Predicting critical closing pressure in children with obstructive sleep apnea using fluid structure interaction simulations

Surgical treatment of obstructive sleep apnea (OSA) in children requires knowledge of upper airway dynamics, including the closing pressure (Pcrit), a measure of airway collapsibility. We applied a Flow-Structure Interaction (FSI) computational model to estimate Pcrit in patient-specific upper airway models obtained from magnetic resonance imaging (MRI) scans. We sought to examine the agreement between measured and estimated Pcrit from FSI models in children with Down syndrome. We hypothesized that the estimated Pcrit would accurately reflect measured Pcrit during sleep and therefore reflect the severity of OSA as measured by the obstructive apnea hypopnea index (AHI). All participants (n=41) underwent polysomnography and sedated sleep MRI scans. We used Bland Altman Plots to examine the agreement between measured and estimated Pcrit. We determined associations between estimated Pcrit and OSA severity, as measured by AHI, using regression models. The agreement between passive and estimated Pcrit showed a fixed bias of -1.31 (CI=-2.78, 0.15) and a non-significant proportional bias. A weaker agreement with active Pcrit was observed. A model including AHI, gender, an interaction term for AHI and gender and neck circumference explained the largest variation (R2 = 0.61) in the relationship between AHI and estimated Pcrit. (P <0.0001). Overlap between the areas of the airway with lowest stiffness, and areas of collapse on dynamic MRI, was 77.4%±30% for the nasopharyngeal region and 78.6%±33% for the retroglossal region. The agreement between measured and estimated Pcrit and the significant association with AHI supports the validity of Pcrit estimates from the FSI model.

Errors Associated With the Application of the G-Function Plot for Analyzing Pressure Fall-Off Tests in Water Injection Wells

Castillo1 suggested the use of the G-Function plot based on the work of Nolte2. It has been a standard practice in the fracturing community to estimate the fracture closing pressure from a tangent to the G*dp/dg plot. In this analysis technique, the assumption is that a fracture has already developed under the high-pressure fracturing fluid. Then when the pumping is relaxed, one can estimate the fracture closing pressure. In many California waterfloods, the issue of maximum allowable injection gradient has been debated. Various solutions have been proposed to calculate a safe injection gradient. One method that has been promoted is the application of the G-function plot. In this paper, we maintain that this application can be misleading using the prescribed cartesian G function plots. We present the results of an extensive research study for analyzing pressure fall-off data using the G-Plot function. We studied a reappraisal of the G function plot using waterflood conditions where no prior fractures had formed, and no fracture closing pressure was meaningful or applicable. We show from analysis of generated data, using both numerical reservoir modeling and analytical derivations for a radial flow system, that fall-off tests analyzed using the cartesian G function can generate false indications of fracture closing where in fact, the entire injection has been based on radial flow homogeneous injection systems. We also studied systems with a pre-existing fracture before injection. We show that if such a reservoir system is subjected to injection and fall-off tests, again, one may compute a false indication of the irrelevant fracture closure pressure. We discuss how the cartesian scale used for the G function plot can be misleading for the analysis of fall-off test data.

Cerebral critical closing pressure and resistance-area product: the influence of dynamic cerebral autoregulation, age, and sex

Instantaneous arterial pressure-flow (or velocity) relationships indicate the existence of a cerebral critical closing pressure (CrCP), with the slope of the relationship expressed by the resistance-area product (RAP). In 194 healthy subjects (20–82 years, 90 female), cerebral blood flow velocity (CBFV, transcranial Doppler), arterial blood pressure (BP, Finapres) and end-tidal CO2 (EtCO2, capnography) were measured continuously for five minutes during spontaneous fluctuations of BP at rest. The dynamic cerebral autoregulation (CA) index (ARI) was extracted with transfer function analysis from the CBFV step response to the BP input and step responses were also obtained for the BP-CrCP and BP-RAP relationships. ARI was shown to decrease with age at a rate of −0.025 units/year in men (p = 0.022), but not in women (p = 0.40). The temporal patterns of the BP-CBFV, BP-CrCP and BP-RAP step responses were strongly influenced by the ARI (p < 0.0001), but not by sex. Age was also a significant determinant of the peak of the CBFV step response and the tail of the RAP response. Whilst the RAP step response pattern is consistent with a myogenic mechanism controlling dynamic CA, further work is needed to explore the potential association of the CrCP step response with the flow-mediated component of autoregulation.