A controlled trial to investigate whether the orientation of the bevel and angle of approach determine the side of endobronchial intubation in an adult manikin.

One of the commonest complications of endotracheal intubation occurs when the tip of the endotracheal tube passes distal to the carina and enters one of the main bronchi. The perioperative practitioner may observe high airway pressures, hypoxia or even pneumothorax. The most common reason given for the high incidence of right endobronchial intubation is that the right main bronchus comes off the trachea at a more acute angle from the midline. We sought, however, to explore two other factors which may explain this phenomenon ­ the angle of the tube's bevel and its trajectory of approach. We conducted a prospective controlled trial in which doctors from our department intubated the trachea of an adult manikin in three distinct sets using standard tube, reversed tubes and reversed laryngoscope blades. We found that the angle of the bevel and trajectory of approach determines the side of endobronchial intubation in an adult manikin.

Nitrous oxide emission from riparian buffers in relation to vegetation and flood frequency.

The nitrate (NO(3)(-)) removal capacity of riparian zones is well documented, but information is lacking with regard to N(2)O emission from riparian ecosystems and factors controlling temporal dynamics of this potent greenhouse gas. We monitored N(2)O fluxes (static chambers) and measured denitrification (C(2)H(2) block using soil cores) at six riparian sites along a fourth-order stretch of the White River (Indiana, USA) to assess the effect of flood regime, vegetation type, and forest maturity on these processes. The study sites included shrub/grass, aggrading (<15 yr-old), and mature (>80 yr) forests that were flooded either frequently (more than four to six times per year), occasionally (two to three times per year), or rarely (every 20 yr). While the effect of forest maturity and vegetation type (0.52 and 0.65 mg N(2)O-m(-2) d(-1) in adjacent grassed and forested sites) was not significant, analysis of variance (ANOVA) revealed a significant effect ( < 0.01) of flood regime on N(2)O emission. Among the mature forests, mean N(2)O flux was in this order: rarely flooded (0.33) < occasionally flooded (0.99) < frequently flooded (1.72). Large pulses of N(2)O emission (up to 80 mg N(2)O-m(-2) d(-1)) occurred after flood events, but the magnitude of the flux enhancement varied with flood event, being higher after short-duration than after long-duration floods. This pattern was consistent with the inverse relationship between soil moisture and mole fraction of N(2)O, and instances of N(2)O uptake near the river margin after flood events. These results highlight the complexity of N(2)O dynamics in riparian zones and suggest that detailed flood analysis (frequency and duration) is required to determine the contribution of riparian ecosystems to regional N(2)O budget.

Electromobility of plasmid DNA in tumor tissues during electric field-mediated gene delivery.

Interstitial transport is a crucial step in plasmid DNA-based gene therapy. However, interstitial diffusion of large nucleic acids is prohibitively slow. Therefore, we proposed to facilitate interstitial transport of DNA via pulsed electric fields. To test the feasibility of this approach to gene delivery, we developed an ex vivo technique to quantify the magnitude of DNA movement due to pulsed electric fields in two tumor tissues: B16.F10 (a mouse melanoma) and 4T1 (a mouse mammary carcinoma). When the pulse duration and strength were 50 ms and 233 V/cm, respectively, we found that the average plasmid DNA movements per 10 pulses were 1.47 microm and 0.35 microm in B16.F10 and 4T1 tumors, respectively. The average plasmid DNA movements could be approximately tripled, ie to reach 3.69 microm and 1.01 microm, respectively, when the pulse strength was increased to 465 V/cm. The plasmid DNA mobility was correlated with the tumor collagen content, which was approximately eight times greater in 4T1 than in B16.F10 tumors. These data suggest that electric field can be a powerful driving force for improving interstitial transport of DNA during gene delivery.

Changes in anisotropic conduction caused by remodeling cell size and the cellular distribution of gap junctions and Na(+) channels.

Because gene therapy presents a new frontier in the treatment of arrhythmias, it has become important to know how manipulation of the cellular distribution of proteins changes electrical events within individual cells, and whether these cellular changes affect conduction at the larger macroscopic size scale. However, experimental limitations in cardiac bundles prevent measurement of conduction delays across specific gap junctions, as well as the intracellular distribution of the maximum rate of rise of the action potential (V(max)). In view of these limitations, we used immunohistochemical morphological results as a basis to develop two-dimensional cellular models of neonatal and mature canine ventricular muscle in order to obtain insight into the electrophysiological effects of changes in the cellular distribution of proteins; eg, the major protein of cardiac gap junctions, connexin43. Morphological results showed that when the cells enlarged after birth, the gap junctions shifted from the sides to the ends of ventricular myocytes. At birth, V(max) was not different during longitudinal and transverse propagation. However, growth hypertrophy produced a selective increase in mean transverse V(max) with no significant change in longitudinal V(max). Two-dimensional cellular computational models of neonatal and mature ventricular muscle showed that the observed changes in the cellular distribution of the gap junctions and change in cell size accounted for the experimental results. The results unexpectedly showed that cellular scaling (cell size) is as important (or more so) as changes in gap junction distribution in determining the properties of transverse propagation. The results suggest that in pathological states that are arrhythmogenic, maintenance of cell size during remodeling the distribution of gap junctions is important in sustaining a maximum rate of rise of the action potential.

Electrophysiological effects of remodeling cardiac gap junctions and cell size: experimental and model studies of normal cardiac growth.

The increased incidence of arrhythmias in structural heart disease is accompanied by remodeling of the cellular distribution of gap junctions to a diffuse pattern like that of neonatal cardiomyocytes. Accordingly, it has become important to know how remodeling of gap junctions due to normal growth hypertrophy alters anisotropic propagation at a cellular level (V(max)) in relation to conduction velocities measured at a macroscopic level. To this end, morphological studies of gap junctions (connexin43) and in vitro electrical measurements were performed in neonatal and adult canine ventricular muscle. When cells enlarged, gap junctions shifted from the sides to the ends of ventricular myocytes. Electrically, normal growth produced different patterns of change at a macroscopic and microscopic level. Although the longitudinal and transverse conduction velocities were greater in adult than neonatal muscle, the anisotropic velocity ratios were the same. In the neonate, mean V(max) was not different during longitudinal (LP) and transverse (TP) propagation. However, growth hypertrophy produced a selective increase in mean TP V(max) (P<0.001), with no significant change in mean LP V(max). Two-dimensional neonatal and adult cellular computational models show that the observed increases in cell size and changes in the distribution of gap junctions are sufficient to account for the experimental results. Unexpectedly, the results show that cellular scaling (cell size) is as important (or more so) as changes in gap junction distribution in determining TP properties. As the cells enlarged, both mean TP V(max) and lateral cell-to-cell delay increased. V(max) increased because increases in cell-to-cell delay reduced the electric current flowing downstream up to the time of V(max), thus enhancing V(max). The results suggest that in pathological substrates that are arrhythmogenic, maintaining cell size during remodeling of gap junctions is important in sustaining a maximum rate of depolarization.

Extracellular discontinuities in cardiac muscle: evidence for capillary effects on the action potential foot.

It has become of fundamental importance to understand variations in the shape of the upstroke of the action potential in order to identify structural loading effects. One component of this goal is a detailed experimental analysis of the time course of the foot of the cardiac action potential (Vm foot) during propagation in different directions in anisotropic cardiac muscle. To this end, we performed phase-plane analysis of transmembrane action potentials during anisotropic propagation in adult working myocardium. The results showed that during longitudinal propagation there was initial slowing of Vm foot that resulted in deviations from a simple exponential; corollary changes occurred at numerous sites during transverse propagation. We hypothesized that the effect on Vm foot observed in the experimental data was created by the microscopic structure, especially the capillaries. This hypothesis predicts that the phase-plane trajectory of Vm foot will deviate from linearity in the presence of a high density of capillaries, and that a linear trajectory will occur in the absence of capillaries. Comparison of the results of Fast and Kléber (Circ Res. 1993;73:914-925) in a monolayer of neonatal cardiac myocytes, which is devoid of capillaries, and our results in newborn ventricular muscle, which is rich in capillaries, showed drastic differences in Vm foot as predicted. Because this comparison provided experimental support for the capillary hypothesis, we explored the underlying biophysical mechanisms due to interstitial electrical field effects, using a "2-domain" model of myocytes and capillaries separated by interstitial space. The model results show that a propagating interstitial electrical field induces an inward capacitive current in the inactive capillaries that causes a feedback effect on the active membrane (source) that slows the initial rise of its action potential. The results show unexpected mechanisms related to extracellular structural loading that may play a role in selected conduction disturbances, such as in a reperfused ischemic region surrounded by normal myocardium.

Threshold variability in fibers with field stimulation of excitable membranes.

The central focus of this report is the evolution of transmembrane potentials following initiation of a point-source field stimulus, particularly when the stimulus is short and the stimulating electrode is close to the fiber. The transmembrane voltage threshold in response to a point-source field stimulus was determined in a numerical model of a single unmyelinated fiber. Both nerve (Hodgkin-Huxley) and cardiac (Ebihara-Johnson [1]) models of the fiber membrane were evaluated. A central question is whether it is possible to know in advance whether a stimulus of specific magnitude, duration, and location will result in a subsequent action potential. Such determination can be based on the membrane's "voltage threshold." In contrast to the commonly held view, the voltage threshold was found to vary markedly depending on the duration and location of the field stimulus. Voltage thresholds ranged from about 8 mV above baseline to more than 100 mV above baseline, the higher thresholds occurring with shorter stimuli and electrode locations closer to the membrane. A related question is whether the passive membrane response can be used as a tool in determining whether a subsequent action potential is elicited. If the answer is affirmative, this finding can be very useful, since passive properties are linear and thereby much simpler to evaluate than active ones. The results show that the passive response tracks active responses long enough to be a good estimator of subsequent action potential development. Examples show that the evaluation of Vm at 0.2-0.5 msec after stimulus initiation, times chosen on the basis of membrane characteristics, was a better predictor of subsequent excitation than was either initial transmembrane current or Vm at the time when the stimulus ends. Most of the circumstances analyzed here with electric field stimulation also appear likely to be valid with magnetic field stimulation.

Electric field stimulation of excitable tissue.

This paper examines the transmembrane voltage response of an unmyelinated fiber to a stimulating electric field from a point current source. For subthreshold conditions, analytic expressions for the transmembrane potential, vm, are developed that include the specific effects of fiber-source distance, h, and time from the onset of the stimulus, T. Suprathreshold effects are determined for two examples by extending the analytical results with a numerical model. The vm response is a complex evolution in time, especially for small h, that differs markedly from the "activating function." In general, the subthreshold response is a good predictor of the wave shape of the suprathreshold vm, but a poor predictor of its magnitude. The subthreshold response also is a good (but not a precise) predictor of the region where excitation begins.

Electrocardiographic inverse solution for ectopic origin of excitation in two-dimensional propagation model.

Inverse calculations were examined that sought the origin of a cardiac ectopic excitation sequence. Cardiac anatomy and its geometric relationships to sites on the body surface were adapted from human cross-sectional images to form a two-dimensional model, which included ventricular muscle and a primitive conduction system. The surrounding volume conductor was modelled in a simplified way as unbounded, homogeneous and isotropic. In a series of tests, one ectopic origin was designated the 'true' origin. The ECG for this true origin was compared to ECGs for 197 ectopic 'trial' origins, and differences between the wave forms for true versus trial origins were determined. Core issues were the magnitudes of changes in ECG wave forms as a function of the site of origin, whether these changes were sufficient to imply uniqueness, and what spatial resolution might be expected, in the presence of realistic noise levels. For a noise level of 10 microV RMS, the origin of excitation was localised to a single region of the muscle using one wave form from the body surface, with a resolution of 10 mm. The resolution was not improved significantly with a second electrode on the body surface, but was substantially improved with an endocardial electrode.

Unidirectional block in a computer model of partially coupled segments of cardiac Purkinje tissue.

The initiation of a reentrant circuit requires a zone of slow conduction and a zone of unidirectional block. This study used computer model conditions under which partial coupling between segments of cardiac Purkinje tissue resulted in unidirectional block. The structure used was one-dimensional and divided into three segments: a middle segment of variable length coupled to two long (semi-infinite in concept) segments. The DiFrancesco-Noble equations represented the ionic currents of the membrane. The results show that the possibility of unidirectional block depends on the size of the middle segment and the coupling resistances between the segments. No combination of coupling resistances allowed unidirectional block for middle segments with a length of two space constants (4 mm) or longer. Unidirectional block occurred for many combinations of coupling resistances as the length of the middle segment decreased to around half a space constant (1 mm). The number of length combinations that caused unidirectional block decreased again as segment length further decreased. These results provide a possible mechanism of unidirectional block for situations where islands of viable tissue are connected through nonviable tissue, such as in a healed myocardial infarction.

Reflection after delayed excitation in a computer model of a single fiber.

Reflection (reflected reentry) is a case of reentry in a one-dimensional structure, divided into proximal and distal segments, in which tissue excited by a wave front propagating in a forward direction is reexcited by electrical activity coming backward from the original direction of propagation. Cases of reflection have been demonstrated in Purkinje fibers and in ventricular muscle preparations containing multiple fibers. Several mechanisms possibly responsible for reflected reentry have been proposed. However, the difficulty in the interpretation of the experimental results, as well as the limited number of different conditions in which reflection was obtained, has kept open the question about conditions and mechanisms for reflection. We have developed a computer model in which reflection occurs. The model involves a single fiber and uses the DiFrancesco-Noble equations for the Purkinje fiber to model the ionic currents. The results show that reflection is possible in a single fiber and that diastolic depolarization (automaticity) is not a requirement for reflection. Active membrane responses to a just-above-threshold stimulus were important for achieving the necessary time delay. Systematic simulations showed further that reflection occurred only when the right coupling conditions linked a short or long proximal fiber to a short distal segment.

Electrophysiological interaction through the interstitial space between adjacent unmyelinated parallel fibers.

The influence of interstitial or extracellular potentials on propagation usually has been ignored, often through assuming these potentials to be insignificantly different from zero, presumably because both measurements and calculations become much more complex when interstitial interactions are included. This study arose primarily from an interest in cardiac muscle, where it has been well established that substantial interstitial potentials occur in tightly packed structures, e.g., tens of millivolts within the ventricular wall. We analyzed the electrophysiological interaction between two adjacent unmyelinated fibers within a restricted extracellular space. Numerical evaluations made use of two linked core-conductor models and Hodgkin-Huxley membrane properties. Changes in transmembrane potentials induced in the second fiber ranged from nonexistent with large intervening volumes to large enough to initiate excitation when fibers were coupled by interstitial currents through a small interstitial space. With equal interstitial and intracellular longitudinal conductivities and close coupling, the interaction was large enough (induced Vm approximately 20 mV peak-to-peak) that action potentials from one fiber initiated excitation in the other, for the 40-microns radius evaluated. With close coupling but no change in structure, propagation velocity in the first fiber varied from 1.66 mm/ms (when both fibers were simultaneously stimulated) to 2.84 mm/ms (when the second fiber remained passive). Although normal propagation through interstitial interaction is unlikely, the magnitudes of the electrotonic interactions were large and may have a substantial modulating effect on function.

Propagation model using the DiFrancesco-Noble equations. Comparison to reported experimental results.

Propagation, re-entry and the effects of stimuli within the conduction system can be studied effectively with computer models when the pertinent membrane properties can be represented accurately in mathematical form. To date, no membrane models have been shown to be accurate representations during repolarisation and recovery of excitability, although for the Purkinje membrane the DiFrancesco-Noble (DN) model has become a possibility. The paper examines the DN model, restates its equations and compares simulated waveforms in a number of propagation contexts to experimental measurements reported in the literature. The objective is to determine whether or not the DN model reproduced phenomena such as supernormality, shortening in action potential duration during pacing rate increases, alternation of duration with changes in rhythm, graded responses and 'all-or-none' repolarisation in a quantitatively realistic way, as each of these come from time and space dependencies not directly a part of the ionic current measurements on which the DN model is based. The results show that the DN equations correctly simulate these situations and support the goal of having a model that is broadly applicable to Purkinje tissue, including refractory period properties and response to electrical stimulation.

Computer simulations of activation in an anatomically based model of the human ventricular conduction system.

Simulations of the electrical activity during excitation were performed in an anatomically based model of the human ventricular conduction system. Each of the 33,000 elements of this model represented a unit bundle of Purkinje or atrioventricular nodal tissue. The Ebihara-Johnson model for sodium defined the active membrane characteristics. Using a combination of new and existing modeling techniques, simulations of excitation were completed in approximately 5 min CPU time on an IBM 3090 at the Cornell National Supercomputer Facility. Activation times at sites in the model were compared to experimental measurements for the excitation of the ventricular myocardium on the endocardial surface. These "literature-based" times were estimated from a number of reported human heart mapping studies. Initially, the times fit poorly. The major factor for the discrepancy was the conduction velocities of the elements, which were a result of the physical and electrical parameters derived from a review of histologic and electrical properties studies. In addition, there was a latency between activation of the system in the left ventricle of the model and that in the right ventricle when compared to the experimental work. When the times were scaled to adjust for the conduction velocity and ventricular latency effects, the match between the simulation and literature-based times was much improved. Quantitative comparison between normalized times resulted in correlation coefficients CCF = 0.76 for the right ventricle and CCF = 0.64 for the left ventricle.

One-dimensional model of cardiac defibrillation.

The response of a single strand of cardiac cells to a uniform defibrillatory shock assuming steady-state linear conditions is examined. It is argued that the effect of this current is quantitatively described by the induced transmembrane potential even under passive conditions. The characteristics of the single strand are those that would exist if the heart was a system of equivalent parallel pathways from apex to base. It is shown that essentially every cell is both hyperpolarized and depolarised from the shock by an amount proportional to the stimulus intensity and the intercellular junctional resistance. For physiological values of model parameters the evaluated depolarisations are consistent with levels necessary to affect electrophysiological behaviour.

The construction of an anatomically based model of the human ventricular conduction system.

The ventricular conduction system is a complicated network of specialized muscle cells responsible for the transmission of electrical activity between the atria and the ventricles of the human heart. It has been the focus of numerous electrical and anatomical studies at both the microscopic and macroscopic levels. An understanding of its behavior at both levels is considered important, because it is primarily responsible for the spread of excitation in the ventricles. Previous computer models have been very simple ones that have been primarily adjuncts to models of the ventricles. This paper describes a strategy for the construction of conduction system models which is based on real microscopic and macroscopic features, although the model still is much simpler than reality. The model contains almost 35,000 individual cylindrical elements, each of whose physical dimensions approximate unit bundles of Purkinje and atrioventricular nodal cells. The model, whose physical appearance closely resembles that of the conduction system, was generated from limited anatomical data in less than 2 min CPU time on an IBM 3090 at the Cornell National Supercomputer Facility.