Tracheal-innominate artery fistula caused by the endotracheal tube tip: case report and investigation of a fatal complication of prolonged intubation.

CASE REPORT: A patient with extensive burns was intubated with an 8.0 mm internal diameter endotracheal tube (ETT) equipped with a subglottic suction port (Mallinckrodt HiLo Evac). The ETT was secured to a left upper molar with wire sutures throughout the hospitalization course to ensure airway stability. On the 40th day of intubation, the patient exsanguinated and died from a tracheo-innominate artery fistula. Postmortem examination revealed a 1 cm lesion of the left anterior tracheal wall at the position of the ETT tip. The prolonged stationary position of the ETT was considered the primary factor responsible for the fistula. Yet tracheo-innominate artery fistula normally is associated with high cuff pressures rather than with the tube tip. The special ETT construction required for the subglottic suction feature was suspected to have increased tube rigidity and may have played a contributory role. METHODS: The rigidity of the Mallinckrodt HiLo Evac was measured with a mechanical model and compared to 5 other commercially-available ETTs. Rigidity was expressed as the force generated by the ETT tip when the tube curvature was altered by 5 cm and 10 cm of flexion from its resting position. RESULTS: The mean force exerted by the Mallinckrodt HiLo Evac was 10.1 +/- 2.8 g at 5 cm of flexion and 17.7 +/- 5.1 g at 10 cm of flexion. This was significantly greater than all other ETT brands tested (by one-way analysis of variance and Student-Newman-Kuels test, p < 0.05). CONCLUSION: This case of fatal tracheo-innominate artery fistula formation associated with an ETT tip was unusual because of the extended duration of endotracheal intubation and the complexity of the patient's airway management problems. Our data suggest that the higher rigidity of the HiLo Evac ETT may have contributed to fistula development at the tube tip. However, we do not believe that the higher rigidity of the HiLo Evac ETT necessarily poses any greater risk than other ETTs under normal circumstances, in which the tube tip is not fixed in a stationary position for an extended period.

Endothelium-dependent, shear-induced vasodilation is rate-sensitive.

OBJECTIVES: To quantify the relative contributions of the rate of change and the magnitude of shear stress to endothelium-mediated arteriolar dilation. METHODS: A feedback control system was designed in which shear stress (tau) and the temporal shear gradient (TSG) were prescribed and dynamically controlled in isolated rat cremaster 1A arterioles. The TSG was the quotient of the maximum shear stress and the ramp duration. This system was used to assess the roles of tau and TSG in the initial, transient vasodilations and the secondary, sustained vasodilations in response to steps and ramps in shear stress. RESULTS: Both step- and ramp-shear experiments revealed time-dependent hiphasic vasodilations that we report for the first time. Application of a step-shear stress of 20 dynes/cm2 elicited an initial transient vasodilation that peaked at about 4 min. When the shear stress was applied as a ramp that reached the maximum value of 20 dynes/cm2 over 5 min, a vasodilation was observed over the ramp period, which reached a peak at the end of the ramp period that was much lower than that observed after step shear. After 20 dynes/cm2 was attained, the vessel diameter decreased despite constant maintenance of the maximum shear stress. In both step- and ramp-shear experiments, after the decrease of the initial vasodilation, a second phase of vasodilation began approximately 15 min after the beginning of the shear application. The second phase of vasodilation reached a steady state that was essentially the same for both the step and the ramp shear. By refining the ramping apparatus further, we varied the TSG up to 40 dynes/cm2 per second and showed that the early vasodilation was highly rate sensitive to TSGs greater than 5 dynes/cm2 per second for a given intermediate value of final shear stress (20 dynes/cm2) and was magnitude sensitive when shear was increased gradually (TSG < 5 dynes/cm2 per second). CONCLUSIONS: Our results suggest that two fundamentally different responses to shear stress are mediated by microvascular endothelium: one vasodilation is elicited by shear stress changes on a time scale of a few seconds or less and another is elicited by shear stress changes on a longer time scale. The former response is potent, transient, and rate sensitive; the latter is more modest, sustained, and magnitude sensitive.

Microvascular thermal equilibration in rat spinotrapezius muscle.

The current study investigates heat exchange in the thermally significant countercurrent paired vessels of the rat spinotrapezius muscle. Detailed tissue surface temperatures under normal (after the microvascular surgery) and pharmacologically vasodilated states were measured using high-resolution infrared thermography. During vasodilation, a measurable thermal disturbance was observed above the first-order feeding vessel pair. The measured tissue temperatures were compared with those predicted by modifying the theoretical model for two-dimensional muscle preparations given by Zhu et al. (Zhu, L., D. E. Lemons, and S. Weinbaum. Ann. Biomed. Eng. 24:109-123, 1996). They were found in good agreement. The Weinbaum-Jiji k(eff) theory (Weinbaum, S., and L. M. Jiji. J. Biomech. Eng. 107:131-139, 1985) for heat exchange between the paired vessels and their surrounding tissue was also examined in this muscle. A close agreement was obtained between the theoretically predicted k(eff) and the measured value calculated using a fin approximation for the tissue layer. This experimental study revealed for the first time the nonequilibration between blood vessels and the surrounding tissue, where the enhancement in k(eff) due to the incomplete countercurrent heat exchange is comparable to the tissue axial conduction.

Defining the boundaries of physiological understanding: the benchmarks curriculum model.

We set out to develop an anatomy and physiology (A&P) curriculum with a content that was relevant and well rationalized. To do this, we developed a benchmarks curriculum process that helps us to determine what our students need to learn in A&P and then to make sure there is alignment between those learning objectives and what we teach and how we assess our students. Using the benchmarks process, we first set the broad skill and content goals of the course. We then prioritize the topic areas to be covered, allocating the course time accordingly, and declare the learning objectives for each topic. To clarify each learning objective, a set of benchmark statements are written that specify in operational terms what is required to demonstrate mastery. After the benchmarks are written, we assemble the learning activities that help students achieve them and write assessment items to evaluate achievement. We have implemented the curriculum in a relational database that allows us to specify the numerous links that exist between its different elements. In the future, the benchmarks model will be used for ongoing A&P curriculum development with geographically distributed contributors accessing it via the World Wide Web. This mechanism will allow for the continuing evolution of the A&P curriculum.

Enhancement in the effective thermal conductivity in rat spinotrapezius due to vasoregulation.

This study was undertaken to gain a better understanding of the countercurrent heat exchange of thermally significant blood vessels in skeletal muscle by measuring the vascular structure and flow in an exteriorized rat spinotrapezius muscle and estimating the enhancement in the effective thermal conductivity of the muscle. Detailed anatomic measurements of the number density and length of countercurrent vessel pairs between 45 and 165 microns diameter were obtained. Moreover, diameter and blood flow in the 1A to 3A vessels were measured for muscles in which pharmacological vasoactive agents were introduced, allowing one to vary the local blood flow Peclet number from 1 to 18 in the major feeding arteries. These combined measurements have been used to estimate the range of possible enhancement in the effective thermal conductivity of the tissue. The newly derived conduction shape factor in Zhu et al. for countercurrent vessels in two-dimensional tissue preparations was used in this analysis. Our experimental data indicated that the value of this conduction shape factor was about one-third to two-thirds the value for two countercurrent vessels of the same size and spacing in an infinite medium. The experiment also revealed that the Weinbaum-Jiji expression for keff was valid for the spinotrapezius muscle when the largest vessels were less than 195 microns diameter. A fivefold increase in keff was predicted for 195 microns diameter vessels. Vasoregulation was also shown to have a dramatic effect on keff. A tissue that exhibits only small increases in keff due to countercurrent convection in its vasoconstricted state can exhibit a more than fivefold increase in keff in its vasodilated state.

Microvascular thermal equilibration in rat cremaster muscle.

A new experimental approach was developed to obtain the first direct measurements of the axial countercurrent thermal equilibration in a microvascular tissue preparation using high resolution infrared thermography. Detailed surface temperature measurements were obtained for an exteriorized rat cremaster muscle in which pharmacological vasoactive agents were used to change the local blood flow Peclet number from 1 to 14 in the feeding artery. Under normal conditions, only the 1A arteries (> 70 microns diameter) showed thermal nonequilibration with the surrounding tissue. The theoretical model developed by Zhu and Weinbaum (28) for a two-dimensional tissue preparation with arbitrarily embedded countercurrent vessels was modified to include axial conduction and the presence of the supporting glass slide. This modified model was used to interpret the experimental results and to relate the surface temperature profiles to the bulk temperature profiles in the countercurrent artery and vein and the local average tissue temperature in the cross-sectional plane. Surface temperature profiles transverse to the vessel axis are shown to depend significantly on the tissue inlet temperature. The eigenfunction for the axial thermal equilibration depends primarily on the blood flow Peclet number and the environmental convective coefficient. The theoretical results predict that when rho(ar)*Pe is less than 1 mm (the range in our experiments), axial conduction is the dominant mode of axial thermal equilibration. For 1 < rho(ar)*PE < 3 mm, countercurrent blood flow becomes comparable to axial conduction, whereas, when rho(ar)*Pe > 3 mm, countercurrent blood flow is the dominant mode of axial thermal equilibration. Therefore, for rho(ar)*Pe > 3 mm the axial equilibration length is proportional to the blood flow Peclet number, as predicted previously by Zhu and Weinbaum in a study in which axial conduction was neglected. It also is shown that the axial decay of the tissue temperature at low perfusion rates can be described by a simple one-dimensional Weinbaum-Jiji equation with a newly derived conduction shape factor.

A non-uniform three-dimensional perfusion model of rat tail heat transfer.

Previous models of rat tail heat transfer have assumed that the tail is uniformly perfused along its length and have introduced questionable assumptions about the heat transfer role of the major axial arteries and the venous blood shunting between the superficial and deep veins. The recent experiments of Lemons and Wu have shown that (i) perfusion of the tail tip is more than tenfold higher than that in the tail base and (ii) the perfusion of the middle region of the tail increases eightfold during heat stress compared to threefold to fourfold in the base and tip. Our anatomical studies have shown that the lateral arteries are a series of radially arcading connections from the ventral artery and probably do not serve as major axial conduit vessels. These observations indicate that current views and models for the blood flow distribution and heat transfer in the major axial arteries and veins and in the rat tail cutaneous circulation need substantial revision. Based on these new experimental findings a new three-dimensional model is developed to determine the heat transfer function of the rat tail at different local and central temperatures. The predictions of the model show good agreement with the axial surface temperature distribution in the rat tail reported by Lemons and Wu. These results, when combined with our anatomical studies, indicate that there is very little shunting of blood between the superficial lateral veins and the deep ventral vein as proposed by Raman et al. Although this model is based on the rat tail anatomy, it can be modified to treat the human limb and digit.

A new approach for predicting the enhancement in the effective conductivity of perfused muscle tissue due to hyperthermia.

This study attempts to measure the hyperthermic response of individual microvessels in skeletal muscle tissue subject to local heating and then to predict the enhancement in thermal conductivity that results from the observed changes in vascular diameter and flow. In contrast to existing studies, which have tried to relate changes in tissue thermal conductivity to local blood perfusion using thermal clearance and self-heated thermistor techniques, we have developed a two-dimensional muscle tissue preparation in which the hyperemic response has been quantified by measuring the in vivo changes in diameter and blood flow of 1A to 4A generation vessels of rat cremaster muscle when the temperature was raised in 2 degrees increments from 34 to 42 degrees C. Only 3A and 4A vessels showed vasodilation when subject to hyperthermia, indicating that the measured increase in flow in the 1A and 2A vessels was the result of a decrease in downstream resistance. Our cremaster muscle preparations have also been used to obtain the first detailed anatomic measurements of the number density and length of countercurrent vessel pairs between 50-200 microns diameter. These combined measurements have been used to establish the limits of validity of the Weinbaum-Jiji theory. Our experimental data indicate that the Weinbaum-Jiji expression for keff is valid in cremaster muscle and cat mesentery tissue for both normal and hyperthermic conditions provided the largest vessels are < 200 microns in diameter. The theory predicts that significant enhancements in keff start to occur for vessels that are 70 microns in diameter or larger, that a 2.5-fold increase in keff can be achieved for a maximally dilated 200 microns diameter 1A vessel pair in cremaster muscle of larger rats, and a 6-fold increase is predicted for maximally dilated 200 microns diameter vessels in the cat mesentery. The experiments also show that maximally dilated 1A to 4A vessels in the microcirculation closely satisfy the condition Q(flow)/(2a)3 = constant, which is consistent with the hypothesis that there is an adaptive regulation of vessel diameter which keeps the wall shear stress nearly constant during temporal changes in flow.

The bleed off perfusion term in the Weinbaum-Jiji bioheat equation.

The microvascular organization and thermal equilibration of the primary and secondary arteries and veins that comprise the bleed off circulation to the muscle fibers from the parent countercurrent supply artery and veins are analyzed. The blood perfusion heat source term in the tissue energy equation is shown to be related to this vascular organization and to undergo a fundamental change in behavior as one proceeds from the more peripheral tissue, where the perfusion term is proportional to the Ta--Tv difference in the parent supply vessels, to the deeper tissue layers where the bleed off vessels themselves form a branching countercurrent system for each muscle tissue cylinder and the venous return temperature can vary between the local tissue temperature and Ta. The consequences of this change in behavior are examined for the Weinbaum-Jiji bioheat equation and a modified expression for the effective conductivity of perfused tissue is derived for countercurrent bleed off exchange.

A three-dimensional variable geometry countercurrent model for whole limb heat transfer.

A new formulation of the combined macro and microvascular model for heat transfer in a human arm developed in Song et al. [1] is proposed using a recently developed approximate theory for the heat exchange between countercurrent vessels embedded in a tissue cylinder with surface convection [2]. The latter theory is generalized herein to treat an arm with an arbitrary variation in cross-sectional area and continuous bleed off from the axial vessels to the muscle and cutaneous tissue. The local microvascular temperature field is described by a "hybrid" model which applies the Weinbaum-Jiji [3] and Pennes [4] equations in the peripheral and deeper tissue layers, respectively. To obtain reliable end conditions at the wrist and other model input parameters, a plethysmograph-calorimeter has been used to measure the blood flow distribution between the arm and hand circulations, and hand heat loss. The predictions of the model show good agreement with measurements for the axial surface temperature distribution in the arm and confirm the minimum in the axial temperature variation first observed by Pennes [4] for an arm in a warm environment.

A combined macro and microvascular model for whole limb heat transfer.

A new prototype model for whole limb heat transfer is proposed wherein the countercurrent heat exchange from the large central arteries and veins in the core of the limb is coupled to microvascular models for the surrounding muscle and the cutaneous tissue layers. The local microvascular temperature field in the muscle tissue is described by the bioheat equation of Weinbaum and Jiji. The new model allows for an arbitrary axial variation of cross-sectional area and blood distribution between the muscle and cutaneous tissue, accounts for the blood flow to and heat loss from the hand and treats the venous return temperature and surface temperature distribution as unknowns that are determined as part of the solution to the overall boundary value problem. Representative solutions are presented for a wide range of environmental conditions for a limb in both the resting state and during exercise.

On the generalization of the Weinbaum-Jiji bioheat equation to microvessels of unequal size; the relation between the near field and local average tissue temperatures.

The extensive series of experiments reported in Lemons et al. [1] show that measureable local tissue temperature fluctuations are observed primarily in the vicinity of the 100-500 micron countercurrent vessels of the microcirculation and thus strongly support the basic hypothesis in the new bioheat equation of Weinbaum and Jiji [2] that these countercurrent microvessels are the principal determinants of local blood-tissue heat transfer. However, the detailed temperature profiles in the vicinity of these vessels indicate that large asymmetries in the local temperature field can result from the significant differences in size between the countercurrent artery and vein. Using the superposition techniques of Baish et al. [9], the paper first presents a solution to the classic problem of an unequal countercurrent heat exchanger with heat loss to the far field. This solution is then used to generalize the Weinbaum-Jiji bioheat equation and the conductivity tensor that appears in this equation to vessels of unequal size. An asymptotic analysis has also been developed to elucidate the relationship between the near field temperature of the artery-vein pair and the local average tissue temperature. This analysis is used to rigorously prove the closure approximation relating the local arterial-venous temperature difference and the mean tissue temperature gradient which had been derived in [2] using a more heuristic approach.

Significance of vessel size and type in vascular heat transfer.

This study was undertaken to gain a better understanding of the fundamental mechanisms of micro- and macrovascular heat transfer by experimentally identifying those vessels most important in the process. Tissue temperature fields around thermally nonequilibrated vessels were determined using a small temperature sensor that was guided through the rabbit thigh to generate a detailed temperature map. The measurements revealed that the lower limit of vessel size for thermal nonequilibration was 100 microns for arteries and 400 microns for veins. Local temperature fields were found around four of the five (80%) arteries that were greater than 300 microns in diameter but in only 3 of the 12 (25%) veins greater than 400 microns. These experimental results are in good agreement with previously published theoretical studies (5) in which it was concluded that thermal equilibration in the branching countercurrent vascular network of the rabbit limb occurs in vessels an order of magnitude larger than the capillaries. In those studies the smallest vessels capable of carrying heat were predicted to be 50 microns ID with the major blood tissue heat exchange occurring in vessels greater than 100 micron ID. These findings contrast with the view that most heat transfer occurs in the capillaries and suggest that vascular heat transfer analysis must take into account the vascular architecture of the 50- to 1,000-micron vessels where most heat transfer occurs.

Theory and experiment for the effect of vascular microstructure on surface tissue heat transfer--Part I: Anatomical foundation and model conceptualization.

A new theoretical model supported by ultrastructural studies and high-spatial resolution temperature measurements is presented for surface tissue heat transfer in a two-part study. In this first paper, vascular casts of the rabbit thigh prepared by the tissue clearance method were serially sectioned parallel to the skin surface to determine the detailed variation of the vascular geometry as a function of tissue depth. Simple quantitative models of the basic vascular structures observed were then analyzed in terms of their characteristic thermal relaxation lengths and a new three-layer conceptual model proposed for surface tissue heat transfer. Fine wire temperature measurements with an 80-micron average diameter thermocouple junction and spatial increments of 20 micrometers between measurement sites have shown for the first time the detailed temperature fluctuations in the microvasculature and have confirmed the fundamental assumptions of the proposed three-layer model for the deep tissue, skeletal muscle and cutaneous layers.

Theory and experiment for the effect of vascular microstructure on surface tissue heat transfer--Part II: Model formulation and solution.

In this paper the conceptual three-layer representation of surface tissue heat transfer proposed in Weinbaum, Jiji and Lemons [1], is developed into a detailed quantitative model. This model takes into consideration the variation of the number density, size and flow velocity of the countercurrent arterio-venous vessels as a function of depth from the skin surface, the directionality of blood perfusion in the transverse vessel layer and the superficial shunting of blood to the cutaneous layer. A closed form analytic solution for the boundary value problem coupling the three layers is obtained. This solution is in terms of numerically evaluated integrals describing the detailed vascular geometry, a capillary bleed-off distribution function and parameters describing the shunting of blood to the cutaneous layer. Representative heat transfer results for typical physiological conditions are presented.

Acclimation, temperature selection, and heat exchange in the turtle, Chrysemys scripta.

Turtles acclimated to temperatures between 3 and 19 degrees C were placed in a thermal gradient. The animals usually selected temperatures above 28 degrees C within 1 h after placement in the gradient, attaining a final thermal preferendum between 31 and 33 degrees C. Turtles placed in the gradient for extended periods of time were more active during the day; the temperature selected was not related to activity or time of day. Turtles were transferred from a constant temperature bath at 10 or 30 degrees C to a calorimeter at 30 or 10 degrees C. Mean body temperature (Tb) and temperatures of the heart (The), brain (Tbr), and cloaca (Tcl) as well as heart rate were continuously monitored. In a 0.76-kg turtle, temperatures increased to two-thirds of the final difference between the initial temperature and the final temperature in the following times (min): Tb, 5.5; The, 6.0; Tcl, 9.0. The increase in Tbr varied depending on whether the head was extended or retracted. Rapid changes in ambient water temperature had relatively little effect on the heart rate of a submerged turtle. Heart rates were closely related to The and were practically independent of brain temperature.