Pulmonary Atelectasis in General Anaesthesia : Clinical Studies on the Counteracting Effects of Positive End-Expiratory Pressure

Partial lung collapse, i.e., pulmonary atelectasis, is common during general anaesthesia. The main causal mechanism is reduced lung volume with airway closure and subsequent gas absorption from preoxygenated alveoli. Atelectasis impairs oxygenation and forms the pathophysiological basis for postoperative pulmonary complications. Positive end-expiratory pressure (PEEP) counteracts the loss in lung volume, but its role in preventing atelectasis during anaesthesia is not clear. All studies included in this thesis were prospective randomized clinical trials. In the first study, oxygenation was used as a surrogate measure of atelectasis in obese patients undergoing laparoscopic gastric bypass. The subsequent studies used single-slice computed tomography (CT) to evaluate atelectasis in healthy patients undergoing non-abdominal surgery. Paper I: We studied the use of continuous positive airway pressure (CPAP) and PEEP during induction of anaesthesia and a reduced inspired oxygen fraction (FiO2) during emergence. Oxygenation was maintained in the group that received CPAP during induction, followed by a PEEP of 10 cmH2O. Postoperative oxygenation was impaired in the group that received a high FiO2 during emergence. Paper II: An early oxygen washout manoeuvre to quickly restore nitrogen levels and thus stabilize the alveoli, had no effect on atelectasis at the end of surgery. Both study groups exhibited small atelectasis after being ventilated with a moderate PEEP of 6-8 cmH2O during anaesthesia. Paper III: The effect of PEEP versus zero PEEP on atelectasis formation and oxygenation at the end of surgery was compared. The PEEP group maintained oxygenation better and exhibited less atelectasis than the zero-PEEP group, with atelectasis involving a median 1.8% of total lung area compared with 4.6% in the zero-PEEP group (P = 0.002). Paper IV: Postoperative atelectasis was compared between a group in which PEEP was maintained during emergence preoxygenation with FiO2 1.0 and a group in which PEEP was withdrawn just before the start of emergence preoxygenation with FiO2 1.0. The two groups had small atelectasis when fully awake at 30 min after extubation, with no statistically significant difference between them.   In conclusion, preserved end-expiratory lung volume is the key to avoiding atelectasis, in particular when an increased oxygen reserve is required during airway manipulation. PEEP is both necessary and sufficient to minimize atelectasis in healthy patients undergoing non-abdominal surgery

Characteristics and definitive outcomes of COVID-19 patients admitted to a secondary hospital intensive care unit in Sweden

Background and Aims Most published reports of COVID-19 Intensive Care Unit (ICU) patients are from large tertiary hospitals and often present short-term or incomplete outcome data. There are reports indicating that ICUs with fewer beds are associated with higher mortality. This study aimed to investigate the definitive outcome and patient characteristics of the complete first wave of COVID-19 patients admitted to ICU in a secondary hospital. Methods In this prospective observational study, all patients with respiratory failure and a positive SARS-CoV-2 test admitted to Vasteras Hospital ICU between 24 March and July 22, 2020 were included. The primary outcome was defined as 90-day mortality. Secondary outcomes included ICU length of stay, hospital length of stay, number of days with invasive ventilation, need for vasopressors/inotropes, and use of renal replacement therapy. Results Fifty-three patients were included. Median age (range) was 59 (33-76) and 74% were men. Obesity and hypertension were the most common comorbidities and 45% of the patients were born outside Europe. Ninety-day mortality was 30%. Median ICU length of stay (interquartile range) was 14 (5-24) days and the duration of invasive mechanical ventilation 16 (12-26) days. No patients received dialysis at 90-day follow-up. Conclusion In this cohort of COVID-19 patients treated in a secondary hospital ICU, mortality rates were low compared to early studies from China, Italy, and the United States, but similar to other government-funded hospitals in Scandinavia. A preparatory reorganization enabled an increase in ICU capacity, hence avoiding an overwhelmed intensive care organization

Minimizing atelectasis formation during general anaesthesia-oxygen washout is a non-essential supplement to PEEP

Background: Following preoxygenation and induction of anaesthesia, most patients develop atelectasis. We hypothesized that an immediate restoration to a low oxygen level in the alveoli would prevent atelectasis formation and improve oxygenation during the ensuing anaesthesia. Methods: We randomly assigned 24 patients to either a control group (n=12) or an intervention group (n=12) receiving an oxygen washout procedure directly after intubation. Both groups were, depending on body mass index, ventilated with a positive end-expiratory pressure (PEEP) of 6-8 cmH(2)O during surgery. The atelectasis area was studied by computed tomography before emergence. Oxygenation levels were evaluated by measuring blood gases and calculating estimated venous admixture (EVA). Results: The atelectasis areas expressed as percentages of the total lung area were 2.0 (1.5-2.7) (median [interquartile range]) and 1.8 (1.4-3.3) in the intervention and control groups, respectively. The difference was non-significant, and also oxygenation was similar between the two groups. Compared to oxygenation before the start of anaesthesia, oxygenation at the end of surgery was improved in the intervention group, mean (SD) EVA from 7.6% (6.6%) to 3.9% (2.9%) (P=.019) and preserved in the control group, mean (SD) EVA from 5.0% (5.3%) to 5.6% (7.1%) (P=.59). .Conclusion: Although the oxygen washout restored a low pulmonary oxygen level within minutes, it did not further reduce atelectasis size. Both study groups had small atelectasis and good oxygenation. These results suggest that a moderate PEEP alone is sufficient to minimize atelectasis and maintain oxygenation in healthy patients