Cardiac output determination by simple one-step rebreathing technique

Effects of a Dietary Beetroot Juice Treatment on Systemic and Cerebral Haemodynamics- A Pilot Study

INTRODUCTION

Beetroot Juice (BJ) contains dietary nitrates that increase the blood Nitric Oxide (NO) level, decrease Blood Pressure (BP), increase athletic performance and improve cognitive functions but the mechanism remains unclear. Ultrasonographic measurement of middle cerebral artery blood flow velocity with computation of Cerebral Augmentation Index (CAIx) is a measure of the reflected flow signal, modulated by changes in cerebrovascular resistance and compliance.

AIM

This pilot study tests the hypothesis that ingestion of an amount of BJ sufficient to raise the blood NO level two-to three-fold, decreases Transcranial Doppler (TCD) measured CAIx.

MATERIALS AND METHODS

Ten healthy young-adult African-American women were studied at two levels of submaximal exercise, 40% and 80% of their predetermined peak oxygen consumptions. The subjects ingested nitrate-free orange juice (OJ, control) and an isocaloric BJ beverage (1.5 mg/mL nitrate, 220 Cal), on different days, 1-2 weeks apart.

RESULTS

The BJ treatment increased blood NO and decreased systolic BP at rest and at the two levels of exercise. The BJ treatment decreased CAIx only at the two levels of exercise (from 79 ± 2% to 62 ± 2% and from 80 ± 2% to 60 ± 3%, p<0.05). Exercise increased TCD-measured resistance and pulsatility indices (RIx, PIx) without changing AIx. The BJ treatment had no effect on RIx and PIx.

CONCLUSION

These findings suggest that decreased CAIx associated with aerobic exercise reflects the change in cerebral haemodynamics resulting from dietary nitrate supplementation. Future studies should determine whether the BJ-induced decrement in CAIx is correlated with an improvement in brain function.

Effects of dietary nitrates on systemic and cerebrovascular hemodynamics

Cerebral blood flow dysregulation is often associated with hypertension. We hypothesized that a beetroot juice (BRJ) treatment could decrease blood pressure and cerebrovascular resistance (CVR). We subjected 12 healthy females to control and BRJ treatments. Cerebrovascular resistance index (CVRI), systolic blood pressure (SBP), total vascular resistance (TVR), and the heart rate-systolic pressure product (RPP) measured at rest and at two exercise workloads were lower after the BRJ treatment. CVRI, SBP, and RPP were lower without a lower TVR at the highest exercise level. These findings suggest improved systemic and cerebral hemodynamics that could translate into a dietary treatment for hypertension.

Effects of acceleration in the Gz axis on human cardiopulmonary responses to exercise

The aim of this paper was to develop a model from experimental data allowing a prediction of the cardiopulmonary responses to steady-state submaximal exercise in varying gravitational environments, with acceleration in the G(z) axis (a (g)) ranging from 0 to 3 g. To this aim, we combined data from three different experiments, carried out at Buffalo, at Stockholm and inside the Mir Station. Oxygen consumption, as expected, increased linearly with a (g). In contrast, heart rate increased non-linearly with a (g), whereas stroke volume decreased non-linearly: both were described by quadratic functions. Thus, the relationship between cardiac output and a (g) was described by a fourth power regression equation. Mean arterial pressure increased with a (g) non linearly, a relation that we interpolated again with a quadratic function. Thus, total peripheral resistance varied linearly with a (g). These data led to predict that maximal oxygen consumption would decrease drastically as a (g) is increased. Maximal oxygen consumption would become equal to resting oxygen consumption when a (g) is around 4.5 g, thus indicating the practical impossibility for humans to stay and work on the biggest Planets of the Solar System.

Once daily dosing of enalapril in congestive heart failure

Enalapril 40 mg or tolerated dose was given once daily to 21 patients with congestive heart failure (CHF), NYHA class III, in addition to treatment with digoxin and/or diuretics. After an 8-week open period, 19 patients were randomized to continue enalapril or to receive a placebo in a double-blind manner. After the first enalapril dose of 10 mg, maximal reduction of blood pressure (BP) occurred after 4 hours (mean 34/17 mmHg; p less than 0.001). No further reduction was found after higher doses. After the open period significant improvement was shown as judged by NYHA class (p less than 0.01), stroke volume (p less than 0.05), maximal working capacity (p less than 0.05), heart volume (p less than 0.01) and maximum rate pressure product (RPPmax) (p less than 0.001). Urinary aldosterone markedly decreased (p less than 0.01), whereas serum potassium and serum creatinine slightly increased (p less than 0.05). At the end of the blind period enalapril was superior to placebo concerning NYHA class (p less than 0.01), heart volume (p less than 0.05) and RPPmax (p less than 0.05). Other parameters, including aldosterone in urine, did not differ between the groups. Carry-over effects may have diminished the differences between enalapril and placebo. Diarrhoea (n = 5) and hypotension (n = 5) were the most common side-effects. Overall, enalapril was well tolerated and seems to be useful in single daily doses in the treatment of CHF.

Techniques of cardiac output measurement during liver transplantation: arterial pulse wave versus thermodilution

In this study, we compared continuous cardiac output (CO) obtained from the femoral arterial pressure by simulation of an aortic input impedance model [model-simulated cardiac output (MCO)] to thermodilution cardiac output (TDCO) determined by bolus injection during liver transplantation. Both variables were measured in 39 adult patients (13 females) every 10th minute during liver transplant surgery. Paired measurements were compared during the 4 phases of surgery-dissection, anhepatic phase, early reperfusion (the first 15 minutes after reperfusion), and late reperfusion (15-60 minutes after reperfusion)-without the detection of any significant difference between the 2 estimates of CO. TDCO ranged from 2.3 to 17.2 L/minute, and the bias (the mean difference between MCO and TDCO) prior to calibration was -0.4 +/- 1.6 L/minute (mean +/- standard deviation; 1309 paired measurements; 95% limits of agreement: -3.4 to 2.6 L/minute). After calibration of the first determined MCO by the simultaneously determined TDCO, the bias was 0.1 +/- 1.5 L/minute, with 57% (n = 744) of the comparisons being less than 1 L/minute and 35% (n = 453) being less than 0.5 L/minute; this was independent of the level of CO, and the mutual correlation coefficient was 0.812 (P < 0.001). This study indicates that during liver transplantation surgery, MCO reflects TDCO throughout the operation. Thus, for CO, this less invasive method appears to provide a reliable uninterrupted measurement during orthotopic liver transplantation.

Simultaneous determination of the accuracy and precision of closed-circuit cardiac output rebreathing techniques

Foreign and soluble gas rebreathing methods are attractive for determining cardiac output (Q(c)) because they incur less risk than traditional invasive methods such as direct Fick and thermodilution. We compared simultaneously obtained Q(c) measurements during rest and exercise to assess the accuracy and precision of several rebreathing methods. Q(c) measurements were obtained during rest (supine and standing) and stationary cycling (submaximal and maximal) in 13 men and 1 woman (age: 24 +/- 7 yr; height: 178 +/- 5 cm; weight: 78 +/- 13 kg; Vo(2max): 45.1 +/- 9.4 ml.kg(-1).min(-1); mean +/- SD) using one-N(2)O, four-C(2)H(2), one-CO(2) (single-step) rebreathing technique, and two criterion methods (direct Fick and thermodilution). CO(2) rebreathing overestimated Q(c) compared with the criterion methods (supine: 8.1 +/- 2.0 vs. 6.4 +/- 1.6 and 7.2 +/- 1.2 l/min, respectively; maximal exercise: 27.0 +/- 6.0 vs. 24.0 +/- 3.9 and 23.3 +/- 3.8 l/min). C(2)H(2) and N(2)O rebreathing techniques tended to underestimate Q(c) (range: 6.6-7.3 l/min for supine rest; range: 16.0-19.1 l/min for maximal exercise). Bartlett's test indicated variance heterogeneity among the methods (P < 0.05), where CO(2) rebreathing consistently demonstrated larger variance. At rest, most means from the noninvasive techniques were +/-10% of direct Fick and thermodilution. During exercise, all methods fell outside the +/-10% range, except for CO(2) rebreathing. Thus the CO(2) rebreathing method was accurate over a wider range (rest through maximal exercise), but was less precise. We conclude that foreign gas rebreathing can provide reasonable Q(c) estimates with fewer repeat trials during resting conditions. During exercise, these methods remain precise but tend to underestimate Q(c). Single-step CO(2) rebreathing may be successfully employed over a wider range but with more measurements needed to overcome the larger variability.

The effects of beta1-adrenergic blockade on cardiovascular oxygen flow in normoxic and hypoxic humans at exercise

At exercise steady state, the lower the arterial oxygen saturation (SaO(2)), the lower the O(2) return (QvO(2)). A linear relationship between these variables was demonstrated. Our conjecture is that this relationship describes a condition of predominant sympathetic activation, from which it is hypothesized that selective beta1-adrenergic blockade (BB) would reduce O(2) delivery (QaO(2)) and QvO(2). To test this hypothesis, we studied the effects of BB on QaO(2) and QvO(2) in exercising humans in normoxia and hypoxia. O(2) consumption VO(2), cardiac output Q, CO(2) rebreathing), heart rate, SaO(2) and haemoglobin concentration were measured on six subjects (age 25.5 +/- 2.4 years, mass 78.1 +/- 9.0 kg) in normoxia and hypoxia (inspired O(2) fraction of 0.11) at rest and steady-state exercises of 50, 100, and 150 W without (C) and with BB with metoprolol. Arterial O(2) concentration (CaO(2)), QaO(2) and QvO(2) were then computed. Heart rate, higher in hypoxia than in normoxia, decreased with BB. At each VO(2), Q was higher in hypoxia than in normoxia. With BB, it decreased during intense exercise in normoxia, at rest, and during light exercise in hypoxia. SaO(2) and CaO(2) were unaffected by BB. The QaO(2) changes under BB were parallel to those in Q.QvO(2) was unaffected by exercise in normoxia. In hypoxia the slope of the relationship between QaO(2) and VO(2) was lower than 1, indicating a reduction of QvO(2) with increasing workload. QvO(2) was a linear function of SaO(2) both in C and in BB. The line for BB was flatter than and below that for C. The resting QvO(2) in normoxia, lower than the corresponding exercise values, lied on the BB line. These results agree with the tested hypothesis. The two observed relationships between QvO(2) and SaO(2) apply to conditions of predominant sympathetic or vagal activation, respectively. Moving from one line to the other implies resetting of the cardiovascular regulation.

Non-invasive pulsatile arterial pressure and stroke volume changes from the human finger

In this paper we review recent developments in the methodology of non-invasive finger arterial pressure measurement and the information about arterial flow that can be obtained from it. Continuous measurement of finger pressure based on the volume-clamp method was introduced in the early 1980s both for research purposes and for clinical medicine. Finger pressure tracks intra-arterial pressure but the pressure waves may differ systematically both in shape and magnitude. Such bias can, at least partly, be circumvented by reconstruction of brachial pressure from finger pressure by using a general inverse anti-resonance model correcting for the difference in pressure waveforms and an individual forearm cuff calibration. The Modelflow method as implemented in the Finometer computes an aortic flow waveform from peripheral arterial pressure by simulating a non-linear three-element model of the aortic input impedance. The methodology tracks fast changes in stroke volume (SV) during various experimental protocols including postural stress and exercise. If absolute values are required, calibration against a gold standard is needed. Otherwise, Modelflow-measured SV is expressed as change from control with the same precision in tracking. Beat-to-beat information on arterial flow offers important and clinically relevant information on the circulation beyond what can be detected by arterial pressure.

Respiratory measurements of cardiac output: from elegant idea to useful test

The measurement of cardiac output was first proposed by Fick, who published his equation in 1870. Fick's calculation called for the measurement of the contents of oxygen or CO2 in pulmonary arterial and systemic arterial blood. These values could not be determined directly in human subjects until the acceptance of cardiac catheterization as a clinical procedure in 1940. In the meanwhile, several attempts were made to perfect respiratory methods for the indirect determination of blood-gas contents by respiratory techniques that yielded estimates of the mixed venous and pulmonary capillary gas pressures. The immediate uptake of nonresident gases can be used in a similar way to calculate cardiac output, with the added advantage that they are absent from the mixed venous blood. The fact that these procedures are safe and relatively nonintrusive makes them attractive to physiologists, pharmacologists, and sports scientists as well as to clinicians concerned with the physiopathology of the heart and lung. This paper outlines the development of these techniques, with a discussion of some of the ways in which they stimulated research into the transport of gases in the body through the alveolar membrane.

Revised one-step method for determination of cardiac output

Cardiac output (Q) is a determinant of blood pressure and O(2) delivery and is critical in the maintenance of homeostasis, particularly during environmental stress and exercise. Cardiac output can be determined invasively in patients; however, indirect methods are required for other situations. Soluble gas techniques are widely used to determine (Q). Historically, measurements during a breathhold, prolonged expiration and rebreathing to CO(2) equilibrium have been used; however, with limitations, especially during stress. Farhi and co-workers developed a single-step CO(2) rebreathing method, which was subsequently revised by his group, and has been shown to be reliable and compared closely to direct, invasive measures. V(CO2), P(ACO2), and P(VCO2) are determined during a 12-25s rebreathing, using the appropriate tidal volume, and (Q) is calculated. This method can provide accurate data in laboratory and field experiments during exercise, increased or decreased gravity, water immersion, lower body pressure, head-down tilt, altered ambient pressure or changes in inspired gas composition.

The interplay of central and peripheral factors in limiting maximal O2 consumption in man after prolonged bed rest

1. The effects of bed rest on the cardiovascular and muscular parameters which affect maximal O2 consumption (VO2,max) were studied. The fractional limitation of VO2,max imposed by these parameters after bed rest was analysed. 2. The VO2,max, by standard procedure, and the maximal cardiac output (Qmax), by the pulse contour method, were measured during graded cyclo-ergometric exercise on seven subjects before and after a 42-day head-down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arterialized blood gas analysis were determined at the highest work load. 3. Muscle fibre types, oxidative enzyme activities, and capillary and mitochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of muscle cross-sectional area (CSA) by NMR imaging at the level of biopsy site allowed computation of muscle oxidative capacity and capillary length. 4. The VO2,max was reduced after bed rest (-16.6%). The concomitant decreases in Qmax (-30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O2 delivery (-39.7%). 5. Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (-17%). The volume density of mitochondria was reduced after bed rest (-16.6%), as were the oxidative enzyme activities (-11%). The total mitochondrial volume was reduced by 28.5%. Capillary density was unchanged. Total capillary length was 22.2% lower after bed rest, due to muscle atrophy. 6. The interaction between these muscular and cardiovascular changes led to a smaller reduction in VO2,max than in cardiovascular O2 transport. Yet the latter appears to play the greatest role in limiting VO2,max after bed rest (> 70% of overall limitation), the remaining fraction being shared between peripheral O2 diffusion and utilization.

Regulation of perfusive O2 transport during exercise in humans: effects of changes in haemoglobin concentration

1. Recently it was suggested that submaximal cardiac output (Q) could vary in response to changes in arterial O2 concentration (Ca,O2), so that arterial O2 delivery (Qa,O2 = Q x Ca,O2, in ml min-1) is kept constant. 2. This hypothesis was tested on eight healthy male subjects, at rest and during exercise (50, 100 and 150 W) in three conditions: normaemia (N), after 6 weeks of endurance training (T), and 2 days after subsequent autologous blood reinfusion (P). 3. Measured variables were oxygen consumption (VO2), by open circuit method, Q, by a CO2 rebreathing method, and haemoglobin concentration ([Hb]), by a photometric method. Ca,O2 was calculated as the product of [Hb], arterial O2 saturation (0.97), and the O2 binding coefficient. 4. [Hb] and thus Ca,O2 increased by 2.6% (T vs. N) and subsequently by further 5.8% (P vs. T). VO2 and Qa,O2 were linear functions of power (w), both relationships being unaffected by changes in Ca,O2. As a consequence, the linear Q vs. VO2 relationships were shifted downward as Ca,O2 increased. 5. The VO2 vs. w and the Qa,O2 vs. w relationships had the same slope. Therefore, the difference between Qa,O2 (w) and VO2 (w), equal to O2 flow in mixed venous blood (Qv,O2), was constant. 6. In conclusion, the tested hypothesis was supported by the present results. The observed constancy of Qv,O2 suggested that Qv,O2 may play a key role in regulating the cardiovascular response to exercise.

Non-invasive pulmonary blood flow measurement by means of CO2 analysis of expiratory gases

Two different methods of CO2-derived non-invasive assessment of the pulmonary blood flow were evaluated. The principle of the formula, as proposed by Gedeon et al., is based on a rapid change in arterial CO2 content and subsequent changes in endtidal PCO2 and CO2 elimination. Both methods were compared to thermodilution cardiac output in 44 postoperative patients after CABG. The first method consisted of a short period of hyperventilation followed by hypoventilation. Comparison with the thermodilution cardiac output showed a low correlation coefficient: using a measured arterial--end-tidal PCO2 difference (E) r = 0.397 was found. Entering a fixed E of 0.53 kPa resulted in r = 0.454. These disappointing figures may be explained by procedural mistakes. The second method, based on partial rebreathing by means of adding an additional dead space of 220 ml for 30-45 s, correlated very well with the thermodilution findings. Correlation coefficients of r = 0.925 (measured E) and r = 0.925 (fixed E) were found. Considering the simplicity of the method, the additional dead space approach seems to be an easy and reliable way to determine pulmonary blood flow.

A program in Quickbasic for the estimation of cardiac output

The current program in Quickbasic provides a valid and reliable computational method for the estimation of cardiac output, using the CO2 rebreathe method. In addition, this program will save time through speed of execution. Furthermore, the program can be used with IBM microcomputers as well as IBM compatible microcomputers.

Spacelab Life Sciences flight experiments: an integrated approach to the study of cardiovascular deconditioning and orthostatic hypotension

The microgravity environment of spaceflight produces rapid cardiovascular changes which are adaptive and appropriate in that setting, but are associated with significant deconditioning and orthostatic hypotension on return to Earth's gravity. The rapidity with which these space flight induced changes appear and disappear provides an ideal model for studying the underlying pathophysiological mechanisms of deconditioning and orthostatic hypotension, regardless of etiology. Since significant deconditioning is seen after flights of very short duration, muscle atrophy due to inactivity plays, at most, a small role. These changes in circulatory control associated with cephalad fluid shifts, rather than inactivity per se, are probably more important factors. In order to test this hypothesis in a systematic way, a multidisciplinary approach which defines and integrates inputs and responses from a wide variety of circulatory sub-systems is required. The cardiovascular experiments selected for Spacelab Life Sciences flights 1 and 2 provide such an approach. Both human and animal models will be utilized. Pre- and post-flight characterization of the payload crew includes determination of maximal exercise capacity (bicycle ergometry), orthostatic tolerance (lower body negative pressure), alpha and beta adrenergic sensitivity (isoproterenol and phenylephrine infusions), baroreflex sensitivity (ECG-gated, stepwise changes in carotid artery transmural pressure with a pneumatic neck collar), and responses to a 24 h period of 5 deg head-down tilt. Measurements of cardiac output (CO2 and C2H2 rebreathing), cardiac chamber dimensions (phased-array 2-dimensional echocardiography), direct central venous pressure, leg volume (Thornton sock), limb blood flow and venous compliance (occlusion plethysmography), blood and plasma volumes, renal plasma flow and glomerular filtration rates, and various hormonal levels including catecholamines and atrial natriuretic factor will also be obtained. The central venous catheter will be inserted immediately pre-launch and monitored with heart rate and blood pressure in-flight until cardiac output, respiratory gas exchange and quantitative 2D echocardiography measurements can be performed. In-flight hemodynamic measurements will be repeated at rest and during submaximal exercise daily and also during maximal exercise midway through the flight to document the timecourse and extent of cardiovascular changes in the payload crew. Parallel studies are planned for the animals. In addition to measurements of right atrial and aortic pressures and cardiac output, a dorsal micro-circulatory chamber will allow determinations of changes in capillary and venular architecture and function in six of the rats. The techniques and findings from many of the SLS-1 and 2 supporting studies have already yielded significant information about circulatory regulation in patients with both hypo- and hypertension. The flight experiments themselves will provide new data to test the validity of both animal and human models currently used for simulating the fluid shifts of a micro-gravity environment. The development of effective countermeasures, not only for short and long duration space travellers, but also for Earth-bound medical patients can then be physiologically based on experimental data rather than anecdote.

Influence of breathing frequency and tidal volume on cardiac output

The aim of our experiment was to investigate the influence of increasing either breathing frequency or tidal volume on cardiac output (Q), in normocapnia. We measured Q with a CO2 rebreathing method in 6 men and 6 women in the sitting and the supine position, imposing different breathing patterns: in one set of experiments tidal volume was kept constant at 1 L while breathing frequency was randomly changed between 20, 30 and 40 breaths/min; in another breathing frequency was kept constant at 30 breaths/min while tidal volume was randomly altered between 1, 1.5 and 2 L. Switching from open circuit breathing to rebreathing (for measurement of Q) required no change in breathing pattern. From the beginning, CO2 was added to the inspired gas to maintain end-tidal FCO2 at 0.054, so as to obtain steady state conditions throughout the measurements. Q rose significantly when tidal volume was increased (938 ml/L rise in tidal volume when sitting, and 743 ml/L when supine). Breathing frequency had an insignificant effect (213 ml/10 breaths frequency increase when sitting and 142 ml/10 breaths when supine). The greater influence of ventilation on Q when sitting than when supine is best explained by the fact that in the latter position venous return is already high. There are no demonstrable differences in this effect between males and females.

Left ventricular structure and function by echocardiography in ultraendurance athletes

To determine left ventricular (LV) structural and functional changes induced by ultraendurance exercise training, M-mode LV echograms and Doppler recordings of LV inflow velocity in 26 triathletes and 17 normal subjects were studied. All triathletes trained 20 to 40 hours/week in swimming, cycling and running for more than 2 years. Structurally, triathletes had normal LV systolic and diastolic cavity dimensions, but increased wall thickness (1.05 +/- 0.26 vs 0.80 +/- 0.27 cm in normal subjects, p less than 0.001), increased relative wall thickness, or h/R ratio (0.41 +/- 0.10 cm vs 0.33 +/- 0.11 cm in normal subjects, p less than 0.001), and increased LV mass (226 +/- 60 vs 143 +/- 54 g in normal subjects, p less than 0.001). LV mass correlated closely with mean exercise blood pressure during an 8-hour exercise test in 14 triathletes (r = 0.88). Systolic function at rest was similar in both groups, with no differences in fractional shortening or end-systolic stress. Diastolic LV function measured by digitized M-mode echo was similar in normal subjects and triathletes, with no differences in peak rates of cavity enlargement and wall thinning by echocardiogram. In contrast, the Doppler-derived ratio of early-to-late LV inflow velocities was slightly increased in triathletes (p less than 0.05). It is concluded that ultraendurance training produces a physiologic pattern of moderate pressure overload LV hypertrophy, in proportion to the hemodynamic load imposed during prolonged exercise. Unlike the abnormal hypertrophy of systemic hypertension, early diastolic function remains normal in the triathlete heart.

Non-invasive assessment of cardiac output and stroke volume in patients during exercise. Evaluation of a CO2-rebreathing method

A one-step CO2 rebreathing method for the determination of cardiac output and stroke volume (SV) has been evaluated by comparison with the direct Fick technique during recumbent exercise (10-90 W) in 13 patients. In an initial analysis, the influence of different rebreathing times and of correction for haemoglobin concentration was studied. The best correlation with the direct Fick technique was obtained with the longest analysis time, i.e. 21 s, and correction for variations in haemoglobin concentration further improved the correlation. Consequently, an analysis time of 21 s and correction for haemoglobin have been used. At low cardiac outputs, the CO2-rebreathing method overestimated the flow compared to the Fick technique. The correlation between the methods, however, was so good that a valid estimate of cardiac output could be obtained from the CO2 rebreathing method with appropriate corrections (Cardiac output, CO2 method = 2.7 + 0.77. Cardiac output, Fick; r = 0.91; Residual Standard deviation (SD res) = 0.77 l X min-1). Stroke volumes measured with the CO2 rebreathing method did not differ significantly from those obtained with the direct Fick technique, although there was a tendency to overestimate stroke volume with the CO2 rebreathing method (SV, CO2 method = 12 + 0.89 X SV, Fick; r = 0.82; SD res = 11 ml).

Noninvasive assessment of valve area in patients with aortic stenosis

A noninvasive method for quantification of aortic orifice area in patients with aortic stenosis is presented and compared with cardiac catheterization data in 24 patients (mean age 67 years). A continuous wave 2 MHz Doppler ultrasound instrument was used to measure the maximal velocity of the aortic jet, and time-averaged pressure drop was obtained by planimetry from the maximal velocity spectral recording using a simplified Bernoulli equation. Left ventricular ejection time was also measured from the spectral recording. Stroke volume was determined with a carbon dioxide-rebreathing method. Noninvasively determined aortic valve areas showed a close correlation with those determined at cardiac catheterization, but mean pressure gradients measured noninvasively were slightly but significantly higher than those measured at catheterization, leading to an underestimation of valve areas with the noninvasive technique, especially when valve areas were large. Neglect of blood flow velocity in the left ventricular outflow tract and recovery of static pressure downstream from the aortic orifice contribute to the difference in the pressure measurements. All patients with a valve area less than 1 cm2 at catheterization, however, also had an area less than 1 cm2 at the noninvasive investigation. This noninvasive approach to the evaluation of the severity of aortic stenosis seems promising for routine clinical use.

Role of changes in insulin and glucagon in glucose homeostasis in exercise

This experiment was performed to determine if plasma glucose homeostasis is maintained in normal human volunteers during light exercise (40% maximal oxygen consumption [VO2 max]) when changes in insulin and glucagon are prevented. Hormonal control was achieved by the infusion of somatostatin, insulin, and glucagon. Glucose kinetics and oxidation rates were determined with stable isotopic tracers of glucose, and by indirect calorimetry. Two different rates of replacement of insulin and glucagon were used; in one group, insulin was clamped at 19.8 +/- 2.6 microU/ml (high-insulin group), and in the other group insulin was clamped at 9.2 +/- 1.3 microU/ml (low-insulin group). Glucagon was maintained at 261 +/- 16.2 and 124 +/- 6.4 pg/ml, respectively, in the high-insulin and low-insulin groups. Without hormonal control, plasma glucose homeostasis was maintained during exercise because the increase in glucose uptake was balanced by a corresponding increase in glucose production. When changes in insulin and glucagon were prevented, plasma glucose concentration fell, particularly in the high-insulin group. Glucose uptake increased to a greater extent than when hormones were not controlled, and glucose production did not increase sufficiently to compensate. The increase in glucose uptake in the hormonal control groups was associated with an increased rate of glucose oxidation. When euglycemia was maintained by glucose infusion in the hormonal control subjects, the modest increase in glucose production that otherwise occurred was prevented. It is concluded that during light exercise there must be a reduction in insulin concentration and/or an increase in glucagon concentration if plasma glucose homeostasis is to be maintained. If such changes do not occur, hypoglycemia, and hence exhaustion, may occur.

An improved algorithm and a computer program for the analysis of capillary gas exchange

In most models of capillary gas exchange, the binding curves for O2 and CO2 are represented by simple analytical expressions, and the interactions among the haemoglobin ligands are either neglected or are assigned fixed values independent of PO2, PCO2, pH and red cell DPG. We here present algorithms and a computer program in which the binding curves are described in a near-rigorous manner. This enables solution of a set of typical equations for a unit of blood which undergoes gaseous and proton exchange. We have applied the algorithms to the problem of calculating pulmonary blood flow from the gaseous exchange in the lung by the single-breath method of Kim et al. (1966), where the CO2 binding curves of arterial and mixed venous blood are approximated by straight lines. The application of the algorithms shows that this approximation introduces significant errors in the calculated pulmonary blood flow.

Ventilation and CO2 response during +Gz acceleration

During foot-to-head acceleration (+Gz) ventilation increases despite a drop in alveolar PCO2. In order to investigate the underlying mechanisms, we measured ventilation (VE), VO2, VCO2 and PACO2, cardiac output (Q) and mixed venous CO2 concentration (CVCO2) using non-invasive techniques in 5 subjects breathing either air or a gas mixture containing 5% CO2 at +1, +2 and +3 Gz in a human centrifuge. Arterial PCO2 was calculated from Fick's equation, using CVCO2, Q and VCO2. VE increased from 8.7 to 18.0 L/min during air breathing and from 19.6 to 36.9 L/min during CO2 breathing at +1 and +3 Gz, respectively. The corresponding values for PACO2 are 37.9 vs 26.9 Torr and 47.8 vs 46.4 Torr. Q dropped from 5.9 to 4.8 L/min during air breathing and remained the same during CO2 breathing (6.7 vs 6.5 L/min). As the decrease of PaCO2 almost paralleled that of PACO2, the arterio-alveolar CO2 difference increased only slightly. The CO2 response curve shifts gradually to the left with an increase in +Gz, a fact that does not support the hypothesis that foot-to-head acceleration increases CO2 sensitivity.

Noninvasive pulmonary blood flow for optimal PEEP

Recent data in the literature suggest that effective pulmonary blood flow (Qp) measured invasively as cardiac output minus shunt flow, assumes its largest value at a PEEP level that is optimal in the sense that it also provides close to maximal oxygen delivery. Qp obtained noninvasively from simultaneous measurements of mixed expired and end-tidal CO2-concentrations is therefore proposed as a simple clinical indicator of the effects of PEEP on the cardio-pulmonary system. It is shown that modern respiratory equipment can be easily adapted to implement such a noninvasive evaluation of PEEP therapy. If the method is proven useful an automatic search for optimal PEEP becomes a future possibility.

Pulmonary blood flow determination with selective rebreathing of CO2

A new CO2-based non-invasive method for pulmonary blood flow has been developed. Selective rebreathing of CO2 was obtained in an open-circuit system by measuring the expired instantaneous CO2 flux in the expired air and by mixing pure CO2 into the inspired air. The time course of the inspired PCO2 was altered so that end-tidal PCO2 changed as a linear function of time (ramp). Pulmonary blood flow was computed as the ratio between the rate of change in net CO2 elimination (or uptake) and the concomitant rate of change of estimated arterial CO2 content. In comparison to simultaneously determined cardiac output by means of the O2 Fick method, the proposed CO2 ramp method underestimated cardiac output by some 20 per cent in 22 supine sedated patients with valvular heart disease. Differences in PCO2 between end-tidal gas and the gas in perfused alveoli are thought to be the main cause of this underestimation.

Orthostatic hypertension. Pathogenetic studies

Among 1800 referred hypertensive patients, 181 had recumbent diastolic blood pressures (DBP) below 90 mm Hg and standing DBP above 90 mm Hg. Orthostatic increments in DBP were greater in these orthostatic hypertensive patients than in 181 persistently hypertensive patients and 134 normotensive subjects. In 12 patients with orthostatic hypertension, the orthostatic fall in cardiac output (27.3 +/- 2.9%, measured by a respiratory method) was double that in 8 normotensive subjects (13.3 +/- 3.7%, p less than 0.01). An inflated pressure suit over the pelvis and lower limbs prevented the excessive fall in cardiac output and significantly reduced (p less than 0.02) the excessive rise in standing DBP in orthostatic hypertensive patients. Gravitational pooling of blood in the legs and reduction of blood in the head was measured by external gamma counting of autologous erythrocytes labeled with sodium pertechnetate Tc 99m through ports in fixed positions over the leg and the temple. Orthostatic intravascular pooling was significantly greater (p less than 0.01) in orthostatic hypertensive subjects than in normotensive subjects, and the magnitudes of orthostatic pooling and orthostatic increases in DBP were closely correlated (r = +0.85). Plasma norepinephrine concentrations were similar in recumbency and after sustained handgrip exercise, but significantly greater (p less than 0.01) after 5 to 60 mins of standing in orthostatic hypertensive subjects than in normotensive subjects. Our results indicate that orthostatic hypertension is common and that its mechanism in representative patients involves excessive orthostatic blood pooling, which results in decreased venous return, decreased cardiac output, increased sympathetic stimulation (presumably through low-pressure cardiopulmonary receptors), and excessive arteriolar, but not venular, constriction.

Non-invasive determination of effective stroke volume. Evaluation of a CO2-rebreathing method in normal subjects and patients

A CO2-rebreathing method for the determination of stroke volume (SV) was evaluated at rest by comparison with the direct Fick technique in 50 randomly selected patients with valvular heart disease. Patients with intracardiac shunts were excluded. Objective criteria for acceptance of a measurement were set to ensure reliable results. Forty-six of the 50 patients fulfilled these criteria. The rebreathing manoeuvre is, in itself, an effort for the patient, leading to a change in steady state which excludes simultaneous comparison with the direct Fick method. Day-to-day variation of the SV measured with the CO2-method was therefore assessed first, and found to be low. Because of this low day-to-day variation, a comparison of stroke volumes measured one day with the CO2-method and next day with the direct Fick technique was found to be acceptable. In the determination of SV in the supine position, there was no significant difference between the two methods (SVCO2 = 5.2 + 0.90 X SVFick, r = 0.90, SDres = 9.4 ml, n = 46), while cardiac output was significantly higher when measured with the CO2 technique than with the direct Fick method (22%, P less than 0.001). Ten of 12 patients with signs of obstructive lung disease managed to produce registrations which fulfilled the criteria of acceptance. The method is well suited for clinical use.

Quantitative description of whole blood CO2 dissociation curve and Haldane effect

A simple procedure is presented to describe accurately the whole blood CO2 dissociation curve on linear content (CCO2) and pressure (PCO2) coordinates with an exponential equation (CCO2 = K . PCO2b). A single coordinate and the hemoglobin concentration, Hb, are required. Whole blood CCO2 can be calculated from values for pH, PCO2, Hb and O2 saturation by empirically accurate equations. The mathematical description of the CO2 curve was employed to quantitate the in vivo Haldane factor (fH) from simultaneous arterial and mixed venous blood samples in 20 healthy exercising subjects. The mean +/- SE was 0.28 +/- 0.03 (vol. % delta CCO2/vol. % delta HbO2). In 20 patients with severe obstructive lung disease fH was 0.29 +/- 0.08 when calculated from arterial samples while breathing air and 100% O2. Values for fH were not related significantly to acid-base status or Hb as suggested by previous workers. By assuming these or other values for fH, the in vivo change in blood PCO2 resulting from a given change in oxygenation can be predicted.

Effect of alteration of pheripheral blood flow on the central circulation in man during supine cycling in different ambient temperatures

Whether the alteration of peripheral circulation caused by changing ambient temperature (Ta) affects central circulatory changes in man during supine cycling was investigated in four well-trained men, who exercised at two levels (117.7 or 176.6 W). Exercise metabolic rate (VO2) in cold (0 degree C or 10 degrees C) was the same as it was at 20 degrees C, whereas the cardiac output (CO; CO2 rebreathing technique) and heart rate were significantly lower (e.g.,176.6 W at 0 degree C, both p < 0.01). In heat (30 degrees C or 40 degrees C), the VO2 reduced with falling CO and mean arterial blood pressure from those at 20 degrees C (e.g., 176.6 W at 40 degrees C, all cases p < 0.01), whereas the peak post-exercise calf blood (CBFp) increased (p < 0.01). The VO2 and stroke volume (SV) were inversely proportional to the ratio of CBFp to CO/kg body weight (CBFp/CO) (r > -0.78, p < 0.001). Total peripheral resistance (TPR) was related to arteriovenous oxygen difference (A-VO2 difference) (r > 0.78, p < 0.001). The TPR and A-VO2 difference decreased as Ta rose, while CBFp/CO was almost the same. As CBFp/CO had exceeded 50 and further progressed, however, the two parameters elevated until the same level as that at 0 degree C. The present results suggest that during moderately prolonged (16--60 min) supine cycling in different Ta's the central circulatory changes are mainly affected by the altered peripheral blood flow in competing between skin and muscle for blood flow.

Cardiopulmonary readjustments during graded immersion in water at 35 degrees C

Six normal male volunteers, aged 25 to 34, suspended vertically in a harness that allowed them to completely relax their postural muscles, were studied in four randomly ordered conditions, namely in air at 28 degrees C, and immersed in water at 35 degrees C to the level of the hips, the xiphoid, or the chin. In each situation, several variables were measured by noninvasive techniques. Cardiac output rose from 5.11 min-1 (air) to 8.31-min-1 (chin), the increase in each of the three steps being significant at the 0.001 level. Heart rate dropped from 76 to 68 min-1 (P less than 0.001) from air to xiphoid immersion, but appeared to rise again (P less than 0.02) during chest immersion. Functional residual capacity decreased marginally during lower limb submergence, and considerably in each of the following stages. Pulmonary capillary blood volume rose significantly only during abdomen immersion. The arterial-endtidal PCO2 difference was minimally reduced as water reached hip level and then remained steady. Mixed venous PO2 increased during abdomen submergence, and PVCO2, was unaltered throughout. Analysis of the step-to-step changes demonstrates that some variables are set by a combination of processes which may counteract each other, and explains the difference between results obtained by previous investigators.