Peter M.J.M. de Vries

Monitoring of recruitment and derecruitment by electrical impedance tomography in a model of acute lung injury

ObjectiveTo evaluate a noninvasive system for obtaining information about alveolar recruitment and derecruitment in a model of acute lung injury. DesignProspective experimental study. SettingAnimal research laboratory. SubjectsNine anesthetized pigs. InterventionsElectrical impedance tomography measurements were performed. Electrical impedance tomography is an imaging technique that can register the ventilation-induced impedance changes in different parts of the lung. In nine anesthetized pigs, repeated lung lavages were performed until a Pao2 of <80 mm Hg was reached. Thereafter, the lungs were recruited according to two different recruitment protocols: the open lung approach and the open lung concept. Five time points for measurements were chosen: healthy (reference), lavage (atelectasis), recruitment, derecruitment, and maintain recruited (final). Measurements and Main ResultsAfter lavage, there was a significant increase in the impedance ratio, defined as the ventilation-induced impedance changes of the anterior part of the lung divided by that of the posterior part (from 1.75 ± 0.63 to 4.51 ± 2.22;p < .05). The impedance ratio decreased significantly after performing the recruitment protocol (from 4.51 ± 2.22 to 1.18 ± 0.51). During both recruitment procedures, a steep increase in baseline impedance change was seen. Furthermore, during derecruitment, a decrease in the slope in baseline impedance change was seen in the posterior part of the lung, whereas the anterior part showed no change. ConclusionElectrical impedance tomography is a technique that can show impedance changes resembling recruitment and derecruitment of alveoli in the anterior and posterior parts of the lung. Therefore, electrical impedance tomography may help in determining the optimal mechanical ventilation in a patient with acute lung injury.

PULMONARY PERFUSION MEASURED BY MEANS OF ELECTRICAL IMPEDANCE TOMOGRAPHY

Electrical impedance tomography (EIT) is a recent imaging technique based on electrical impedance, offering the possibility of measuring pulmonary perfusion. In the present study the influence of several pulmonary haemodynamical parameters on the EIT signal were investigated. First, the influence on the systolic wave of the EIT signal (delta Zsys) of stroke volume, large pulmonary artery distensibility (both assessed by means of MRI) and the extent of the pulmonary peripheral vascular bed in 11 emphysematous patients (reduced peripheral vascular bed) and 9 controls (normal peripheral vascular bed) was investigated. Second, the influence of hypoxic pulmonary vasoconstriction on delta Zsys was examined in 14 healthy subjects. Finally, the origin of the diastolic wave was examined in three patients with atrioventricular dissociation. Multiple regression analysis showed that delta Zsys was only dependent on the variable emphysema (p < 0.02), but not dependent on stroke volume (p < 0.3) or pulmonary artery distensibility (p > 0.9). The mean value of delta Zsys for emphysematous patients (131 +/- 32 arbitrary units (AU)) was significantly lower (p < 0.001) than in the control group (200 +/- 39). In the group of healthy subjects delta Zsys decreased significantly (p < 0.001) during hypoxia (193 +/- 38 AU) compared with rest measurements (260 +/- 62 AU). The absence of the diastolic wave in the cardiological patients suggests the influence of reverse venous blood flow on the EIT signal. It is concluded that volume changes in the small pulmonary vessels contribute significantly to the EIT signal. Moreover, the hypoxia induced decrease in delta Zsys indicates the potential of EIT for measuring pulmonary vascular responses to external stimuli.

Determination of stroke volume by means of electrical impedance tomography

ECG-gated electrical impedance tomography (EIT) is a non-invasive imaging technique, developed to monitor blood volume changes. This study is the first in comparing this non-invasive technique in measuring stroke volume with established techniques. The objective of this study was to validate EIT variables derived from the EIT images with paired obtained stroke volume measurements by thermodilution and MRI. After right cardiac catheterization, EIT measurements were performed in 25 patients. Regression analysis was used to analyse the relation between the EIT results and stroke volume determined by thermodilution. From the regression line an equation was derived to estimate stroke volume (in ml) by EIT. A strong correlation was found between EIT and stroke volume measured by the thermodilution method (r = 0.86). In a group of 11 healthy subjects this equation was validated to MRI. The mean and standard deviation of the difference between EIT and MRI was 0.7 ml and 5.4 ml respectively. These data indicate that EIT is a valid and reproducible method for the assessment of stroke volume.

Determinants of pulmonary perfusion measured by electrical impedance tomography

Electrical impedance tomography (EIT) is a non-invasive imaging technique for detecting blood volume changes that can visualize pulmonary perfusion. The two studies reported here tested the hypothesis that the size of the pulmonary microvascular bed, rather than stroke volume (SV), determines the EIT signal. In the first study, the impedance changes relating to the maximal pulmonary pulsatile blood volume during systole (ΔZsys) were measured in ten healthy subjects, ten patients diagnosed with chronic obstructive pulmonary disease, who were considered to have a reduced pulmonary vascular bed, and ten heart failure patients with an assumed low cardiac output but with a normal lung parenchyma. Mean ΔZsys (SD) in these groups was 261 (34)×10−5, 196 (39)×10−5 (P<0.001) and 233 (61)×10−5 arbitrary units (AU) (P=NS), respectively. In the second study, including seven healthy volunteers, ΔZsys was measured at rest and during exercise on a recumbent bicycle while SV was measured by means of magnetic resonance imaging. The ΔZsys at rest was 352 (53)×10−5 and 345 (112)×10−5 AU during exercise (P=NS), whereas SV increased from 83 (21) to 105 (34) ml (P<0.05). The EIT signal likely reflects the size of the pulmonary microvascular bed, since neither a low cardiac output nor a change in SV of the heart appear to influence EIT.

Prediction of pulmonary capillary wedge pressure and assessment of stroke volume by noninvasive impedance cardiography

Early recognition of heart failure is important because early treatment reduces mortality and hospitalization rates. In screening for this disease, there is a need for a simple, safe, and cost-effective method to obtain cardiovascular variables. Therefore we developed a noninvasive impedance cardiographic method to predict the pulmonary capillary wedge pressure (PCWP) from the impedance cardiogram. The impedance cardiographic technique, though, was originally designed for stroke volume (SV) determination. The objectives of this study were to validate both variables by comparison with the paired, invasively obtained equivalents. PCWP, measured with a pulmonary artery catheter, was related to the O/C ratio from the impedance cardiogram. The O/C ratio was calculated as the amplitude of the impedance cardiogram during diastole (O) divided by the maximum height during systole (C). Stroke volume was also calculated from the impedance cardiogram according to the equation of Kubicek (SVIC) and compared with thermodilution (SVTD). Data analysis was performed in 24 stable patients who underwent diagnostic heart catheterization. Linear regression analysis showed that the O/C ratio was strongly related to the invasively measured PCWP over a range of 3 to 30 mm Hg (r = 0.92, standard error of the estimate, 3.2 mm Hg). Between SVIC and SVTD a moderate correlation was established (r = 0.69), but after exclusion of the data from patients with an aortic valve disorder (n = 5), the correlation increased considerably (r = 0.87). No significant differences between SVIC and SVTD were found (mean difference +/- 2 SD = 1.8 +/- 28.8 ml). These preliminary observations suggest that impedance cardiography can predict PCWP and measure SV over a wide range of clinically relevant values. The combined measurement of SV and PCWP by impedance cardiography might be a clinical useful tool in screening for heart failure.

Assessment of the pulmonary volume pulse in idiopathic pulmonary arterial hypertension by means of electrical impedance tomography.

Background: Electrical impedance tomography (EIT) is a non-invasive imaging technique which can be used to measure the blood volume changes in the pulmonary vascular bed during the cardiac cycle. Study Objectives: This study was performed to evaluate the differences in the EIT signal of the pulmonary vascular bed between healthy subjects and patients with idiopathic pulmonary arterial hypertension (IPAH), who are known to have a remodelled pulmonary vascular bed. Patients and Methods: Twenty-one patients (17 females, 4 males) with IPAH and 30 healthy controls (5 females, 25 males) were measured. EIT measurements were performed in duplicate, on the same day as right heart catheterization to obtain haemodynamic data. The maximal impedance change during systole (Δ Zsys) was used as a measure of the pulmonary volume pulse and expressed in arbitrary units (AU). Total lung capacity, spirometric values and diffusion capacity for carbon monoxide were measured as well. Results: Mean Δ Zsys was 215 ± 58 × 10–2 AU (95% CI 193 × 10–2 to 236 × 10–2) in the healthy subjects and 78 ± 27 × 10–2 AU (95% CI 66 × 10–2 to 91 × 10–2) in the IPAH patient group (p < 0.0001). No significant correlation was found between Δ Zsys and any of the haemodynamic or lung function data. Conclusion: The impedance pulsation of the pulmonary vascular bed is reduced in IPAH in comparison with controls, indicating a reduced volume pulse. This might represent the reduced cross section area, as well as the reduced compliance and number of the pulmonary vessels in these patients.

The intra- and interobserver variability of impedance cardiography in patients at rest and during exercise

We studied the intra- and interobserver variability in the calculation of stroke volume by the impedance technique, using the recently proposed refinements in the electrode configuration and signal processing. Three groups of patients were included in this study: ten stable cardiac patients who underwent a diagnostic heart catheterization, ten patients 24-26 h after coronary artery bypass surgery and ten patients with severe chronic obstructive pulmonary disease (COPD). The first two groups were studied at rest and the COPD group during submaximal exercise. The intra-observer variability was 4.2%, 3.9-4.0% and 6.0-6.9% for the catheterized, surgical and COPD groups, respectively. The interobserver variability was 4.3%, 2.6% and 2.4%, respectively. It is concluded that highly reproducible data can be obtained with the newly proposed impedance technique in patients at rest and exercise which may be comparable or superior to other techniques used in clinical settings.

The adjustment of post dialytic dry weight based on non-invasive measurement of extracellular fluid and blood volumes.

One of the major problems in the clinical practice of hemodialysis is an incorrect estimation of post dialytic (PD) dry weight. Underestimation of dry weight leads to hypovolemia induced hypotension, and overestimation to hypertension, pulmonary edema, and left ventricular hypertrophy. Because of the insensitivity of clinical variables to estimate dry weight, a more accurate technique is warranted. For this purpose and for the continuous surveillance of changes in blood volume (BV) during hemodialysis, two non-invasive techniques were applied. Based on post dialytically obtained extracellular fluid volume (EFV) values, measured by means of a conductivity method, 30 stable hemodialysis patients were divided into three groups for further analysis: de- (n = 9), normo- (n = 15), and overhydrated (n = 6). Using an on-line optical reflection method, changes in BV were measured continuously during therapy. Mean BV decrease, corrected for UF, differed slightly between the three groups (0 = 1.84 +/- 2.06, N = 3.20 +/- 1.80, D = 4.20 +/- 1.60 %/L). However, eight hypotensive episodes occurred in group D versus none in groups N and O. These hypotensive episodes were characterized by a greater reduction of BV--corrected for ultrafiltration--from the start of treatment until the moment of hypotension (6.96 +/- 2.21 %/L), compared with the 22 non hypotensive controls (2.16 +/- 2.01 %/L, p < 0.001). Based on the PD EFV dry weight of the overhydrated and dehydrated patients was decreased and increased, respectively, by 500 g after each session, until PD EFV was within normal bounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Doxazosin in the treatment of patients with mild or moderate hypertension and mild or moderate renal insufficiency.

The antihypertensive efficacy and safety of doxazosin, a selective alpha 1-inhibitor, were assessed in 23 hypertensive patients with renal insufficiency. The study involved three phases: (1) a 2-week baseline period, (2) a 10-week period during which patients received doxazosin, 1 to 16 mg, once daily, and (3) a 4-week maintenance period. After 14 weeks of active treatment, systolic/diastolic blood pressures of efficacy evaluable patients were reduced by 8.9/9.2 and 4.6/9.1 mm Hg to final values of 153/90 and 149/91 mm Hg in the supine and standing positions, respectively. The mean dose of the efficacy evaluable patients was 9.8 mg/day. Eleven patients experienced one or more side effects, most of which were mild or moderate and disappeared or were tolerated with continued therapy. No clinically significant laboratory changes were apparent, and no trends were observed with regard to organ systems or correlations with dose or duration of treatment. There were no significant differences in serum creatinine levels between baseline and final visits. The overall lipid profile indicated a decrease in total cholesterol with increases in high-density lipoprotein cholesterol and the high-density lipoprotein/total cholesterol ratio. From baseline to final visit there was a highly significant reduction of 19% (p less than 0.05) in calculated risk scores for coronary heart disease on the basis of the Framingham equation.

Noninvasive measurement of cardiac output : Two methods compared in patients with mitral regurgitation

In search for the origin of the less reliable cardiac output (CO) estimations by means of electrical impedance cardiography (EIC), the authors hypothesized that cardiac valve pathology might be one of the reasons. Twenty-six patients were examined by means of echo Doppler (ED) and EIC. The cardiac valve status was obtained by means of echocar diography and color Doppler flow, while CO was obtained by means of both methods. Seventeen patients had no valve pathology (nVP) while nine patients had mild to moderate mitral regurgitation (MVR). The overall correlation between the calculation of CO by means of the two methods was good (r = 0.85, p < 0.001, mean difference and standard deviation: 0.20 ± 0.74 L/min), while there was no significant difference between the paired values. After division into an nVP and an MVR population, the results showed an even closer agreement between the CO values in the nVP population (r = 0.88, p < 0.001, mean difference and standard deviation: 0.15 ± 0.68 L/min). Further more, significant differences were found in the first derivative of the impedance (dZ/dt) signals of these groups. Although the agreement between ED and EIC was slightly lower in the MVR population, EIC reliably estimated CO, even in case of MVR. The impedance signal itself gave an indication for the existence of MVR.