Respiratory gating improves correlation between pulse wave transit time and pulmonary artery pressure in experimental pulmonary hypertension

Objective. Since pulse wave transit time (PWTT) shortens as pulmonary artery pressure (PAP) increases it was suggested as a potential non-invasive surrogate for PAP. The state of tidal lung filling is also known to affect PWTT independently of PAP. The aim of this retrospective analysis was to test whether respiratory gating improved the correlation coefficient between PWTT and PAP.Approach. In each one of five anesthetized and mechanically ventilated pigs two high-fidelity pressure catheters were placed, one directly behind the pulmonary valve, and the second one in a distal branch of the pulmonary artery. PAP was raised using the thromboxane A2 analogue U46619 and animals were ventilated in a pressure controlled mode (I:E ratio 1:2, respiratory rate 12/min, tidal volume of 6 ml kg-1). All signals were recorded using the multi-channel platform PowerLab®. The arrival of the pulse wave at each catheter tip was determined using a MATLAB-based modified hyperbolic tangent algorithm and PWTT calculated as the time interval between these arrivals.Main results. Correlation coefficient for PWTT and mean PAP wasr= 0.932 for thromboxane. This correlation coefficient increased considerably when heart beats either at end-inspiration (r= 0.978) or at end-expiration (r= 0.985) were selected (=respiratory gating).Significance. The estimation of mean PAP from PWTT improved significantly when taking the respiratory cycle into account. Respiratory gating is suggested to improve for the estimation of PAP by PWTT.

Pressure- vs. volume-controlled ventilation and their respective impact on dynamic parameters of fluid responsiveness: a cross-over animal study

BACKGROUND AND GOAL OF STUDY

Pulse pressure variation (PPV) and stroke volume variation (SVV), which are based on the forces caused by controlled mechanical ventilation, are commonly used to predict fluid responsiveness. When PPV and SVV were introduced into clinical practice, volume-controlled ventilation (VCV) with tidal volumes (VT) ≥ 10 ml kg- 1 was most commonly used. Nowadays, lower VT and the use of pressure-controlled ventilation (PCV) has widely become the preferred type of ventilation. Due to their specific flow characteristics, VCV and PCV result in different airway pressures at comparable tidal volumes. We hypothesised that higher inspiratory pressures would result in higher PPVs and aimed to determine the impact of VCV and PCV on PPV and SVV.

METHODS

In this self-controlled animal study, sixteen anaesthetised, paralysed, and mechanically ventilated (goal: VT 8 ml kg- 1) pigs were instrumented with catheters for continuous arterial blood pressure measurement and transpulmonary thermodilution. At four different intravascular fluid states (IVFS; baseline, hypovolaemia, resuscitation I and II), ventilatory and hemodynamic data including PPV and SVV were assessed during VCV and PCV. Statistical analysis was performed using U-test and RM ANOVA on ranks as well as descriptive LDA and GEE analysis.

RESULTS

Complete data sets were available of eight pigs. VT and respiratory rates were similar in both forms. Heart rate, central venous, systolic, diastolic, and mean arterial pressures were not different between VCV and PCV at any IVFS. Peak inspiratory pressure was significantly higher in VCV, while plateau, airway and transpulmonary driving pressures were significantly higher in PCV. However, these higher pressures did not result in different PPVs nor SVVs at any IVFS.

CONCLUSION

VCV and PCV at similar tidal volumes and respiratory rates produced PPVs and SVVs without clinically meaningful differences in this experimental setting. Further research is needed to transfer these results to humans.

Regional ventilation in spontaneously breathing COVID-19 patients during postural maneuvers assessed by electrical impedance tomography

BACKGROUND

Gravity-dependent positioning therapy is an established concept in the treatment of severe acute respiratory distress syndrome and improves oxygenation in spontaneously breathing patients with hypoxemic acute respiratory failure. In patients with coronavirus disease 2019, this therapy seems to be less effective. Electrical impedance tomography as a point-of-care functional imaging modality for visualizing regional ventilation can possibly help identify patients who might benefit from positioning therapy and guide those maneuvers in real-time. Therefore, in this prospective observational study, we aimed to discover typical patterns in response to positioning maneuvers.

METHODS

Distribution of ventilation in 10 healthy volunteers and in 12 patients with hypoxemic respiratory failure due to coronavirus disease 2019 was measured in supine, left, and right lateral positions using electrical impedance tomography.

RESULTS

In this study, patients with coronavirus disease 2019 showed a variety of ventilation patterns, which were not predictable, whereas all but one healthy volunteer showed a typical and expected gravity-dependent distribution of ventilation with the body positions.

CONCLUSION

Distribution of ventilation and response to lateral positioning is variable and thus unpredictable in spontaneously breathing patients with coronavirus disease 2019. Electrical impedance tomography might add useful information on the immediate reaction to postural maneuvers and should be elucidated further in clinical studies. Therefore, we suggest a customized individualized positioning therapy guided by electrical impedance tomography.

Ventilation Induces Changes in Pulse Wave Transit Time in the Pulmonary Artery

Pulse wave transit time (PWTT) shortens as pulmonary artery pressure (PAP) increases and was therefore suggested as a surrogate parameter for PAP. The aim of this analysis was to reveal patterns and potential mechanisms of ventilation-induced periodic changes in PWTT under resting conditions. To measure both PWTT and PAP in five healthy pigs, two pulmonary artery Mikro-Tip™ catheters were inserted into the pulmonary vasculature: one with the tip placed in the pulmonary artery trunk, and a second one placed in a distal segment of the pulmonary artery. Animals received pressure-controlled mechanical ventilation. Ventilation-dependent changes were seen in both variables, PWTT and mean PAP; however, changes in PWTT were not synchronous with changes in PAP. Thus, plotting the value of PWTT for each heartbeat over the respective PAP revealed a characteristic hysteresis. At the beginning of inspiration, PAP rose while PWTT remained constant. During further inspiration, PWTT started to decrease rapidly as mPAP was about to reach its plateau. The same time course was observed during expiration: while mPAP approached its minimum, PWTT increased rapidly. During apnea this hysteresis disappeared. Thus, non-synchronous ventilation-induced changes in PWTT and PAP were found with inspiration causing a significant shortening of PWTT. Therefore, it is suggested that the respiratory cycle should be considered when using PWTT as a surrogate for PAP.

Overlapping roles of pocket proteins in the myocardium are unmasked by germ line deletion of p130 plus heart-specific deletion of Rb

The pocket protein family of tumor suppressors, and Rb specifically, have been implicated as controlling terminal differentiation in many tissues, including the heart. To establish the biological functions of Rb in the heart and overcome the early lethality caused by germ line deletion of Rb, we used a Cre/loxP system to create conditional, heart-specific Rb-deficient mice. Mice that are deficient in Rb exclusively in cardiac myocytes (CRbL/L) are born with the expected Mendelian distribution, and the adult mice displayed no change in heart size, myocyte cell cycle distribution, myocyte apoptosis, or mechanical function. Since both Rb and p130 are expressed in the adult myocardium, we created double-knockout mice (CRbL/L p130-/-) to determine it these proteins have a shared role in regulating cardiac myocyte cell cycle progression. Adult CRbL/L p130-/- mice demonstrated a threefold increase in the heart weight-to-body weight ratio and showed increased numbers of bromodeoxyuridine- and phosphorylated histone H3-positive nuclei, consistent with persistent myocyte cycling. Likewise, the combined deletion of Rb plus p130 up-regulated myocardial expression of Myc, E2F-1, and G1 cyclin-dependent kinase activities, synergistically. Thus, Rb and p130 have overlapping functional roles in vivo to suppress cell cycle activators, including Myc, and maintain quiescence in postnatal cardiac muscle.