The Effects of Positive End Expiratory Pressure and Lung Volume on Diaphragm Thickness and Thickening

Exploring ultrasonographic diaphragmatic function in perioperative anesthesia setting: A comprehensive narrative review

The ultrasound study of diaphragm function represents a valid method that has been extensively studied in recent decades in various fields, especially in intensive care, emergency, and pulmonology settings. Diaphragmatic function is pivotal in these contexts due to its crucial role in respiratory mechanics, ventilation support strategies, and overall patient respiratory outcomes. Dysfunction or weakness of the diaphragm can lead to respiratory failure, ventilatory insufficiency, and prolonged mechanical ventilation, making its assessment essential for patient management and prognosis in critical care and emergency medicine. While several studies have focused on diaphragmatic functionality in the context of intensive care, there has been limited attention within the field of anesthesia. The ultrasound aids in assessing diaphragmatic dysfunction (DD) by measuring muscle mass and contractility and their potential variations over time. Recent advancements in ultrasound imaging allow clinicians to evaluate diaphragm function and monitor it during mechanical ventilation more easily. In the context of anesthesia, early studies have shed light on the patho-physiological mechanisms of diaphragm function during general anesthesia. In contrast, more recent research has centered on evaluating diaphragmatic functionality at various phases of general anesthesia and by comparing diverse types of procedures or anatomical position during surgery. The objectives of this current review are to highlight the use of diaphragm ultrasound for the evaluation of diaphragmatic function during perioperative anesthesia and surgery. Specifically, we aim to examine the effects of anesthetic agents, surgical techniques, and anatomical positioning on diaphragmatic function. We explore how ultrasound aids in assessing DD by measuring muscle mass and contractility, as well as their potential variations over time. Additionally, we will discuss recent advancements in ultrasound imaging that allow clinicians to evaluate diaphragm function and monitor it during mechanical ventilation more easily.

Diaphragm ultrasound in patients with prolonged weaning from mechanical ventilation

Background

Several publications have examined diaphragmatic ultrasound using two-dimensional (2D) parameters in the context of weaning from mechanical ventilation (MV) and extubation. However, the studied cohorts had rather short duration of ventilation. Examinations on patients with prolonged weaning after long-term ventilation were missing. It was the aim of this study to assess of the diaphragm and peripheral musculature of patients undergoing prolonged weaning creating a chronological sequence of ultrasonic parameters during the course of weaning.

Methods

This study was carried out as a monocentric, prospective observational cross-sectional study. Patients in prolonged weaning who were transferred to a specialized weaning unit were eligible for inclusion if they were ventilated invasively by means of an endotracheal tube or tracheal cannula and if their expected treatment period was at least 5 days. Diaphragmatic function and one representative peripheral muscle were examined in 50 patients between March 2020 and April 2021. The 2D sonographic parameters of diaphragm and diaphragmatic function consisted of diaphragmatic thickness (Tdi) at the end of inspiration and expiration, the fractional thickening (FT) and the diaphragmatic excursion. Additionally, the M. quadriceps femoris was sonographically assessed at two locations. The difference of measurements between the first and the last measuring timepoint were examined using the Wilcoxon signed-rank test. For a longer chronological sequence, the Friedman's rank sum test with subsequent Wilcoxon-Nemenyi-McDonald-Thompson test for multiple comparisons was carried out.

Results

Fifty patients with prolonged weaning were included. The median duration of MV before transfer to the weaning unit was 11.5 [interquartile range (IQR) 10] days. Forty-one patients could be assessed over the full course of weaning, with 38 successfully weaned. Within these 41 patients, the sonographic parameters of the diaphragm slightly increased over the course of weaning indicating an increase in thickness and mobility. Especially parameters which represented an active movement reached statistical significance, i.e., inspiratory Tdi when assessed under spontaneous breathing [begin 3.41 (0.99) vs. end 3.43 (1.31) mm; P=0.01] and diaphragmatic excursion [begin 0.7 (0.8) vs. end 0.9 (0.6) cm; P=0.01]. The presence of positive end-expiratory pressure (PEEP) and pressure support did not influence the sonographic parameters significantly. The M. quadriceps femoris, in contrast, decreased slightly but constantly over the time [lower third: begin 1.36 (0.48) vs. end 1.28 (0.36) cm; P=0.054].

Conclusions

The present study is the first one to longitudinally analyse diaphragmatic ultrasound in patients with prolonged weaning. Sonographic assessment showed that Tdi and excursion increased over the course of prolonged weaning, while the diameter of a representative peripheral muscle decreased. However, the changes are rather small, and data show a wide dispersion. To allow a potential, standardized use of diaphragm ultrasound for diagnostic decision support in prolonged weaning, further studies in this specific patient group are required.

Setting positive end-expiratory pressure: role in diaphragm-protective ventilation

PURPOSE OF REVIEW

With mechanical ventilation, positive end-expiratory pressure (PEEP) is applied to improve oxygenation and lung homogeneity. However, PEEP setting has been hypothesized to contribute to critical illness associated diaphragm dysfunction via several mechanisms. Here, we discuss the impact of PEEP on diaphragm function, activity and geometry.

RECENT FINDINGS

PEEP affects diaphragm geometry: it induces a caudal movement of the diaphragm dome and shortening of the zone of apposition. This results in reduced diaphragm neuromechanical efficiency. After prolonged PEEP application, the zone of apposition adapts by reducing muscle fiber length, so-called longitudinal muscle atrophy. When PEEP is withdrawn, for instance during a spontaneous breathing trial, the shortened diaphragm muscle fibers may over-stretch which may lead to (additional) diaphragm myotrauma. Furthermore, PEEP may either increase or decrease respiratory drive and resulting respiratory effort, probably depending on lung recruitability. Finally, the level of PEEP can also influence diaphragm activity in the expiratory phase, which may be an additional mechanism for diaphragm myotrauma.

SUMMARY

Setting PEEP could play an important role in both lung and diaphragm protective ventilation. Both high and low PEEP levels could potentially introduce or exacerbate diaphragm myotrauma. Today, the impact of PEEP setting on diaphragm structure and function is in its infancy, and clinical implications are largely unknown.

Setting positive end-expiratory pressure: lung and diaphragm ultrasound

PURPOSE OF REVIEW

The purpose of this review is to summarize the role of lung ultrasound and diaphragm ultrasound in guiding ventilator settings with an emphasis on positive end-expiratory pressure (PEEP). Recent advances for using ultrasound to assess the effects of PEEP on the lungs and diaphragm are discussed.

RECENT FINDINGS

Lung ultrasound can accurately diagnose the cause of acute respiratory failure, including acute respiratory distress syndrome and can identify focal and nonfocal lung morphology in these patients. This is essential in determining optimal ventilator strategy and PEEP level. Assessment of the effect of PEEP on lung recruitment using lung ultrasound is promising, especially in the perioperative setting. Diaphragm ultrasound can monitor the effects of PEEP on the diaphragm, but this needs further validation. In patients with an acute exacerbation of chronic obstructive pulmonary disease, diaphragm ultrasound can be used to predict noninvasive ventilation failure. Lung and diaphragm ultrasound can be used to predict weaning outcome and accurately diagnose the cause of weaning failure.

SUMMARY

Lung and diaphragm ultrasound are useful for diagnosing the cause of respiratory failure and subsequently setting the ventilator including PEEP. Effects of PEEP on lung and diaphragm can be monitored using ultrasound.

The increase in diaphragm thickness in preterm infants is related to birth weight: a pilot study

Diaphragm ultrasound (DU) has been used in adult and pediatric critical patients in relation to prediction of extubation success or to detect diaphragm dysfunction, but there is a lack of evidence in neonates. Our aim is to study the evolution of diaphragm thickness in preterm infants, as well as related variables. This prospective monocentric observational study included preterm infants born before 32 weeks (PT32). We performed DU to measure right and left inspiratory and expiratory thickness (RIT, LIT, RET, and LET) and calculated the diaphragm-thickening fraction (DTF) in the first 24 h of life and then weekly until 36 weeks postmenstrual age, death, or discharge. Using multilevel mixed-effect regression, we evaluated the influence of time since birth on diaphragm measurements, as well as bronchopulmonary dysplasia (BPD), birth weight (BW), and days of invasive mechanical ventilation (IMV). We included 107 infants, and we performed 519 DUs. All diaphragm thickness increased with time since birth, but the only additional variable that influenced this growth was BW: beta coefficients RIT = 0.00006; RET = 0.00005; LIT = 0.00005; and LET = 0.00004, p < 0.001. Right DTF values remained stable since birth but left DTF increased with time only in infants with BPD.   Conclusion: In our population we found that the higher the BW, the higher diaphragm thicknesses at birth and follow-up. Contrary to the previously published findings in adult and pediatric settings, we were unable to describe a relationship between days of IMV and diaphragm thickness in PT32. The final diagnosis of BPD does not influence this increase either, but it does increase left DTF. What is Known: • Diaphragm thickness and diaphragm thickening fraction have been related to the time on invasive mechanical ventilation in adults and pediatric patients, as well as with extubation failure. • Very few evidence is yet available on the use of diaphragmatic ultrasound in preterm infants. What is New: • Birth weight is the only variable related to diaphragm thickness in preterm infants born before 32 weeks postmenstrual age. • Days of invasive mechanical ventilation do not influence diaphragm increase in thickness in preterm infants.

The Potential Role of Neutrophil-Reactive Intensity (NEUT-RI) in the Diagnosis of Sepsis in Critically Ill Patients: A Retrospective Cohort Study

The diagnosis of sepsis is often difficult and belated, substantially increasing mortality in affected patients. Its early identification allows for us to choose the most appropriate therapies in the shortest time, improving patients' outcomes and eventually their survival. Since neutrophil activation is an indicator of an early innate immune response, the aim of the study was to evaluate the role of Neutrophil-Reactive Intensity (NEUT-RI), which is an indicator of their metabolic activity, in the diagnosis of sepsis. Data from 96 patients consecutively admitted to the Intensive Care Unit (ICU) were retrospectively analyzed (46 patients with and 50 without sepsis). Patients with sepsis were further divided between sepsis and septic shock according to the severity of the illness. Patients were subsequently classified according to renal function. For the diagnosis of sepsis, NEUT-RI showed an AUC of >0.80 and a better negative predictive value than Procalcitonin (PCT) and C-reactive protein (CRP) (87.4% vs. 83.9% and 86.6%, p = 0.038). Unlike PCT and CRP, NEUT-RI did not show a significant difference within the "septic" group between patients with normal renal function and those with renal failure (p = 0.739). Similar results were observed among the "non-septic" group (p = 0.182). The increase in NEUT-RI values could be useful in the early ruling-out of sepsis, and it does not appear to be influenced by renal failure. However, NEUT-RI has not proved to be efficient in discriminating the severity of sepsis at the time of admission. Larger, prospective studies are needed to confirm these results.