Effect of lower body negative pressure and gravity on regional lung ventilation determined by EIT

The influence of gravity on electrical impedance tomography measurements during upper body position change

Objective

The aim of the study was to examine the influence of gravity on regional ventilation measured by electrical impedance tomography (EIT) with the standard electrode belt position at the 5th intercostal space during tilting from supine to sitting positions.

Methods

A total of 30 healthy volunteers were examined prospectively in supine position during quiet tidal breathing. Subsequently, the bed was tilted so that the upper body of the subjects achieved 30, 60 and 90° every 3 min. Regional ventilation distribution and end-expiratory lung impedance (EELI) were monitored with EIT throughout the whole experiment. Absolute tidal volumes were measured with spirometry and the volume-impedance ratio was calculated for each position.

Results

The volume-impedance ratio did not differ statistically between the studied body positions but 11 subjects exhibited a large change in ratio at one of the positions (outside 99.3% coverage). In general, ventilation distribution became more heterogeneous and moved towards dorsal regions as the upper body was tilted to 90-degree position. EELI increased and tidal volume decreased. The lung regions identified at various positions differed significantly.

Conclusion

Gravity has non-negligible influence on EIT data, as the upper body tilted from supine to sitting positions. The standard electrode belt position might be reconsidered if ventilation distribution is to be compared between supine and sitting positions.

Lower body negative pressure to safely reduce intracranial pressure

KEY POINTS

During long-term missions, some astronauts experience structural and functional changes of the eyes and brain which resemble signs/symptoms experienced by patients with intracranial hypertension. Weightlessness prevents the normal cerebral volume and pressure 'unloading' associated with upright postures on Earth, which may be part of the cerebral and ocular pathophysiology. By placing the lower body in a negative pressure device (LBNP) that pulls fluid away from cranial compartments, we simulated effects of gravity and significantly lowered pressure within the brain parenchyma and ventricle compartments. Application of incremental LBNP demonstrated a non-linear dose-response curve, suggesting 20 mmHg LBNP as the optimal level for reducing pressure in the brain without impairing cerebral perfusion pressure. This non-invasive method of reducing pressure in the brain holds potential as a countermeasure in space as well as having treatment potential for patients on Earth with traumatic brain injury or other pathology leading to intracranial hypertension.

ABSTRACT

Patients with elevated intracranial pressure (ICP) exhibit neuro-ocular symptoms including headache, papilloedema and loss of vision. Some of these symptoms are also present in astronauts during and after prolonged space-flight where lack of gravitational stress prevents daily lowering of ICP associated with upright posture. Lower body negative pressure (LBNP) simulates the effects of gravity by displacing fluid caudally and we hypothesized that LBNP would lower ICP without compromising cerebral perfusion. Ten cerebrally intact volunteers were included: six ambulatory neurosurgical patients with parenchymal ICP-sensors and four former cancer patients with Ommaya-reservoirs to the frontal horn of a lateral ventricle. We applied LBNP while recording ICP and blood pressure while supine, and during simulated intracranial hypertension by 15° head-down tilt. LBNP from 0 to 50 mmHg at increments of 10 mmHg lowered ICP in a non-linear dose-dependent fashion; when supine (n = 10), ICP was decreased from 15 ± 2 mmHg to 14 ± 4, 12 ± 5, 11 ± 4, 10 ± 3 and 9 ± 4 mmHg, respectively (P < 0.0001). Cerebral perfusion pressure (CPP), calculated as mean arterial blood pressure at midbrain level minus ICP, was unchanged (from 70 ± 12 mmHg to 67 ± 9, 69 ± 10, 70 ± 12, 72 ± 13 and 74 ± 15 mmHg; P = 0.02). A 15° head-down tilt (n = 6) increased ICP to 26 ± 4 mmHg, while application of LBNP lowered ICP (to 21 ± 4, 20 ± 4, 18 ± 4, 17 ± 4 and 17 ± 4 mmHg; P < 0.0001) and increased CPP (P < 0.01). An LBNP of 20 mmHg may be the optimal level to lower ICP without impairing CPP to counteract spaceflight-associated neuro-ocular syndrome in astronauts. Furthermore, LBNP holds clinical potential as a safe, non-invasive method for lowering ICP and improving CPP for patients with pathologically elevated ICP on Earth.

Aging modifies the effect of cardiac output on middle cerebral artery blood flow velocity

An association between cerebral blood flow (CBF) and cardiac output (CO) has been established in young healthy subjects. As of yet it is unclear how this association evolves over the life span. To that purpose, we continuously recorded mean arterial pressure (MAP; finger plethysmography), CO (pulse contour; CO‐trek), mean blood flow velocity in the middle cerebral artery (MCAV; transcranial Doppler ultrasonography), and end‐tidal CO₂ partial pressure (PetCO₂) in healthy young (19–27 years), middle‐aged (51–61 years), and elderly subjects (70–79 years). Decreases and increases in CO were accomplished using lower body negative pressure and dynamic handgrip exercise, respectively. Aging in itself did not alter dynamic cerebral autoregulation or cerebrovascular CO₂ reactivity. A linear relation between changes in CO and MCAV_mean was observed in middle‐aged (P < 0.01) and elderly (P = 0.04) subjects but not in young (P = 0.45) subjects, taking concurrent changes in MAP and PetCO₂ into account. These data imply that with aging, brain perfusion becomes increasingly dependent on CO.

The cerebrovascular response to lower-body negative pressure vs. head-up tilt

Lower-body negative pressure (LBNP) has been proposed as a MRI-compatible surrogate for orthostatic stress. Although the effects of LBNP on cerebral hemodynamic behavior have been considered to reflect those of orthostatic stress, a direct comparison with actual orthostasis is lacking. We assessed the effects of LBNP (-50 mmHg) vs. head-up tilt (HUT; at 70°) in 10 healthy subjects (5 female) on transcranial Doppler-determined cerebral blood flow velocity (CBFv) in the middle cerebral artery and cerebral perfusion pressure (CPP) as estimated from the blood pressure signal (finger plethysmography). CPP was maintained during LBNP but decreased after 2 min in response to HUT, leading to an ~15% difference in CPP between LBNP and HUT (P ≤ 0.020). Mean CBFv initially decreased similarly in response to LBNP and for HUT, but, from minute 3 on, the decline became ~50% smaller (P ≤ 0.029) during LBNP. The reduction in end-tidal Pco₂ partial pressure (Pet_CO2 ) was comparable but with an earlier return toward baseline values in response to LBNP but not during HUT (P = 0.008). We consider the larger decrease in CBFv during HUT vs. LBNP attributable to the pronounced reduction in Pet_CO2 and to gravitational influences on CPP, and this should be taken into account when applying LBNP as an MRI-compatible orthostatic stress modality.NEW & NOTEWORTHY Lower-body negative pressure (LBNP) has the potential to serve as a MRI-compatible surrogate of orthostatic stress but a comparison with actual orthostasis was lacking. This study showed that the pronounced reduction in end-tidal Pco₂ together with gravitational effects on the brain circulation lead to a larger decline in cerebral blood flow velocity in response to head-up tilt than during lower-body negative pressure. This should be taken into account when employing lower-body negative pressure as MRI-compatible alternative to orthostatic stress.

Determination of respiratory gas flow by electrical impedance tomography in an animal model of mechanical ventilation

Background

A recent method determines regional gas flow of the lung by electrical impedance tomography (EIT). The aim of this study is to show the applicability of this method in a porcine model of mechanical ventilation in healthy and diseased lungs. Our primary hypothesis is that global gas flow measured by EIT can be correlated with spirometry. Our secondary hypothesis is that regional analysis of respiratory gas flow delivers physiologically meaningful results.

Methods

In two sets of experiments n = 7 healthy pigs and n = 6 pigs before and after induction of lavage lung injury were investigated. EIT of the lung and spirometry were registered synchronously during ongoing mechanical ventilation. In-vivo aeration of the lung was analysed in four regions-of-interest (ROI) by EIT: 1) global, 2) ventral (non-dependent), 3) middle and 4) dorsal (dependent) ROI. Respiratory gas flow was calculated by the first derivative of the regional aeration curve. Four phases of the respiratory cycle were discriminated. They delivered peak and late inspiratory and expiratory gas flow (PIF, LIF, PEF, LEF) characterizing early or late inspiration or expiration.

Results

Linear regression analysis of EIT and spirometry in healthy pigs revealed a very good correlation measuring peak flow and a good correlation detecting late flow. PIF_EIT = 0.702 · PIF_spiro + 117.4, r² = 0.809; PEF_EIT = 0.690 · PEF_spiro-124.2, r² = 0.760; LIF_EIT = 0.909 · LIF_spiro + 27.32, r² = 0.572 and LEF_EIT = 0.858 · LEF_spiro-10.94, r² = 0.647. EIT derived absolute gas flow was generally smaller than data from spirometry. Regional gas flow was distributed heterogeneously during different phases of the respiratory cycle. But, the regional distribution of gas flow stayed stable during different ventilator settings. Moderate lung injury changed the regional pattern of gas flow.

Conclusions

We conclude that the presented method is able to determine global respiratory gas flow of the lung in different phases of the respiratory cycle. Additionally, it delivers meaningful insight into regional pulmonary characteristics, i.e. the regional ability of the lung to take up and to release air.

Visualization of multidimensional and multimodal tomographic medical imaging data, a case study

Multidimensional tomographic datasets contain physical properties defined over four-dimensional (e.g. spatial-temporal, spatial-spectral), five-dimensional (e.g. spatial-temporal-spectral) or even higher-dimensional domains. Multimodal tomographic datasets contain physical properties obtained with different imaging modalities. In medicine, four-dimensional data are widely used, five-dimensional data are emerging, and multimodal data are being used more often every day. Visualization is vital for medical diagnosis and surgical planning to interpret the information included in imaging data. Visualization of multidimensional and multimodal tomographic imaging data is still a challenging task. As a case study, our work focuses on the visualization of five-dimensional (spatial-temporal-spectral) brain electrical impedance tomography (EIT) data. In this paper, a task-based subset definition scheme is proposed: a task model named Cubic Task Explorer (CTE) is derived to support the visualization task exploration for medical imaging data, and a structured method for visualization system development called Task-based Multi-Dimensional Visualization (TMDV) is proposed. A prototype system named EIT5DVis is developed using the CTE model and TMDV method to visualize five-dimensional brain EIT data.

Comparison of different methods to define regions of interest for evaluation of regional lung ventilation by EIT

The measurement of regional lung ventilation by electrical impedance tomography (EIT) has been evaluated in many experimental studies. However, EIT is not routinely used in a clinical setting, which is attributable to the fact that a convenient concept for how to quantify the EIT data is missing. The definition of region of interest (ROI) is an essential point in the data analysis. To date, there are only limited data available on the different approaches to ROI definition to evaluate regional lung ventilation by EIT. For this survey we examined ten patients (mean age +/- SD: 60 +/- 10 years) under controlled ventilation. Regional tidal volumes were quantified as pixel values of inspiratory-to-expiratory impedance differences and four types of ROIs were subsequently applied. The definition of ROI contours was based on the calculation of the pixel values of (1) standard deviation from each pixel set of impedance data and (2) the regression coefficient from linear regression equations between the individual local (pixel) and average (whole scan) impedance signals. Additionally, arbitrary ROIs (four quadrants and four anteroposterior segments of equal height) were used. Our results indicate that both approaches to ROI definition using statistical parameters are suitable when impedance signals with high sensitivity to ventilation-related phenomena are to be analyzed. The definition of the ROI contour as 20-35% of the maximum standard deviation or regression coefficient is recommended. Simple segmental ROIs are less convenient because of the low ventilation-related signal component in the dorsal region.