Assessing effects of PEEP and global expiratory lung volume on regional electrical impedance tomography.

OBJECTIVE: This study was performed to assess the value of electrical impedance tomography (EIT) as an indicator of tidal (V(T)) and end expiratory lung volume (EELV). METHODS: EIT measurements were performed in seven healthy piglets during constant tidal volume ventilation at incremental and decremental positive end-expiratory pressure (PEEP) levels. Tidal impedance changes were calibrated to volume using V(T) calculated from flow at the airway opening. Simultaneously, calibrated respiratory inductive plethysmography was used to measure EELV changes, and used as a reference standard. RESULTS: EIT systematically underestimated both V(T) and EELV changes when EELV deviated from the level at which it was calibrated. Calculated over the entire pressure-volume curve, EIT systematically underestimated V(T) by 28 ml, with a precision from -16 to 72 ml. EELV was systemically underestimated by 406 ml, with a precision of -38 to 849 ml. Nonlinear recruitment in the ventral regions of the lungs was the main cause of this underestimation. CONCLUSIONS: Tidal and end-expiratory changes in pulmonary impedance reflect corresponding changes in lung volume, but the increasing underestimation with increasing lung volume should be taken into account in the analysis of EIT data.

Respiratory inductive plethysmography accuracy at varying PEEP levels and degrees of acute lung injury.

BACKGROUND AND OBJECTIVE: This study was performed to assess the accuracy of respiratory inductive plethysmographic (RIP) estimated lung volume changes at varying positive end-expiratory pressures (PEEP) during different degrees of acute respiratory failure. METHODS: Measurements of inspiratory tidal volume were validated in eight piglets during constant volume ventilation at incremental and decremental PEEP levels and with increasing severity of pulmonary injury. RIP accuracy was assessed with calibration from the healthy state, from the disease state as the measurement error was assessed, and at various PEEP levels. RESULTS: Best results (bias 3%, precision 7%) were obtained in healthy animals. RIP accuracy decreased with progressing degrees of acute respiratory failure and was PEEP dependent, unless RIP was calibrated again. When calibration was performed in the disease state as the measurement error was assessed, bias was reduced but precision did not improve (bias -2%, precision 9%). CONCLUSIONS: RIP accuracy is within the accuracy range found in monitoring devices currently in clinical use. Most reliable results with RIP are obtained when measurements are preceded by calibration in pulmonary conditions that are comparable to the measurement period. When RIP calibration is not possible, fixed weighting of the RIP signals with species and subject size adequate factors is an alternative. Measurement errors should be taken into account with interpretation of small volume changes.

Endoluminal aortic shunting for distal perfusion during thoracic aortal cross-clamping in a pig model.

PURPOSE OF THE STUDY: To investigate the haemodynamic properties of a direct endovascular aortic shunt to maintain distal aortic perfusion as an alternative of a distal shunt (left-left-, Gott shunt) in thoracic aortic aneurysm repair. METHODS: A shunt was developed and tested in an in vitro model which should be capable of transporting a flow of 3-4 L/min with a decrease in blood pressure < 20 mmHg. Thereupon the shunt was tested in an in vivo experiment in six pigs to assess the possibility of its use with normal distal blood pressure. The shunt was inserted in the thoracic aorta and stayed in place for 1.5 h. Parameters were measured at six time intervals to assess organ perfusion, -function, cardiac output, proximal- and distal blood pressure and aortic- and shunt flow. PRINCIPLE FINDINGS: The mean blood flow through the shunt was 2.5 L/min. The difference of the mean blood pressure over the shunt was on average 14.20 mmHg. Parameters for coagulation disturbance and organ ischaemia were tested. The decrease in mean thrombocyte count was 299-158 (p<0.02). The venous lactate and the venous mesenteric lactate as parameters for intestinal ischemia did not increase significantly. No significant changes occurred in angiotensin II levels. Pulsatile flow was maintained but significantly suppressed (60%) distal from the shunt. The clamp time needed to insert the shunt and the venous mesenteric lactate, as well as the venous lactate, showed high correlation, r(s) = 0.9 (p<0.05) and r(s) = 0.94 (p<0.01). This also accounted for the 2nd clamp time, both r(s) = 0.95 (p<0.05). CONCLUSION: The shunt is capable of transporting a blood flow of 2-4 L/min with an acceptable decrease in distal blood pressure. However, the time, needed to insert the shunt, was significantly associated with parameters of organ ischaemia.

A system for integrated measurement of ventilator settings, lung volume change and blood gases during high-frequency oscillatory ventilation.

To describe and validate a system for integrated measurement of ventilator settings and dependent physiological variables during high-frequency oscillatory ventilation (HFOV). A custom interface was built for data acquisition. Lung volume change was determined by respirator inductive plethysmography (RIP), modified to sampling rates of 140 Hz. Blood gas analysis was obtained using a continuous intra-arterial blood gas monitoring system. FIO2 was measured by means of an electrochemical sensor. Pressure at the airway opening and trachea (microtip transducer) were sampled. The data acquired were sent to a laptop computer for analysis, display and storage. The system was tested during a lung recruitment procedure in an animal model of respiratory distress. Linearity of the RIP was checked by gas volume injection using a supersyringe. The system operated successfully. Agreement between RIP-measured volume with injected volume was excellent; bias was 5 ml; limits of agreement were 1-9 ml. Graphs were obtained, showing the relationship between imposed mean airway pressure and lung volume change, and oxygenation. The integration of ventilator settings and dependent physiological variables may provide useful information for clinical, instructional and research application.

Reduction of oscillatory pressure along the endotracheal tube is indicative for maximal respiratory compliance during high-frequency oscillatory ventilation: a mathematical model study.

We hypothesized that during high-frequency oscillatory ventilation (HFOV), a reduction of peak-to-peak oscillatory pressure along the endotracheal tube is maximal when respiratory system compliance is maximal. We made a mathematical model of the endotracheal tube and the respiratory system of a neonate suffering from idiopathic respiratory distress syndrome (IRDS). The model consisted of linear viscous and inertive elements, a non-linear endotracheal tube resistance, and a non-linear compliance allowing for alveolar recruitment and overdistention. Respiratory compliance was maximal at the transition between maximal recruitment and minimal overdistention. A new variable, the oscillatory pressure ratio (OPR), was defined as the ratio between peak-to-peak oscillatory pressures at the distal end and the proximal opening of the endotracheal tube, respectively. The respiratory variables of four patients were fed into the model, and the relationship between respiratory system compliance and OPR was determined. OPR decreased as compliance increased, except for very low compliances below where 0.08 mL. cm H2O(-1), and OPR increased with increasing compliance. The relationship between mean airway pressure P(aw) and OPR revealed that the minimal OPR (range, 0.37-0.78) and maximal respiratory compliance coincided at the same P(aw). However, the relationship did depend on oscillation frequency, applied oscillatory pressure, and endotracheal tube resistance, parameters that may change during clinical application of HFOV. When 81 permutations of nominal and extreme respiratory variables were used in the model, the minimum OPR (0.60 +/- 0.23) and maximum compliance coincided in all cases. These model experiments support our hypothesis. The results indicate that the OPR may be a useful index to optimize lung expansion, where lung recruitment is maximal and overdistention minimal. In vivo tests will be needed to reveal the feasibility and reliability of such an index for biomedical and clinical application.

The visceral perfusion system and distal bypass during thoracoabdominal aneurysm surgery: an alternative for physiological blood flow?

There are potential benefits to addition of visceral organ perfusion, by means of a 9-Fr. catheter system (octopus), to distal aortic perfusion during thoracoabdominal aneurysm surgery. However, in the literature there are reports of adverse effects. The authors therefore compared two groups of patients who underwent thoracoabdominal aneurysm surgery with and without visceral organ perfusion. In the group in which the visceral perfusion was applied, the use of platelets (26 versus 11 units; P < 0.05), fresh frozen plasma (3.4 versus 1.5 units; P < 0.05) and packed cells (20 versus 8 units, P < 0.05) was significantly increased. An equal number of patients in both groups developed renal failure postoperatively. An explanation for this adverse effect can be found in the high shear rates in the catheters used, mainly as a result of the small diameter. High shear rates cause haemolysis. Also, the flow through the catheters is insufficient to maintain adequate perfusion of the visceral organs. A higher flow in these catheters would result in an even higher shear rate. It is therefore concluded that coagulopathy and insufficient bloodflow is caused by the small internal diameter of the catheters, which renders the device insufficient.

[Atrioventricular conduction time in premature infants is about half of that in adults]. / Atrioventriculaire geleidingstijd bij te vroeg geborenen ongeveer de helft van die bij volwassenen.

OBJECTIVE: To determine the atrioventricular (AV) conduction time in prematurely born infants as part of a comparative electrocardiological study of conduction times versus heart size. DESIGN: Recording and analysis of electrocardiograms. SETTING: Department of Neonatology, University Hospital of the Free University of Amsterdam, the Netherlands. METHODS: Using bipolar precordial leads of standard monitoring equipment in 28 babies, born at a gestational age of 26-36 weeks, ECGs were recorded as soon after birth as possible. The ECGs were analysed and relevant conduction times, such as PR intervals and QRS durations, were measured by hand. These data were related to the birth weights of the infants. (The heart weight amounts to approximately 0.6% of body weight.) RESULTS: Average birth weight of the babies was 1374 g (SD: 491), average PR interval 93 ms (9), QRS duration 40 ms (4), and average heart rate 148/min (14). CONCLUSION: Human hearts weighing 6-10 g have conduction times half that of the adult human heart which weighs 50 times as much. The contribution of the AV node to the total AV conduction time increases with diminishing heart size.

Bench test assessment of dosage accuracy and measurement inaccuracy in nitric oxide inhalational therapy during high frequency oscillatory ventilation.

OBJECTIVE: The objective of this study is to determine the accuracy and precision of chemiluminescence and electrochemical nitric oxide (NO) measurements and accuracy of NO dosage with electronic mass flow controllers (MFC) versus rotameters during NO inhalational therapy. METHODS: NO flow was delivered to a high frequency oscillator and mixed with ventilator flow. NO and NO2 concentrations were measured simultaneously with a standard chemiluminescence analyzer and a modified electrochemical analyzer. Dosage accuracy was assessed with gas flows adjusted with either MFC's or rotameters. Accuracy of both analyzers was validated with both NO and ventilator flow regulated with a MFC. RESULTS: In dry air, without pulsatile pressure, MFC controlled NO and ventilator flow resulted in an accuracy expressed as the ratio of calculated concentration to measured concentration (RCM) of 0.995 (CI: 0.983-0.988) when measured with chemiluminescence. When the ventilator rotameter was used instead of a MFC, RCM was 0.856 (CI: 0.835-0.877). With a rotameter for both NO and ventilator flow, RCM increased to 1.175 (CI: 0.793-1.740) with an increase of confidence interval limits. Chemiluminescence was sensitive to humidification of the ventilatory gases (p < 0.05), slightly sensitive to the addition of oxygen and to pulsatile pressure (not significant). RCM obtained with the modified electrochemical analyzer was in close agreement with chemiluminescence RCM, although 95% CI were wider with electrochemical analysis. CONCLUSIONS: During high frequency oscillatory ventilation (HFOV), standard rotameter flow control of both NO and ventilator flow results in unpredictable NO concentrations that would be clinically unacceptable. When one MFC was used for NO flow control, with ventilator flow controlled with a rotameter, this resulted in moderate dosage accuracy. To achieve a still higher accuracy, MFC flow control for both NO and ventilator flow is indicated. During HFOV, standard chemiluminescence analyzers cannot be considered to be the gold standard for determination of the NO concentration delivered. Measurement of NO concentration may not be mandatory for determination of inhaled NO dose during HFOV, but may be used to monitor for unsafe or unwanted events.

Bisferiens peaks in the radial artery pressure wave during patent ductus arteriosus in newborn infants: relationship with ascending aortic flow.

Previously, we found evidence that bisferiens peaks in the radial artery pressure wave in the newborn infant may suggest the presence of a left-to-right shunt through a patent ductus arteriosus (PDA). The purpose of the present study was to analyze the origin of this pulsus bisferiens. Starting from the assumption that the radial artery pressure wave form is similar to the aortic pressure wave form, as described previously, we attempted to explain the bisferiens peaks on the basis of echocardiographically obtained ascending aortic flow. We studied 11 preterm mechanically ventilated infants with a left-to-right shunt through a PDA and 7 without. Aortic volume flow was established echocardiographically, and radial artery blood pressure measurement was performed with a high fidelity cathetermanometer system. Ascending aortic peak flow during PDA was significantly higher in the case of PDA, compared with the case without PDA. An augmented peak flow with an abrupt decline after the high peak in PDA, resulting in a sharp pressure peak with a steep decline after the peak, was thought to explain the first sharp peak of pulsus bisferiens. An abrupt decline of flow after peak flow is thought to be due to the fast runoff of blood through the ductus. According to the pulsatile pressure dynamics theories, which state that pressure wave forms consist of forward and backward wave forms, the second peak of the pulsus bisferiens can be explained by the return of the reflected (backward) wave form when the forward wave form has already considerably decreased. We conclude that the bisferiens peaks found in PDA result from a combination of large stroke volume (augmented first peak) and large runoff (quick decline of the forward wave) before the return of the reflected wave.

Accuracy of oscillometric blood pressure measurement in critically ill neonates with reference to the arterial pressure wave shape.

OBJECTIVE: To perform further evaluation of the oscillometric device for neonatal arterial blood pressure (ABP) measurement, using a catheter-manometer system (CMS) for accurate intraarterial measurement. We aimed to describe the influence of the radial artery wave shape on oscillometric ABP determination, as pressure wave-shape influences the relationships between systolic arterial pressure (SAP), diastolic arterial pressure (DAP) and mean arterial pressure (MAP) in the wave. These relationships are part of the algorithms contributing to the final ABP determination in the oscillometric device. DESIGN: Intra-patient comparison of two blood pressure measurement systems. SETTING: Neonatal intensive care unit. PATIENTS: In 51 critically ill newborn infants, ABP was determined oscillometrically in the brachial artery and, simultaneously, invasively in the radial artery using a high-fidelity CMS. Clinical data of the infants were: gestational age: 29 (25-41) weeks; birthweight: 1200 (500-3675) g, postnatal age: 6 (2-46) h. METHODS: Statistical analysis was performed with the paired Student's t-test. Multiple regression analysis was used to determine the influence of birthweight and height of the blood pressure on the results. MEASUREMENTS AND MAIN RESULTS: In 51 infants, 255 paired values of SAP, DAP and MAP were recorded. In all recordings, we determined the relationship between SAP, DAP and MAP, using the equation: MAP = alpha%(SAP - DAP) + DAP. For SAP, DAP, MAP and alpha, we computed mean differences (bias) and the limits of agreement (precision). Biases for SAP, DAP, MAP and alpha were significantly different from zero (P < 0.001) and the limits of agreement for SAP, DAP and MAP were wide: 18.8 mmHg, 17.2 mmHg and 15.2 mmHg respectively. The relationship between invasive and noninvasive values is only partly (7-19%) influenced by the height of the blood pressure; low values of SAP, DAP and MAP tend to give overestimated oscillometric values. In the relationship between SAP, DAP and MAP, alpha was found to be 47% invasively (as generally found in the radial artery in newborns) and 34% noninvasively (as generally found in the brachial/radial artery in adults). CONCLUSIONS: Inaccuracy of the oscillometric device may be partly explained by the incorporation of an inappropriately fixed algorithm for final ABP determination in newborns. Care should be taken when interpreting the oscillometrically derived values in critically ill newborn infants.

Bisferiens peaks in the radial artery pressure wave in newborn infants: a sign of patent ductus arteriosus.

Previously, we found evidence that radial artery pressure wave forms in newborns represent central aortic wave forms, provided that pressure is measured with adequate accuracy. Therefore, we postulated that the neonatal radial artery wave form, like the adult aortic wave form, may contribute to cardiovascular diagnosis. We investigated whether radial artery wave forms in infants suffering from patent ductus arteriosus (PDA) are different from the wave forms as seen without the presence of PDA. We studied 34 newborn infants with a radial artery line and with the possible clinical diagnosis of PDA with left-to-right shunt. On the basis of echocardiographic examination to assess PDA, these infants were divided in two groups: infants with PDA (n = 24) and without PDA (n = 10). In 15 out of 24 infants with PDA, recordings were repeated after ductal closure. Blood pressure measurement was performed with a high fidelity cathetermanometer system using a tip-transducer (natural frequency 95 Hz, damping coefficient 0.15). Contour analysis was performed by describing morphology of the waves during PDA and without PDA. In 23 out of 24 infants with PDA, a pulsus bisferiens was present: two peaks separated by a deep cleft. The average pressure difference between the first pressure peak and the cleft [delta Ppeak1] was 0.35 +/- 0.19 kPa, and the average difference between the cleft and the second pressure peak [delta Ppeak2] was 0.44 +/- 0.23 kPa. the ratio of mean magnitude of delta Ppeak1 and delta Ppeak2 was 0.81 +/- 0.26. None of the 10 infants without PDA showed pulsus bisferiens.(ABSTRACT TRUNCATED AT 250 WORDS)

Calculated mean arterial pressure in the posterior tibial and radial artery pressure wave in newborn infants.

Mean arterial pressure (MAP) is the area under the pressure wave averaged over the cardiac cycle, and therefore depends on pressure wave contour. A generally used rule of thumb to estimate MAP of peripheral arteries in adults is adding one-third of the arterial pulse pressure (PP) to diastolic arterial pressure (DAP). As peripheral pressure wave forms in neonates do not resemble adult peripheral wave forms, it may be expected that this rule of thumb does not hold for neonates. Previously, we found that MAP can be calculated by adding 50% PP to DAP in radial artery waves in neonates. In the present study, we investigated in neonates how MAP in the posterior tibial artery depends on systolic and diastolic pressure and we compared these findings to those found in the radial artery. Forty infants admitted for intensive care were studied. We analyzed 5000 invasively and accurately obtained blood pressure waves in the posterior tibial artery of 20 neonates and another 5000 waves similarly obtained from the radial artery in another group of 20 neonates. We found that MAP in posterior tibial artery waves is well approximated by adding 41.5 +/- 2.0% of PP to DAP, whereas MAP in radial artery waves can be calculated by adding 46.7 +/- 1.7% of PP to DAP. These values are significantly different (p < 0.0001). In conclusion, the rule of thumb as used in the adult to find MAP, where 33% PP is added to DAP, does not hold for the newborn. We recommend to calculate MAP in the tibial artery by adding 40% of PP to DAP and in the radial artery by adding 50% of PP to DAP.

Intra-arterial pressure measurement in neonates: dynamic response requirements.

A computer simulation of a catheter manometer system was used to quantify measurement errors in neonatal blood pressure parameters. Accurate intra-arterial pressure recordings of 21 critically ill newborns were fed into this simulated system. The dynamic characteristics, natural frequency and damping coefficient, were varied from 2.5 to 60 Hz and from 0.1 to 1.4, respectively. As a result, errors in systolic, diastolic and pulse arterial pressure were obtained as a function of natural frequency and damping coefficient. Iso-error curves for 2%, 5% and 10% were constructed. Using these curves, the maximum inaccuracy of any neonatal catheter manometer system can be determined and used in the clinical setting.

Radial artery blood pressure measurement in neonates: an accurate and convenient technique in clinical practice.

To achieve accurate blood pressure measurement through radial artery catheters in infants, we previously developed an experimental high-fidelity catheter-manometer system (CMS). As this system lacks facilities for flushing and for blood sampling, we aimed to further develop this technique in order to make the system suitable for clinical practice. In addition, we aimed to develop methods to automate processing of the pressure wave forms. The high-fidelity system to be improved consisted of a 24 Gauge catheter, a threeway stopcock and a tip-manometer. We inserted this system in the catheter-manometer system as routinely used i.e. the remaining end of the stopcock was connected to the fluid-filled CMS as used routinely. This combined system became clinically applicable, since blood samples could be obtained and flushing could be performed. The measurement chain was completed by application of a modified physiological monitor and a computerized method to analyze pressure wave forms. In this manner accurate beat-to-beat pressure parameters were obtained. This technique was applied to 25 neonates admitted for intensive care and requiring arterial access. Gestational age of these infants ranged from 25-40 (median 29) weeks and birth weight ranges from 500-3375 (median 1060) grams. In all infants the technique was found to be convenient and the high-fidelity blood pressure measurements were performed without any problems. The advantage of the present system is the potential for both correct intermittent recordings of arterial wave forms in close relation to clinical condition and for the establishment of accurate radial artery beat-to-beat pressure values in clinical practice.

Prevention of air introduction in catheter-manometer systems for accurate neonatal blood pressure measurement: an in vitro study.

OBJECTIVE: Our objective was to find an optimum filling technique to prevent air entrapment in catheter-transducer systems. Ultimately, this may help achieve more accurate neonatal blood pressure measurement. METHODS: We first assembled a catheter-transducer system with a minimum of components fulfilling clinical requirements in neonatology. Then, we tested in vitro different filling techniques: flushing with CO2, flushing with alcohol, use of degassed filling liquid, and a combination of all three methods. After the filling procedure, dynamic response was determined by applying sinusoidal pressures. We calculated natural frequency (fn), damping coefficient (D), and the maximum frequency (fmax) up to which the amplitude response is uniform (+/- 10%). RESULTS: With the system filled in the usual clinical way, fmax was 27 Hz (fn = 94 Hz; D = 0.13). With application of the three methods separately, fmax increased to 34 to 39 Hz. With all methods combined, fmax increased to 51 Hz (fn = 182 Hz; D = 0.14). These techniques were not always successful. CONCLUSION: A clinical system can be assembled to fulfill the dynamic requirements for neonatal use. Dynamic response can be improved by special filling techniques. We fell that an in vivo quality test needs to be developed and evaluated in neonates to ensure accurate blood pressure measurements.

Carbon-fiber electrodes and leads for electrocardiography during MR imaging.

In an effort to minimize distortion and artifacts on magnetic resonance images obtained with electrocardiographic (ECG) gating, the authors tested the use of ECG electrodes and leads made of carbon fiber. These materials caused no image degradation and, because the leads were reinforced with plastic, were less vulnerable to bending than leads made of graphite.

Computer-assisted capnogram analysis.

Characteristic abnormal carbon dioxide waveforms from patients with mechanically ventilated lungs are observed when, for example, valves are incompetent, the airway is obstructed, the breathing circuit becomes disconnected, or a patient overrides mechanical ventilation with spontaneous breaths. Automated observation of the carbon dioxide waveform provides a uniform, concise, and consistent interpretation of the capnogram. This article describes a computer algorithm for analyzing and classifying capnograms as normal or as belonging to one of the categories above. The algorithm also generates a diagnostic message when the capnogram deviates from a learned norm for at least three consecutive waveforms (and thus reduces the influence of artifacts). Clinical experience shows reliable waveform recognition by the algorithm.