Intraoperative Ventilation/Perfusion Mismatch and Postoperative Pulmonary Complications after Major Noncardiac Surgery: A Prospective Cohort Study.

BACKGROUND: Postoperative pulmonary complications can increase hospital length of stay, postoperative morbidity, and mortality. Although many factors can increase the risk of postoperative pulmonary complications, it is not known whether intraoperative ventilation/perfusion (V/Q) mismatch can be associated with an increased risk of postoperative pulmonary complications after major noncardiac surgery. METHODS: This study enrolled patients undergoing general anesthesia for noncardiac surgery and evaluated intraoperative V/Q distribution using the automatic lung parameter estimator technique. The assessment was done after anesthesia induction, after 1 h from surgery start, and at the end of surgery. Demographic and procedural information were collected, and intraoperative ventilatory and hemodynamic parameters were measured at each timepoint. Patients were followed up for 7 days after surgery and assessed daily for postoperative pulmonary complication occurrence. RESULTS: The study enrolled 101 patients with a median age of 71 [62 to 77] years, a body mass index of 25 [22.4 to 27.9] kg/m2, and a preoperative Assess Respiratory Risk in Surgical Patients in Catalonia (ARISCAT) score of 41 [34 to 47]. Of these patients, 29 (29%) developed postoperative pulmonary complications, mainly acute respiratory failure (23%) and pleural effusion (11%). Patients with and without postoperative pulmonary complications did not differ in levels of shunt at T1 (postoperative pulmonary complications: 22.4% [10.4 to 35.9%] vs. no postoperative pulmonary complications:19.3% [9.4 to 24.1%]; P = 0.18) or during the protocol, whereas significantly different levels of high V/Q ratio were found during surgery (postoperative pulmonary complications: 13 [11 to 15] mmHg vs. no postoperative pulmonary complications: 10 [8 to 13.5] mmHg; P = 0.007) and before extubation (postoperative pulmonary complications: 13 [11 to 14] mmHg vs. no postoperative pulmonary complications: 10 [8 to 12] mmHg; P = 0.006). After adjusting for age, ARISCAT, body mass index, smoking, fluid balance, anesthesia type, laparoscopic procedure and surgery duration, high V/Q ratio before extubation was independently associated with the development of postoperative pulmonary complications (odds ratio, 1.147; 95% CI, 1.021 to 1.289; P = 0.02). The sensitivity analysis showed an E-value of 1.35 (CI, 1.11). CONCLUSIONS: In patients with intermediate or high risk of postoperative pulmonary complications undergoing major noncardiac surgery, intraoperative V/Q mismatch is associated with the development of postoperative pulmonary complications. Increased high V/Q ratio before extubation is independently associated with the occurrence of postoperative pulmonary complications in the first 7 days after surgery.

Hypoxia in the pulmonary vein increases pulmonary vascular resistance independently of oxygen in the pulmonary artery.

INTRODUCTION: Hypoxic pulmonary vasoconstriction (HPV) can be a challenging clinical problem. It is not fully elucidated where in the circulation the regulation of resistance takes place. It is often referred to as if it is in the arteries, but we hypothesized that it is in the venous side of the pulmonary circulation. METHODS: In an open thorax model, pigs were treated with a veno-venous extra corporeal membrane oxygenator to either oxygenate or deoxygenate blood passing through the pulmonary vessels. At the same time the lungs were ventilated with extreme variations of inspired air from 5% to 100% oxygen, making it possible to make combinations of high and low oxygen content through the pulmonary circulation. A flow probe was inserted around the main pulmonary artery and catheters in the pulmonary artery and in the left atrium were used for pressure monitoring and blood tests. Under different combinations of oxygenation, pulmonary vascular resistance (PVR) was calculated. RESULTS: With unchanged level of oxygen in the pulmonary artery and reduced inspired oxygen fraction lowering oxygen tension from 29 to 6.7 kPa in the pulmonary vein, PVR was doubled. With more extreme hypoxia PVR suddenly decreased. Combinations with low oxygenation in the pulmonary artery did not systematic influence PVR if there was enough oxygen in the inspired air and in the pulmonary veins. DISCUSSION: The impact of hypoxia occurs from the alveolar level and forward with the blood flow. The experiments indicated that the regulation of PVR is mediated from the venous side.

Four different models for simulation-based training of bronchoscopic procedures.

BACKGROUND: Flexible bronchoscopy procedures require detailed anatomical knowledge and advanced technical skills. Simulation-based training offers a patient-safe training environment that can be more efficient than patient-based training. Physical models are cheaper than virtual reality simulators and allow trainees to be acquainted with the equipment used in the clinic. The choice of a physical model for training depends on the local context. The aim of this study was to compare four different bronchoscopy models for flexible bronchoscopy training. METHODS: The BronchoBoy manikin, the Koken manikin, a human cadaver, and a preserved porcine lung were included in the study. Seven physicians experienced in bronchoscopy performed a bronchoscopic airway inspection, bronchoalveolar lavage (BAL), and tissue sampling on all four models with performance evaluated by observation and participant evaluation of models by questionnaire. RESULTS: Nineteen segments were identified in all human anatomy models, and the only significant difference found was that only the Thiel embedded cadaver allowed all participants to enter RB1 with an instrument in the working channel (p = 0.001). The Thiel embedded cadaver and the BronchoBoy manikin had low fluid return on BAL (22 and 52 ml), whereas the Koken manikin and the preserved porcine lung had high return (132 and 134 ml), (p = 0.017). Tissue samplings were only completed in the preserved porcine lung and the Thiel embedded cadaver (p < 0.001). CONCLUSIONS: An anatomically correct bronchoscopy is best simulated with the Koken manikin or the Thiel embedded cadaver. Bronchoalveolar lavage should be simulated with the Koken manikin or the preserved porcine lung. Tissue sampling procedures are best simulated using the Thiel embedded cadaver or the preserved porcine lung.

The metabolic cost of inspiratory muscle training in mechanically ventilated patients in critical care.

BACKGROUND: Diaphragmatic dysfunction is well documented in patients receiving mechanical ventilation. Inspiratory muscle training (IMT) has been used to facilitate weaning by strengthening the inspiratory muscles, yet the optimal approach remains uncertain. Whilst some data on the metabolic response to whole body exercise in critical care exist, the metabolic response to IMT in critical care is yet to be investigated. This study aimed to quantify the metabolic response to IMT in critical care and its relationship to physiological variables. METHODS: We conducted a prospective observational study on mechanically ventilated patients ventilated for ≥ 72 h and able to participate in IMT in a medical, surgical, and cardiothoracic intensive care unit. 76 measurements were taken on 26 patients performing IMT using an inspiratory threshold loading device at 4 cmH2O, and at 30, 50 and 80% of their negative inspiratory force (NIF). Oxygen consumption (VO2) was measured continuously using indirect calorimetry. RESULTS: First session mean (SD) VO2 was 276 (86) ml/min at baseline, significantly increasing to 321 (93) ml/min, 333 (92) ml/min, 351(101) ml/min and 388 (98) ml/min after IMT at 4 cmH2O and 30, 50 and 80% NIF, respectively (p = 0.003). Post hoc comparisons revealed significant differences in VO2 between baseline and 50% NIF and baseline and 80% NIF (p = 0.048 and p = 0.001, respectively). VO2 increased by 9.3 ml/min for every 1 cmH2O increase in inspiratory load from IMT. Every increase in P/F ratio of 1 decreased the intercept VO2 by 0.41 ml/min (CI - 0.58 to - 0.24 p < 0.001). NIF had a significant effect on the intercept and slope, with every 1 cmH2O increase in NIF increasing intercept VO2 by 3.28 ml/min (CI 1.98-4.59 p < 0.001) and decreasing the dose-response slope by 0.15 ml/min/cmH2O (CI - 0.24 to - 0.05 p = 0.002). CONCLUSIONS: IMT causes a significant load-dependent increase in VO2. P/F ratio and NIF impact baseline VO2. The dose-response relationship of the applied respiratory load during IMT is modulated by respiratory strength. These data may offer a novel approach to prescription of IMT. TAKE HOME MESSAGE: The optimal approach to IMT in ICU is uncertain; we measured VO2 at different applied respiratory loads to assess whether VO2 increased proportionally with load and found VO2 increased by 9.3 ml/min for every 1 cmH2O increase in inspiratory load from IMT. Baseline NIF has a significant effect on the intercept and slope, participants with a higher baseline NIF have a higher resting VO2 but a less pronounced increase in VO2 as the inspiratory load increases; this may offer a novel approach to IMT prescription. Trial registration ClinicalTrials.gov, registration number: NCT05101850. Registered on 28 September 2021, https://clinicaltrials.gov/ct2/show/NCT05101850.

Physiological effects of two driving pressure-based methods to set positive end-expiratory pressure during one lung ventilation.

During one-lung ventilation (OLV), titrating the positive end-expiratory pressure (PEEP) to target a low driving pressure (∆P) could reduce postoperative pulmonary complications. However, it is unclear how to conduct PEEP titration: by stepwise increase starting from zero PEEP (PEEPINCREMENTAL) or by stepwise decrease after a lung recruiting manoeuvre (PEEPDECREMENTAL). In this randomized trial, we compared the physiological effects of these two PEEP titration strategies on respiratory mechanics, ventilation/perfusion mismatch and gas exchange. Patients undergoing video-assisted thoracoscopic surgery in OLV were randomly assigned to a PEEPINCREMENTAL or PEEPDECREMENTAL strategy to match the lowest ∆P. In the PEEPINCREMENTAL group, PEEP was stepwise titrated from ZEEP up to 16 cm H2O, whereas in the PEEPDECREMENTAL group PEEP was decrementally titrated, starting from 16 cm H2O, immediately after a lung recruiting manoeuvre. Respiratory mechanics, ventilation/perfusion mismatch and blood gas analyses were recorded at baseline, after PEEP titration and at the end of surgery. Sixty patients were included in the study. After PEEP titration, shunt decreased similarly in both groups, from 50 [39-55]% to 35 [28-42]% in the PEEPINCREMENTAL and from 45 [37-58]% to 33 [25-45]% in the PEEPDECREMENTAL group (both p < 0.001 vs baseline). The resulting ∆P, however, was lower in the PEEPDECREMENTAL than in the PEEPINCREMENTAL group (8 [7-11] vs 10 [9-11] cm H2O; p = 0.03). In the PEEPDECREMENTAL group the PaO2/ FIO2 ratio increased significantly after intervention (from 140 [99-176] to 186 [152-243], p < 0.001). Both the PEEPINCREMENTAL and the PEEPDECREMENTAL strategies were able to decrease intraoperative shunt, but only PEEPDECREMENTAL improved oxygenation and lowered intraoperative ΔP.Clinical trial number NCT03635281; August 2018; "retrospectively registered".

Physiologic Evaluation of Ventilation Perfusion Mismatch and Respiratory Mechanics at Different Positive End-expiratory Pressure in Patients Undergoing Protective One-lung Ventilation.

BACKGROUND: Arterial oxygenation is often impaired during one-lung ventilation, due to both pulmonary shunt and atelectasis. The use of low tidal volume (VT) (5 ml/kg predicted body weight) in the context of a lung-protective approach exacerbates atelectasis. This study sought to determine the combined physiologic effects of positive end-expiratory pressure and low VT during one-lung ventilation. METHODS: Data from 41 patients studied during general anesthesia for thoracic surgery were collected and analyzed. Shunt fraction, high V/Q and respiratory mechanics were measured at positive end-expiratory pressure 0 cm H2O during bilateral lung ventilation and one-lung ventilation and, subsequently, during one-lung ventilation at 5 or 10 cm H2O of positive end-expiratory pressure. Shunt fraction and high V/Q were measured using variation of inspired oxygen fraction and measurement of respiratory gas concentration and arterial blood gas. The level of positive end-expiratory pressure was applied in random order and maintained for 15 min before measurements. RESULTS: During one-lung ventilation, increasing positive end-expiratory pressure from 0 cm H2O to 5 cm H2O and 10 cm H2O resulted in a shunt fraction decrease of 5% (0 to 11) and 11% (5 to 16), respectively (P < 0.001). The PaO2/FIO2 ratio increased significantly only at a positive end-expiratory pressure of 10 cm H2O (P < 0.001). Driving pressure decreased from 16 ± 3 cm H2O at a positive end-expiratory pressure of 0 cm H2O to 12 ± 3 cm H2O at a positive end-expiratory pressure of 10 cm H2O (P < 0.001). The high V/Q ratio did not change. CONCLUSIONS: During low VT one-lung ventilation, high positive end-expiratory pressure levels improve pulmonary function without increasing high V/Q and reduce driving pressure.

Diffusion capacity of the lung for carbon monoxide - A potential marker of impaired gas exchange or of systemic deconditioning in chronic obstructive lung disease?

Gas exchange impairment is primarily caused by ventilation-perfusion mismatch in chronic obstructive pulmonary disease (COPD), where diffusing capacity of the lungs for carbon monoxide (DLCO) remains the clinical measure. This study investigates whether DLCO: (1) can predict respiratory impairment in COPD, that is, changes in oxygen and carbon dioxide (CO2); (2) is associated with combined risk assessment score for COPD (Global Initiative for Chronic Obstructive Lung Disease (GOLD) score); and (3) is associated with blood glucose and body mass index (BMI). Fifty patients were included retrospectively. DLCO; arterial blood gas at inspired oxygen (FiO2) = 0.21; oxygen saturation (SpO2) at FiO2 = 0.21 (SpO2 (21)) and FiO2 = 0.15 (SpO2 (15)) were registered. Difference between arterial and end-tidal CO2 (ΔCO2) was calculated. COPD severity was stratified according to GOLD score. The association between DLCO, SpO2, ΔCO2, GOLD score, blood glucose, and BMI was investigated. Multiple regression showed association between DLCO and GOLD score, BMI, and glucose level (R (2) = 0.6, p < 0.0001). Linear and multiple regression showed an association between DLCO and SpO2 (21) (R (2) = 0.3, p = 0.001 and p = 0.03, respectively) without contribution from SpO2 (15) or ΔCO2. A stronger association between DLCO and GOLD score than between DLCO and SpO2 could indicate that DLCO is more descriptive of systemic deconditioning than gas exchange in COPD patients. However, further larger studies are needed. A weaker association is seen between DLCO and SpO2 (21) without contribution from SpO2 (15) and ΔCO2. This could indicate that DLCO is more descriptive of systemic deconditioning than gas exchange in COPD patients. However, further larger studies are needed.