt heart ventricle and its arterial load: a quantitative solution.

Piene and Sund (Cardiovasc. Res., 1980) have recently demonstrated that the performance of the ventricle can be expressed by relating the principal variables pressure (P), volume (V), and time after onset of contraction (t): P = function (V,t). The function can be mapped experimentally as a three-dimensional surface, denoted the PVt surface, and can thereafter be approximated by an empirical equation. The vascular load of the ventricle is defined by the arterial input impedance vs. frequency graph, Z(f), which can be converted to the time domain by inverse Fourier transformation. This report describes how flow and pressure generated by the ventricle can be calculated when the independent expressions for the ventricle and for the load are given. The method is applied on right ventricular contractions of isolated cat hearts, and excellent fit between calculated and observed flow and pressure curves was obtained.

The influence of pulmonary blood flow rate on vascular input impedance and hydraulic power in the sympathetically and noradrenaline stimulated cat lung.

This study was designed to evaluate the influence of sympathetic nerve stimulation (SN) and alpha-adrenergic receptor stimulation (alphaS) on the pulmonary vascular input impedance and hydraulic power output of the right heart during variations of cardiac output (CO). An open chest cat preparation was used and pulsatile pressure and flow in the pulmonary artery were measured by high frequency response transducers. Calculations showed that vascular resistance (VR) was inversely dependent on CO, but input impedance of the unstimulated lung was not influenced by CO variations. NS or alphaS increased VR and input impedance significantly, and the relation pulsatile hydraulic power/total hydraulic power (Wp/Wt) increased 40%, indicating that such stimulation has larger relative influence on impedance than on resistance. The reduction of arterial compliance during NS (maximal stimulus) was calculated to be 60%, independent of CO. Input impedance during NS or alphaS was reduced by CO elevations, probably because the concomitant distension of the arterial bed reduced arterial resistance and inertance. The ratio Wp/CO, which expresses the fraction of pulsatile hydraulic power lost per ml mean arterial flow, was found to be flow dependent both in control and stimulated conditions: Wp/CO was positively correlated to CO in control condition and weakly negatively correlated to CO during stimulation. At high CO the arterial vessels could be stimulated and stiffened without much extra load on the right heart.

Influence of vessel distension and myogenic tone on pulmonary arterial input impedance. A study using a computer model of rabbit lung.

A computer model of the pulmonary arterial (PA) bed of rabbit lungs was designed in order to test experimental observations of changes in PA input impedance and pulsatile hydraulic power (cap.) during increased PA pressure. The computer model was based on a simple 3-component analog representation of single vessels (i.e. resistance, inertance and compliance). 16 generations of arterial vessels, from PA to 60 mum diameter, were combined to calculate PA input impedance. Input impedance was found to mimic closely that observed experimentally. Both venous pressure elevation and arteriolar constriction was found to reduce input impedance and Wp. By combining arteriolar constriction with increased myogenic tone of the larger arteries, Wp was found to show a minimum level at a certain PA pressure, dependent on the degree of arterial stiffening. Wp was found to follow changes in arterial volume and resistance during stimulated vasoconstriction. Wp dissipation in arterial vessels was calculated to approx. 50% of total imput Wp at physiological pressure conditions, and could be reduced by one half after PA pressure increase from 20 to 50 cm H2O, despite a concurrent halving of arterial compliance. Arterial vessels smaller than 200 mum diameter were found to have negligible direct influence on PA input impedance.

Some physical properties of the pulmonary arterial bed deduced from pulsatile arterial flow and pressure.

This study aimed to quantify changes of vascular compliance and resistance of the proximal and the peripheral pulmonary arterial vessels when vascular smooth muscle was stimulated. These above vascular characteristics were derived from registrations of pulsatile pressure and flow in the pulmonary artery (PA). An in situ cat lung preparation was used, with the right heart by-passed by a pulsatile blood pump. Vascular input impedance was derived from PA pulsatile pressure and flow recordings, and impedance characteristics were used for calculation of the variables of a simple lumped analog representation of the arterial bed. PA smooth muscle was stimulated by infusions of collagen suspension, by general hypoxia and by noradrenaline injections. Collagen caused 40% reduction of vascular compliance (C), no changes in proximal arterial resistance (R1) and 180% increase in peripheral vascular resistance (R2). Hypoxia caused 50% reduced C, 20% increased R1 and 7u% increased R2. Noradrenaline caused 20% reduced C and 30% increased R1 and R2. These results, together with results derived from simulation of the observed impedance changes in a computer model of the lung arterial bed, indicated that collagen infusion elicited contraction of small and medium-sized arteries, with increased arterial volume as result of increased distending pressure. Hypoxia and noradrenaline, seemed both to cause contraction of the total arterial bed. This effect being most pronounced during hypoxia.

Improved left ventricular performance by the transmission of pulse waves through the pulmonary vascular bed.

The influence on left ventricular performance of pulsewaves transmitted through the pulmonary vascular bed was studied in a cat lung preparation with the right heart bypassed by a pulsatile blood pump. The pump worked at a frequency slightly different from the intrinsic heart rate; transmitted pulse waves were thereby forced to arrive the left atrium at different phases of the left heart cycle. Slow fluctuations of left atrial pressure, left ventricular systolic pressure and left ventricular dP/dt were observed. Left ventricular systolic pressure and left ventricular dP/dt were maximum when the transmitted pulsewaves arrived left atrium just prior to the atrial contraction. The observed variation in left ventricular systolic pressure was found to be directly dependent on the magnitude of pulsatile hydraulic power transmitted to the left atrium.

[Criteria for waiting list guarantee. An attempt at precision]. / Kriterier for å få ventelistegaranti. Forsøk på presisering.

A scoring form has been developed, based upon the system of priorities used in Norway when deciding among patients waiting for investigation and treatment in hospital. This scoring form was used to evaluate 40 different case histories in gynaecological, orthopaedic, urologic and oto-rhino-laryngological departments. There was much disagreement when evaluating the different case histories. Using this scoring form did not result in greater agreement among the physicians using it than among another group of physicians who evaluated the same case histories without using the form.

[The waiting list guarantee. Variations in frequency of receiving the waiting list guarantee]. / Ventelistegarantiordningen. Variasjon i andel som får ventelistegaranti.

From 1987, patients waiting for treatment in Norwegian hospitals are given different degrees of priority, from zero to five. As from 1990, patients in priority group two are placed on a special list, and a report is made if they have not been treated within six months. There is a considerable variation in the frequency with which the patients are given this treatment guarantee in the different Norwegians counties. In the case of gynaecological patients the frequency varies from one to 94%, of urological patients from 43 to 100%, of orthopaedic patients from 21 to 89% and of otorhinolaryngological patients from 21 to 89%. The same variations are also seen within a single region of the country, and between departments. The reason for this discrepancy may be varying composition of the population, different extents of day surgery or different economic strategic thinking. Probably the main reason is that the criteria for giving a waiting list guarantee are not accepted as operational. This leads to different medical judgments when evaluating the applications for treatment in hospital.