Lung behavior during a staircase high-frequency oscillatory ventilation recruitment maneuver

BACKGROUND

Lung volume optimization maneuvers (LVOM) are necessary to make physiologic use of high-frequency oscillatory ventilation (HFOV), but lung behavior during such maneuvers has not been studied to determine lung volume changes after initiation of HFOV, to quantify recruitment versus derecruitment during the LVOM and to calculate the time to stabilization after a pressure change.

METHODS

We performed a secondary analysis of prospectively collected data in subjects < 18 years on HFOV. Uncalibrated respiratory inductance plethysmography (RIP) tracings were used to quantify lung recruitment and derecruitment during the LVOM inflation and deflation. The time constant was calculated according to the Niemann model.

RESULTS

RIP data of 51 subjects (median age 3.5 [1.7-13.3] months) with moderate-to-severe pediatric acute respiratory distress syndrome (PARDS) in 85.4% were analyzed. Lung recruitment and derecruitment occurred during the LVOM inflation phase upon start of HFOV and between and within pressure changes. At 90% of maximum inflation pressure, lung derecruitment already started during the deflation phase. Time to stable lung volume (time constant) could only be calculated in 26.2% of all pressure changes during the inflation and in 21.4% during the deflation phase, independent of continuous distending pressure (CDP). Inability to calculate the time constant was due to lack of stabilization of the RIP signal or no change in any direction.

CONCLUSIONS

Significant heterogeneity in lung behavior during a staircase incremental-decremental LVOM occurred, underscoring the need for higher initial inflation pressures when transitioning from conventional mechanical ventilation (CMV) and a longer time between pressure changes to allow for equilibration.

EEG based oculographic analysis of epileptic nystagmus

Epileptic nystagmus (EN) is a subtle seizure semiology, most commonly seen in seizures originating in the posterior cortical regions. EN is broadly categorized into type I and type II. Type I EN consists of contralateral repetitive saccadic eye movements alternating with post-saccadic slow drifts with an overall contralateral deviation. Type II EN is characterized by ipsilateral slow drift alternating with contralateral corrective saccades. In this article, we report a method to perform oculographic analysis of eye movements using EEG only. We used this method to classify the type of EN in three patients with parieto-occipital seizures. In all three patients, the ictal EEG demonstrated repetitive saccadic eye movements, directed contralateral to the seizure onset zone. With prolonged time constant, we were able to identify this eye movement pattern as EN with distinct slow and fast phases. We were able to further characterize the type of EN as type I and type II. In all three patients, the direction of EN (direction of fast phase or saccades) was contralateral to the seizure onset zone. EN can be easily missed on video-electroencephalography (vEEG) recordings because of various reasons. Our study demonstrates a systematic method of eye movement analysis on EEG, which can be used to not only identify EN as seizure semiology but also classify it, without requiring additional electrodes.

Dynamic modeling of Haidinger's brush phenomenon and analysis of the cornea effect based on the model

Starting from the dynamic feature of the vision process and taking into account the time constants of the polarization-dependent process of cones, this study proposed a physical model that can mathematically describe the transient nature of Haidinger's brushes (HB). A saturated exponential growth function was proposed to describe the dynamic process, and the corresponding formulas were derived and discussed. The sensitivities of both static and rotating HB were considered in the model, and the visibility of HB was also investigated. Additionally, the impact of the cornea on the visions of HB was examined by using this model. Analytical results reveal that there exist two types of patterns in the rotating HB phenomenon. The visual perception of the rotating HB is related to the magnitude and interaction of these two patterns, as well as the corneal phase shift. The rotation orientation reversal at large corneal phase shifts observed by some researchers can also be explained by the model. The results are consistent with experimental observations and might help with the clinical diagnosis of macula disorders and corneal abnormalities.

A novel approach to CSF pressure measurement via lumbar puncture that shortens the measurement time with a high level of accuracy

Intracranial pressure (ICP) is an important parameter in clinical management and diagnosis of several neurological diseases which is indirectly measured via lumbar puncture (LP). In routine measurements of cerebrospinal fluid pressure (P_CSF) from lumbar region, a spinal needle and a spinal manometer are used. P_CSF measurement via LP with the use of a spinal manometer may not yield correct P_CSF results due to prolonged times required to obtain an accurate pressure value. Equilibrium pressure may be underestimated in circumstances where spinal manometry procedure is terminated prematurely, with the wrong assumption that equilibrium pressure is reached. Elevated P_CSF levels can lead to visual loss and brain damage when go undiagnosed. In this study, the spinal needle-spinal manometer combination was modelled with a first-order differential equation and a time constant (τ) was defined as the product of the resistance to flow of the needle with the bore area of the manometer divided by the dynamic viscosity of CSF, i.e. τ= RA/ρ_CSF. Each needle/manometer combination had a unique constant as a predictor of the equilibrium pressure. The fluid pressure in the manometer rose in an exponential manner which was tested in a simulated environment using 22G spinal needles namely Braun-Spinocan, Pajunk-Sprotte and M.Schilling. Curve fitting of the manometer readings were obtained with regression coefficients of R² ≥ 0.99 to determine measurement time constants. The residual differences between predicted and true values were less than 1.18 cmH₂O. For a given needle/manometer combination, time required to reach equilibrium pressure was identical for all pressure levels. P_CSF measured at reduced times can easily be interpolated to their equilibrium level allowing clinicians to obtain P_CSF values with high accuracy within seconds. This method can be used as an indirect estimation of ICP in routine clinical practice.

Time constant to determine PEEP levels in mechanically ventilated COVID-19 ARDS: a feasibility study

BACKGROUND

We hypothesized that the measured expiratory time constant (TauE) could be a bedside parameter for the evaluation of positive end-expiratory pressure (PEEP) settings in mechanically ventilated COVID-19 patients during pressure-controlled ventilation (PCV).

METHODS

A prospective study was conducted including consecutively admitted adults (n = 16) with COVID-19-related ARDS requiring mechanical ventilation. A PEEP titration using PCV with a fixed driving pressure of 14 cmH₂O was performed and TauE recorded at each PEEP level (0 to 18 cmH₂O) in prone (n = 29) or supine (n = 24) positions. The PEEP setting with the highest TauE (TauE_MAX) was considered to represent the best tradeoff between recruitment and overdistention.

RESULTS

Two groups of patterns were observed in the TauE plots: recruitable (R) (75%) and nonrecruitable (NR) (25%). In the R group, the optimal PEEP and PEEP ranges were 8 ± 3 cmH₂O and 6-10 cmH₂O for the prone position and 9 ± 3 cmH₂O and 7-12 cmH₂O for the supine position. In the NR group, the optimal PEEP and PEEP ranges were 4 ± 4 cmH₂O and 1-8 cmH₂O for the prone position and 5 ± 3 cmH₂O and 1-7 cmH₂O for the supine position, respectively. The R group showed significantly higher optimal PEEP (p < 0.004) and PEEP ranges (p < 0.001) than the NR group. Forty-five percent of measurements resulted in the most optimal PEEP being significantly different between the positions (p < 0.01). Moderate positive correlation has been found between TauE vs C_RS at all PEEP levels (r² = 0.43, p < 0.001).

CONCLUSIONS

TauE may be a novel method to assess PEEP levels. There was wide variation in patient responses to PEEP, which indicates the need for personalized evaluation.

The (human) respiratory rate at rest

All schoolchildren know how often they breathe, but even experts don’t know exactly why. The aim of this publication is to develop a model of the resting spontaneous breathing rate using physiological, physical and mathematical methods with the aid of the principle that evolution pushes physiology in a direction that is as economical as possible. The respiratory rate then follows from an equation with the parameters \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_{2}$$\end{document}CO2-production rate of the organism, resistance, static compliance and dead space of the lungs, the inspiration duration: expiration duration - ratio and the end-expiratory \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_{2}$$\end{document}CO2 fraction. The derivation requires exclusively secondary school mathematics. Using the example of an adult human or a newborn child, data from the literature then result in normal values for their breathing rate at rest. The reason for the higher respiratory rate of a newborn human compared to an adult is the relatively high \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {CO}_{2}$$\end{document}CO2-production rate together with the comparatively low compliance of the lungs. A side result is the fact that the common alveolar pressure throughout the lungs and the common time constant is a consequence of the economical principle as well. Since the above parameters are not human-specific, there is no reason to assume that the above equation could not also be applicable to many animals breathing through lungs within a thorax, especially mammals. Not only physiology and biology, but also medicine, could benefit: Applicability is being discussed in pulmonary function diagnostics, including pathophysiology. However, the present publication only claims to be a theoretical concept of the spontaneous quiet breathing rate. In the absence of comparable animal data, this publication is intended to encourage further scientific tests.

Interaction of cyclotide Kalata B1 protein with model cellular membranes of varied electrostatics

A uni-molecular layer of lipids at air-water interface mimicking one of the leaflets of the cellular membrane provides a simple model to understand the interaction of any foreign molecules with the membrane. Here, the interactions of protein Kalata B1 (KB1) of cyclotide family with the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG), and 1,2-distearoyl-sn-glycero-3-ethylphosphocholine chloride salt (DSEPC) have been investigated. The addition of KB1 induces a change in pressure of the lipid monolayers. The characteristic time of the change in pressure is found to be dependent on the electrostatic nature of the lipid. Even though the protein is weakly surface active, it is capable of modifying the phase behavior and elastic properties of lipid monolayers with differences in their strength and nature making the layers more floppy. The KB1-lipid interaction has been quantified by calculating the excess Gibb's free energy of interaction and the 1-anilino-8-naphthalenesulfonate (ANS) binding studies. The interaction with zwitterionic DPPC and negatively charged DPPG lipids are found to be thermodynamically favorable whereas the protein shows a weaker response to positively charged DSEPC lipid. Therefore, the long ranged electrostatic is the initial driving force for the KB1 to recognize and subsequently attach to a cellular membrane. Thereafter, the hydrophobic region of the protein may penetrate into the hydrophobic core of the membrane via specific amino acid residues.

Association of transcranial Doppler blood flow velocity slow waves with delayed cerebral ischemia in patients suffering from subarachnoid hemorrhage: a retrospective study

Background

Cerebral vasospasm (VS) and delayed cerebral ischemia (DCI) constitute major complications following subarachnoid hemorrhage (SAH). A few studies have examined the relationship between different indices of cerebrovascular dynamics with the occurrence of VS. However, their potential association with the development of DCI remains elusive. In this study, we investigated the pattern of changes of different transcranial Doppler (TCD)-derived indices of cerebrovascular dynamics during vasospasm in patients suffering from subarachnoid hemorrhage, dichotomized by the presence of delayed cerebral ischemia.

Methods

A retrospective analysis was performed using recordings from 32 SAH patients, diagnosed with VS. Patients were divided in two groups, depending on development of DCI. Magnitude of slow waves (SWs) of cerebral blood flow velocity (CBFV) was measured. Cerebral autoregulation was estimated using the moving correlation coefficient Mxa. Cerebral arterial time constant (tau) was expressed as the product of resistance and compliance. Complexity of CBFV was estimated through measurement of sample entropy (SampEn).

Results

In the whole population (N = 32), magnitude of SWs of ipsilateral to VS side CBFV was higher during vasospasm (4.15 ± 1.55 vs before: 2.86 ± 1.21 cm/s, p < 0.001). Ipsilateral SWs of CBFV before VS had higher magnitude in DCI group (N = 19, p < 0.001) and were strongly predictive of DCI, with area under the curve (AUC) = 0.745 (p = 0.02). Vasospasm caused a non-significant shortening of ipsilateral values of tau and increase in SampEn in all patients related to pre-VS measurements, as well as an insignificant increase of Mxa in DCI related to non-DCI group (N = 13).

Conclusions

In patients suffering from subarachnoid hemorrhage, TCD-detected VS was associated with higher ipsilateral CBFV SWs, related to pre-VS measurements. Higher CBFV SWs before VS were significantly predictive of delayed cerebral ischemia.

Inspiratory respiratory mechanics estimation by using expiratory data for reverse-triggered breathing cycles

BACKGROUND AND OBJECTIVE

Model-based lung mechanics monitoring can provide clinically useful information for guiding mechanical ventilator treatment in intensive care. However, many methods of measuring lung mechanics are not appropriate for both fully and partially sedated patients, and are unable provide lung mechanics metrics in real-time. This study proposes a novel method of using lung mechanics identified during passive expiration to estimate inspiratory lung mechanics for spontaneously breathing patients.

METHODS

Relationships between inspiratory and expiratory modeled lung mechanics were identified from clinical data from 4 fully sedated patients. The validity of these relationships were assessed using data from a further 4 spontaneously breathing patients.

RESULTS

For the fully sedated patients, a linear relationship was identified between inspiratory and expiratory elastance, with slope 1.04 and intercept 1.66. The r value of this correlation was 0.94. No cohort-wide relationship was determined for airway resistance. Expiratory elastance measurements in spontaneously breathing patients were able to produce reasonable estimates of inspiratory elastance after adjusting for the identified difference between them.

CONCLUSIONS

This study shows that when conventional methods fail, typically ignored expiratory data may be able to provide clinicians with the information needed about patient condition to guide MV therapy.

Abdominal obesity linked to a longer cone-mediated dark-adaptation recovery time in healthy eyes

Obesity has been associated with abnormal lipid metabolism and with tissue hypoxia. Human Bruch's membrane (BrM) lipid deposits have been proposed to create a diffusion barrier to metabolic exchange between the choroid and photoreceptors, delaying the regeneration of photopigments. The speed of retinal dark adaptation (DA) is dependent on the regeneration of these photopigments. While the retina is extremely sensitive to hypoxia, the inner retina, which encodes visual contrast, is more affected by hypoxia than the outer retina. This study examines the association between adiposity measures and the time course of DA measured psychophysically through contrast detection to test the functionality of both the outer and inner retina. Cone-mediated DA recovery of contrast threshold (CT) was measured following near-total photopigment bleach for 6 min in 52 healthy eyes of 52 individuals (42.6 ± 18.3 years). Stimuli were sine-wave gratings of low-spatial frequency (1 cycle-per-degree (cpd)) and low luminance (1 cd/m²) generated at the centre of a CRT monitor. CT recovery functions were fitted to an exponential decay model to determine the time constant (τ, seconds) of cone sensitivity recovery, final cone CT (CT_f) and CT elevation (CT₀). Weight, height and waist circumference (WC) were measured and body mass index (BMI) and waist-to-height ratio (WHtR) calculated. Relationships were examined through Spearman correlation and through multiple linear regression using age, optical and adiposity measures as independent variables. The repeatability of cone time constant measurements was estimated by the Bland-Altman method and reported as the coefficient of repeatability (CoR). Mean ± SD of time constant and CT_f were 57.3 ± 27.7 s and -1.78 ± 0.20 log₁₀ units respectively. Cone time constant showed positive Spearman correlation with WC (p = 0.008) and WHtR (p = 0.023) but not with BMI (p = 0.058). Only WHtR emerged as an independent predictor of time constant (p = 0.001). CT_f was not correlated with any adiposity measures. Mean cone time constant was 41 s slower in subjects (25%, n = 13) with abdominal obesity (WHtR≥0.5). Mean CT_f was not significantly different in subjects with or without abdominal obesity. CoR for cone time constant was ±16 s. In adult subjects, greater abdominal obesity (WHtR) was related to a longer contrast recovery time for cone-mediated DA (time to dark-adapt) suggesting outer retinal dysfunction. Final contrast threshold, preferentially processed by inner retinal cells, was unaffected by abdominal obesity.

Advanced myopia, prevalence and incidence analysis

PURPOSE

Various high-percentage high-incidence medical conditions, acute or chronic, start at a particular age of onset t1 (years), accumulate or progress rapidly, with a system time constant t0 (years), typically from 1 week to 5 years, and then level off at a plateau level [Formula: see text], ultimately affecting 10-95% of the population. This report investigates the prevalence and incidence functions for myopia and high myopia as a function of age.

METHODS

Fundamental prevalence versus time and incidence versus time results allow continuous prediction of myopia and high myopia population fractions as a function of age. This is a retrospective study. Nine reports are calculated with N = 444,600 subjects. There were no interventions other than usual regular eye examinations and subsequent indicated refraction change.

RESULTS

The main result is continuous prediction of myopia prevalence-time data along with incidence rate data (%/year), age of onset (years), system plateau level, and system time constant (years). These parameters apply to progressive myopia and high myopia (R < -6 D), useful over several decades.

CONCLUSIONS

The primary finding of this research is that the prevalence ratio of high myopes (R < -6.0 D) to common myopes is expected to increase from 15% entering college to 45% or more after college and graduate school. These statistics are particularly relevant to the many years of study required by M.D., Ph.D., and M.D./Ph.D. programs.

Identification of ankle plantar-flexors dynamics in response to electrical stimulation

Modeling the muscle response to functional electrical stimulation (FES) is an essential step in the design of closed-loop controlled neuroprostheses. This study was aimed at identifying the dynamic response of ankle plantar-flexors to FES during quiet standing. Thirteen healthy subjects stood in a standing frame that locked the knee and hip joints. The ankle plantar-flexors were stimulated bilaterally through surface electrodes and the generated ankle torque was measured. The pulse amplitude was sinusoidally modulated at five different frequencies. The pulse amplitude and the measured ankle torque fitted by a sine function were considered as input and output, respectively. First-order and critically-damped second-order linear models were fitted to the experimental data. Both models fitted similarly well to the experimental data. The coefficient of variation of the time constant among subjects was smaller in the case of the second-order model compared to the first-order model (18.1%vs. 79.9%, p<0.001). We concluded that the critically-damped second-order model more consistently described the relationship between isometric ankle torque and surface FES pulse amplitude, which was applied to the ankle plantar-flexors during quiet standing.

Equivalent linear damping characterization in linear and nonlinear force-stiffness muscle models

In the current research, the muscle equivalent linear damping coefficient which is introduced as the force-velocity relation in a muscle model and the corresponding time constant are investigated. In order to reach this goal, a 1D skeletal muscle model was used. Two characterizations of this model using a linear force-stiffness relationship (Hill-type model) and a nonlinear one have been implemented. The OpenSim platform was used for verification of the model. The isometric activation has been used for the simulation. The equivalent linear damping and the time constant of each model were extracted by using the results obtained from the simulation. The results provide a better insight into the characteristics of each model. It is found that the nonlinear models had a response rate closer to the reality compared to the Hill-type models.