A large proportion of dogs undergoing computed tomography are anaesthetised and receive concurrent supplementary oxygen. Both factors promote the development of pulmonary atelectasis, which may mask or mimic lung pathology and compromise image quality, which is of concern to the radiologist. The aim of the study was firstly to determine whether significant atelectasis would develop using a commonly employed anaesthetic protocol in a typical hospital setting, especially where dogs may have been anaesthetised in lateral recumbency prior to scanning. Secondly, to determine whether a change in body position to sternal recumbency would be sufficient to resolve atelectasis. Six healthy adult Beagles were anaesthetised in sternal recumbency and using a breath-hold technique, baseline helical transverse thoracic images were acquired. Dogs were then placed in either right or left lateral recumbency for 30 minutes, with scans performed at predetermined lung lobe locations and time intervals. Dogs were then repositioned in sternal recumbency for a further 20 minutes, with similar scans performed. The study was repeated two weeks later in the opposite lateral recumbency. Changes in Hounsfield units and cross-sectional area of all lung lobes were measured. Lateral recumbency did not result in true atelectasis in healthy Beagles of normal body condition. Infrequently, patchy increased attenuation, which failed to resolve completely during sternal recumbency, was visualised in the left cranial lobe during left lateral recumbency. The degree of attenuation changes in healthy Beagles was minimal, and thus if dogs were anaesthetised in lateral recumbency prior to computed tomography, this should not preclude scanning. / Dissertation (MMedVet)--University of Pretoria, 2016. / Companion Animal Clinical Studies / MMedVet / Unrestricted

Formation of atelectasis, defined as reversible collapse of aerated lung, often occurs after induction of anaesthesia with mechanical ventilation. As a consequence, there is a risk for hypoxemia, altered hemodynamics and impaired respiratory system mechanics. In certain situations, the risk for atelectasis formation is increased and its consequences may also be more difficult to manage. Anesthesia for bariatric surgery in morbidly obese patients and surgery requiring one-lung ventilation (OLV) with capnothorax are examples of such situations. In Paper I (30 patients with BMI > 40 kg/m2 scheduled for bariatric surgery) a recruit­ment maneuver followed by positive end-expiratory pressure (PEEP) re­duced the amount of atelectasis and improved oxygenation for a prolonged period of time. PEEP or a recruitment maneuver alone did not reduce the amount of atelectasis. In paper II we investigated whether it is possible to predict respiratory function impairment in morbidly obese patients without pulmonary disease from a preoperative lung function test. Patients with mild signs of airway obstruction (reduced end-expiratory flow) in the preoperative spirometry developed less atelectasis during anaesthesia. In paper III we developed an experimental model of sequential OLV with capnothorax using electrical impedance tomography (EIT) that in real-time detected lung separation and dynamic changes in pulmonary ventilation and perfusion distributions. OLV to the left side caused a decrease in cardiac output, arterial oxygenation and mixed venous saturation. In paper IV we used our model of OLV with capnothorax and applied a CO2-insufflation pressure of 16 cm H2O. We demonstrated that a PEEP level of 12-16 cm H2O is needed for optimal oxygenation and lowest possible driving pressure without compromising hemodynamic variables. Thus, the optimal PEEP was closely related to the level of the capnothorax insufflation pressure. With insufficient PEEP, ventilation/perfusion mismatch in the ventilated lung and redistribution of blood flow to the non-ventilated lung occurred.

Os prematuros frequentemente necessitam de ajuda para iniciar a respiração, pois líquido amniótico residual e deficiência de surfactante podem impedir o estabelecimento da CRF. A necessidade de intubação e do uso de ventilação com pressão positiva está relacionada à chamada lesão pulmonar induzida pela ventilação. A intubação endotraqueal é uma ocorrência comum na unidade de terapia intensiva neonatal. A extubação acidental (EA) ou o deslocamento acidental do tubo endotraqueal é um evento inseguro e adverso. Um estudo norteamericano relatou que a EA é o quarto evento adverso mais comum na UTIN. Isso coloca o neonato em risco de morbidade significativa. As hipóteses iniciais para este estudo considerando as características locais do serviço, são que a taxa de extubação acidental deste serviço seria superior ou igual à encontrada na literatura e que as consequências da perda de volume pulmonar causada pela EA geraria uma necessidade de aumento de parâmetros ventilatórios, expondo esses pacientes a um maior risco de desenvolvimento de lesão pulmonar induzida pela ventilação mecânica. O presente estudo teve como objetivo estabelecer a densidade de extubação acidental para cada 100 pacientes-dia e os fatores de risco e as consequências associadas à extubação acidental em uma unidade de terapia intensiva neonatal. A coleta de dados ocorreu o período de fevereiro a dezembro de 2016. Foram elegíveis todos os recém-nascidos intubados com necessidade de ventilação mecânica invasiva e selecionados, para o estudo, os recém-nascidos em assistência ventilatória há pelo menos 12 horas. Nos 10 meses e meio de estudo (318 dias), observou-se uma média de 9,04 pacientes intubados por dia, sendo analisados 2875 pacientes-dia com via aérea artificial. Nos 318 dias de observação ocorreram 45 extubações acidentais, com taxa de EA de 1,57 para cada 100 dias intubação. Os recém-nascidos tinham em média 26,55±35,35 dias de vida na EA, mínimo de 3 dias e máximo de 182 dias. Na amostra estudada, todos os recém-nascidos necessitaram de reintubação após o evento de EA. As características clínicas foram semelhantes nos dois grupos. Assim, os dados apresentados evidenciam que os grupos estudados se apresentaram bastante homogêneos. Foram associados como fatores de risco para EA a utilização de material inadequado para a fixação da cânula orotraqueal (esparadrapo), a carência de pessoal para os cuidados diários (mais de 2 pacientes por funcionário), e mostrou-se ainda a grande importância dos alarmes de saturação sanguínea já que foram responsáveis por 60% diagnóstico da ocorrência do evento. A EA acarretou piora das condições respiratórias com necessidade de aumento do suporte ventilatório ( maior necessidade de oxigênio, pressão inspiratória e PEEP) e recuperação lenta às condições pré- acidente em média 5,7 dias. Concomitante com o aumento da necessidade de suporte respiratório houve maior ocorrência de hiperóxia após o acidente, levantando o temor de efeitos deletérios sinérgicos sobre o pulmão já anteriormente debilitado dos prematuros. É necessária a conscientização da administração e das equipes de profissionais da saúde das UTINs unidades, da enorme importância de se prevenir e diagnosticar precocemente a extubação acidental. / Premature infants often require help in initiating breathing, since residual amniotic fluid and surfactant deficiency may prevent the establishment of functional residual capacity. The need for intubation and the use of positive pressure ventilation is related with ventilator-induced lung injury. Endotracheal intubation is a common occurrence in the neonatal intensive care unit. Accidental extubation (AE) or accidental displacement of the endotracheal tube is an unsafe and adverse event. An American study has reported that EA is the fourth most common adverse event in the NICU. This puts the neonate at risk of significant morbidity. The initial hypotheses for this study considering the local characteristics of the service are that the rate of accidental extubation of this service would be higher than or equal to that found in the literature and that the consequences of loss of lung volume caused by the AE would generate a need for increased ventilatory parameters , exposing these patients to a higher risk of lung injury induced by mechanical ventilation. The present study aimed to establish the accidental extubation density for each 100 patients per day and the risk factors and consequences associated with accidental extubation in a neonatal intensive care unit. Data were collected from February to December 2016. All intubated newborns requiring invasive mechanical ventilation were eligible and the newborns in ventilatory assistance were selected for at least 12 hours for the study. In the 10 and a half months of study (318 days), an average of 9.04 intubated patients per day was observed, with 2875 day-patients with artificial airway being analyzed. In the 318 days of observation, there were 45 accidental extubations, with an AE rate of 1.57 for every 100 days of intubation. The newborns had a mean of 26.55±35.35 days of life in AE, a minimum of 3 days and a maximum of 182 days. In the studied sample, all newborns required reintubation after the AE event. As constitutional aspects were seem in the two groups. Thus, the presented data evidences that the studied groups are quite homogeneous. Risk factors for the use of materials for the fixation of the orotracheal waist (tape), a shortage of personnel for the longest care, and the greater importance of the alarms were included. of blood saturation that was responsible for 60% of the occurrence of the event. AE is one of the largest public health service companies in the world, inspiratory pressure and PEEP, and average recurring expenses an average of 5.7 days. Concomitant with the increased risk of respiratory accident for most cases following the occurrence of an accident, raising the risk of fire and synergy over the previous exercise already weakened of the premature. Immigration is an awareness of NICU management and staff of health professionals, which is important in preventing and early diagnosis of accidental extubation.

A tomografia computadorizada (TC) é o método de diagnóstico de eleição para a avaliação dos campos pulmonares. Porém, a atelectasia pulmonar ou opacificação pulmonar dependente da gravidade, observada em indivíduos submetidos à anestesia ou a longos períodos de decúbito, pode aumentar o risco de se obscurecer lesões pulmonares, como as metástases. O objetivo deste trabalho foi comparar, através do uso da TC, o efeito dos decúbitos dorsal e esternal na formação de opacificação pulmonar gravitacional dependente em cães anestesiados. Foram utilizados dezesseis cães adultos (Canis familiaris), da raça Rottweiller, clinicamente normais. Estes animais foram divididos em quatro grupos, submetidos a dois exames tomográficos, em decúbito dorsal e em decúbito esternal, em diferentes tempos anestésicos: imediatamente após a indução anestésica, 20 minutos após a indução anestésica, 40 minutos e 60 minutos após a indução anestésica. Foram realizados quatro cortes transversais, em pontos pré-determinados do tórax. Foi determinada a densidade pulmonar média setorial, a densidade em unidades de Hounsfield (HU) e área em cm² nas áreas de opacificação pulmonar gravitacional dependente formadas. A densidade média setorial demonstrou que o decúbito dorsal apresentou significante gradiente de atenuação das porções não dependentes para as porções dependentes do tórax, enquanto que no decúbito esternal tal comportamento não foi observado. Com relação às áreas de opacificação pulmonar gravitacional dependente, estas foram observadas em 43,75% dos animais em decúbito dorsal e em 18,75% dos cães em decúbito esternal, com atenuações de –43,95 a –361,94 HU para ambos os decúbitos, área mínima de 0,03 e máxima de 1,55 cm² no decúbito dorsal, e 0,24 a 0,70 cm² no decúbito esternal / Computed tomography (CT) is a diagnostic method of election to the lungs evaluation. However, the pulmonary atelectasis or gravitacional dependent pulmonary densities, wich is observed during general anesthesia or long-term of recumbency, could increase the risk of obscuring pulmonary lesions, such metastasis. The aim of this study was to compare the recumbency effect´s (supine and prone), by CT, on the formation of pulmonary densities in anesthetized dogs. Sixteen healthy Rottweiler dogs (Canis familiaris) were used. These dogs were divided into four groups, wich performed dorsal and prone CT exams each, with differents periods of anesthesia: immediately after induction, 20 minutes after induction, 40 minutes and 60 minutes after induction. There were performed four transverse CT scans, in differents sites of the chest. There were determined mean lung density (sector method), pulmonary densities in Hounsfield units (HU) and area in cm² of dense areas in dependent lung regions. Mean lung density was demonstrated that the supine recumbency had a significant density gradient from non-dependent to dependent lung regions, and the prone position hadn´t any density gradient. Dense areas were observed in 43,75% of dogs in supine recumbency and in 18,75% of dogs in prone recumbency. Atenuation was between – 43,47 to – 361,94 HU in both recumbencies. Area was between 0,03 to 1,55 cm² in supine position, and 0,24 to 0,70 cm² in prone position

Introdução: A hipoxemia é uma complicação frequente no pós-operatório de cirurgia de revascularização miocárdica (RM) com circulação extracorpórea (CEC), normalmente decorrente da formação de atelectasia pulmonar. O presente estudo visa investigar o papel do coração na perda de aeração dos lobos pulmonares inferiores. Métodos: Dezessete pacientes sem disfunção cardíaca foram submetidos a tomografia computadorizada (TC) pré e pósoperatória em apnéia após expiração normal. A massa cardíaca, a pressão exercida pelas protrusões cardíacas direita e esquerda sobre os segmentos pulmonares subjacentes a elas e a fração de tecido pulmonar atelectasiado dos mesmos segmentos e dos segmentos não subjacentes foram avaliados nos cortes tomográficos 1 cm acima do diafragma. Resultados: A massa cardíaca sofreu um aumento de 32% (117 ± 31g vs. 155 ± 35g; p < 0,001) no pós-operatório, levando a um aumento nas pressões exercidas pelas protrusões cardíacas direita (2,2 ± 0,6 g.cm-2 vs 3,2 ± 1,2 g.cm-2; p < 0,05) e esquerda (2,4 ± 0,7 g.cm-2 vs. 4,2 ± 1,8 g.cm-2; p < 0,001) sobre os segmentos pulmonares subjacentes. A fração média de tecido atelectasiado direita e esquerda abaixo das protrusões aumentou substancialmente no pós-operatório (6,7% para 32,9% e 6,2% para 29%, respectivamente; p < 0,001). Este aumento foi menos pronunciado nas áreas não subjacentes (direito: 0,7% para 10,8%, esquerdo 1,5% para 12,6%; p < 0,001). Conclusão: A pressão exercida pelo coração sobre os pulmões aumentou no período pós-operatório, contribuindo de forma importante para a formação de atelectasias nos segmentos pulmonares subjacentes / Introduction: Atelectasis is a major cause of hypoxemia after onpump coronary artery bypass grafting (CABG). General anesthesia, use of high inspiratory oxygen concentration and cardiopulmonary bypass (CPB) are implicated in the genesis of atelectasis. However, the contribution of pressure exerted by the heart to collapse the underlying lung is unknown. The objective of this study was to evaluate the role of heart-induced pulmonary compression after on-pump CABG. Methods: Seventeen patients without cardiac dysfunction underwent pre and postoperative thoracic computed tomography (CT) in apnea conditions after a normal expiration. The cardiac mass, the pressure exerted on the lungs by the right and left heart segments and the collapsed lung fraction of lower lobe segments below and outside of the heart limits were evaluated in a CT section 1 cm above the diaphragm. Results: Cardiac mass increased by 32% (117 ± 31g vs. 155 ± 35g; p <0.001) postoperatively, leading to an increase in the pressures exerted by right (2.2 ± 0.6 g.cm-2 vs. 3.2 ± 1.2 g.cm-2; p <0.05) and left heart segments (2.4 ± 0.7 g.cm-2 vs. 4.2 ± 1.8 g.cm-2; p <0.001) on the lungs. The median fraction of collapsed right (6.7% to 32.9%; p <0.001) and left (6.2% to 29%; p <0.001) lung parenchyma segments beneath the heart greatly increased postoperatively, while it was less important outside of the heart limits, (right: 0.7% to 10.8; p<0,001 and left: 1.5% to 12.6%; p<0,001). Conclusion: The pressure exerted by the heart on the lungs increased postoperatively, importantly contributing to collapse the subjacent pulmonary segments

Introdução: A disfunção respiratória é uma das complicações de maior prevalência no período pós-operatório de pacientes submetidos à cirurgia de revascularização do miocárdio (RM) com circulação extracorpórea (CEC), mesmo na ausência de doença pulmonar prévia1. Por meio de tomografia computadorizada, foram investigadas as alterações pulmonares pósoperatórias e o seu impacto na oxigenação. Métodos: vinte pacientes não hipoxêmicos em programação de cirurgia eletiva de revascularização do miocárdio com CEC foram estudados. Medidas hemodinâmicas, e amostras sanguíneas seriadas foram obtidas antes da cirurgia, após a intubação orotraqueal, após a CEC, na UTI, 12h, 24h e 48h após a cirurgia. Tomografias volumétricas pré e pós-operatórias foram adquiridas em condições de apneia após uma expiração espontânea. Os dados foram analisados usando teste de t Student; o comportamento temporal dos dados hemodinâmicos e outras variáveis fisiológicas foi analisado ao longo do tempo, usando análise de variância de uma via para repetidas medidas, seguido pelo teste de Student-Neumann-Keuls, quando necessário.Resultados: a relação PaO2/FiO2 diminuiu de forma significativa após a indução da anestesia, atingindo o seu nadir após a saída de CEC. Comparando-se com a TC pré-operatória, foi observada uma redução de 31% no volume de gás pulmonar (p<0,001), ao passo que foi observado um aumento do volume de tecido de 19% (p<0,001). A área não aerada dos pulmões aumentou de 253 97 g(p < 0.001), de 3 % a 27 %, após a cirurgia; e a área pobremente aerada pulmonar apresentou aumento de 72 68 g (p < 0.001), de 24 % to 27 %, enquanto a área normalmente aerada pulmonar reduziu de 147 119 g (p < 0.001), que representa 72 % to 46%. Não foram observadas correlações entre a relação PO2/FIO2 ou na fração de shunt nas primeiras 24 horas pós-operatórias Conclusões: A estrutura pulmonar está profundamente modificada após a cirurgia de RM com CEC. Associadas a outros fatores, essas alterações são responsáveis pela ocorrência de hipoxemia resultantes de atelectasias / Introduction: Hypoxemia is a frequent complication after coronary artery bypass graft (CABG) with cardiopulmonary bypass (CPB), usually attributed to atelectasis. Using computed tomography (CT), we investigated pulmonary alterations postoperatively and its impact on oxygenation. Methods: Twenty non-hypoxemic patients with normal cardiac function scheduled to CABG under CPB were studied. Hemodynamic measurements and blood samples were obtained before surgery, after intubation, after CPB, at ICU admission, 12h, 24h and 48h after surgery. Pre and postoperative volumetric thoracic CT scans were acquired in apnea conditions after a spontaneous expiration. Data was analyzed using paired Student t test and two-way repeated measures analysis of variance followed by SNK test when indicated. Results: PaO2/FiO2 ratio was significantly reduced after anesthesia induction, reaching its nadir after CPB and partially ameliorating 12h after surgery. Compared to preoperative CT, there was a postoperative 31 % reduction in pulmonary gas volume (p < 0.001) while tissue volume increased by 19 % (p < 0.001). Non-aerated lung increased by 253 97 g(p < 0.001), from 3 % to 27 %, after surgery and poorly-aerated lung by 72 68 g (p < 0.001), from 24 % to 27 % while normally-aerated lung was reduced by 147 119 g (p < 0.001), from 72 % to 46 %. No correlations were observed between PaO2/FiO2 ratio or shunt fraction at 24 h postoperatively and postoperative lung alterations. Conclusions: Lung structure is profoundly modified after CABG with CPB. Taken together, these multiple alterations occurring in the lungs are responsible for postoperative hypoxemia instead of atelectasis alone

Durante a gravidez o sistema respiratório sofre modificações. Além da elevação do diafragma, provavelmente secundária ao aumento do volume uterino, estudos indicaram que, com a progressão da gestação algumas mudanças em volumes e capacidades pulmonares ocorrem como redução na capacidade residual funcional, volume de reserva expiratório, volume corrente e da capacidade inspiratória. Uma das hipóteses para explicar esses achados é que a elevação do diafragma pode causar colapso pulmonar, que pode ser pior em mulheres que realizaram parto cesariana (PC), quando comparadas as que tiveram parto vaginal (PV). Este estudo avaliou 20 mulheres, 2 a 5 horas após o parto em um Hospital Universitário. Após a obtenção do consentimento livre e esclarecido, 20 mulheres saudáveis, não-fumantes, onde 10 mulheres haviam realizado PV e 10 haviam realizado PC, após o parto foram submetidos à tomografia computadorizada torácica. A imagem foi analisada por um sistema de software (Osíris). A média de atenuação de cada fatia foi expresso em unidades Hounsfield (UH) e medido o percentual da área de colapso pulmonar (em g%). No grupo PV a média foi de -841,90 UH e 3,95 g%, enquanto que no grupo PC a média foi de -765,95 UH e 14,1g%. Estes resultados sugerem que as mulheres submetidas ao PC têm maior área de colapso pulmonar comparado com mulheres submetidas ao PV / During pregnancy the respiratory system undergoes several modifications. Besides the elevation of diaphragm, probably secondary to the increase in uterine volume, studies indicated that with the progression of the gestation some changes in pulmonary volumes and capacities occur, as a reduction in Functional Residual Capacity, Expiratory Reserve Volume, Tidal Volume and Inspiratory Capacity. One of the hypotheses to explain these findings is that the elevation of diaphragm may cause pulmonary collapse, which may be worse in women who underwent a cesarean section when compared to those who had vaginal delivery. This paper evaluates 20 women, 2 to 5 hours after delivery in a University Hospital. After getting informed consent, 20 healthy non-smoking women, 10 who underwent vaginal delivery (VD) and 10 who underwent a cesarean section (CS), were submitted to a chest computed tomography. The images of two different basal lung levels were analyzed by means of a software system (Osiris). The mean attenuation value of each slice in Hounsfield Units (UH) were measured and the percentage area of collapsed lung (in g%) was calculated. In the VD group the mean attenuation was - 841.90 UH and the collapsed area was 3.95 g%, while in the CS group the mean attenuation was -765.95 UH and the collapsed area was 14.1 g%. These results suggested that women who underwent a cesarean section have more collapse at basal lung levels than women who had a vaginal delivery

Introdução: A disfunção respiratória é uma das complicações de maior prevalência no período pós-operatório de pacientes submetidos à cirurgia de revascularização do miocárdio (RM) com circulação extracorpórea (CEC), mesmo na ausência de doença pulmonar prévia1. Por meio de tomografia computadorizada, foram investigadas as alterações pulmonares pósoperatórias e o seu impacto na oxigenação. Métodos: vinte pacientes não hipoxêmicos em programação de cirurgia eletiva de revascularização do miocárdio com CEC foram estudados. Medidas hemodinâmicas, e amostras sanguíneas seriadas foram obtidas antes da cirurgia, após a intubação orotraqueal, após a CEC, na UTI, 12h, 24h e 48h após a cirurgia. Tomografias volumétricas pré e pós-operatórias foram adquiridas em condições de apneia após uma expiração espontânea. Os dados foram analisados usando teste de t Student; o comportamento temporal dos dados hemodinâmicos e outras variáveis fisiológicas foi analisado ao longo do tempo, usando análise de variância de uma via para repetidas medidas, seguido pelo teste de Student-Neumann-Keuls, quando necessário.Resultados: a relação PaO2/FiO2 diminuiu de forma significativa após a indução da anestesia, atingindo o seu nadir após a saída de CEC. Comparando-se com a TC pré-operatória, foi observada uma redução de 31% no volume de gás pulmonar (p<0,001), ao passo que foi observado um aumento do volume de tecido de 19% (p<0,001). A área não aerada dos pulmões aumentou de 253 97 g(p < 0.001), de 3 % a 27 %, após a cirurgia; e a área pobremente aerada pulmonar apresentou aumento de 72 68 g (p < 0.001), de 24 % to 27 %, enquanto a área normalmente aerada pulmonar reduziu de 147 119 g (p < 0.001), que representa 72 % to 46%. Não foram observadas correlações entre a relação PO2/FIO2 ou na fração de shunt nas primeiras 24 horas pós-operatórias Conclusões: A estrutura pulmonar está profundamente modificada após a cirurgia de RM com CEC. Associadas a outros fatores, essas alterações são responsáveis pela ocorrência de hipoxemia resultantes de atelectasias / Introduction: Hypoxemia is a frequent complication after coronary artery bypass graft (CABG) with cardiopulmonary bypass (CPB), usually attributed to atelectasis. Using computed tomography (CT), we investigated pulmonary alterations postoperatively and its impact on oxygenation. Methods: Twenty non-hypoxemic patients with normal cardiac function scheduled to CABG under CPB were studied. Hemodynamic measurements and blood samples were obtained before surgery, after intubation, after CPB, at ICU admission, 12h, 24h and 48h after surgery. Pre and postoperative volumetric thoracic CT scans were acquired in apnea conditions after a spontaneous expiration. Data was analyzed using paired Student t test and two-way repeated measures analysis of variance followed by SNK test when indicated. Results: PaO2/FiO2 ratio was significantly reduced after anesthesia induction, reaching its nadir after CPB and partially ameliorating 12h after surgery. Compared to preoperative CT, there was a postoperative 31 % reduction in pulmonary gas volume (p < 0.001) while tissue volume increased by 19 % (p < 0.001). Non-aerated lung increased by 253 97 g(p < 0.001), from 3 % to 27 %, after surgery and poorly-aerated lung by 72 68 g (p < 0.001), from 24 % to 27 % while normally-aerated lung was reduced by 147 119 g (p < 0.001), from 72 % to 46 %. No correlations were observed between PaO2/FiO2 ratio or shunt fraction at 24 h postoperatively and postoperative lung alterations. Conclusions: Lung structure is profoundly modified after CABG with CPB. Taken together, these multiple alterations occurring in the lungs are responsible for postoperative hypoxemia instead of atelectasis alone

Background: Atelectasis can provoke pulmonary and non-pulmonary complications after general anaesthesia. Unfortunately, there is no instrument to estimate atelectasis and prompt changes of mechanical ventilation during general anaesthesia. Although arterial partial pressure of oxygen (PaO2) and intrapulmonary shunt have both been suggested to correlate with atelectasis, studies yielded inconsistent results. Therefore, we investigated these correlations. Methods: Shunt, PaO2 and atelectasis were measured in 11 sheep and 23 pigs with otherwise normal lungs. In pigs, contrasting measurements were available 12 hours after induction of acute respiratory distress syndrome (ARDS). Atelectasis was calculated by computed tomography relative to total lung mass (Mtotal). We logarithmically transformed PaO2 (lnPaO2) to linearize its relationships with shunt and atelectasis. Data are given as median (interquartile range). Results: Mtotal was 768 (715–884) g in sheep and 543 (503–583) g in pigs. Atelectasis was 26 (16–47)% in sheep and 18 (13–23) % in pigs. PaO2 (FiO2 = 1.0) was 242 (106–414) mmHg in sheep and 480 (437–514) mmHg in pigs. Shunt was 39 (29–51)% in sheep and 15 (11–20) % in pigs. Atelectasis correlated closely with lnPaO2 (R2 = 0.78) and shunt (R2 = 0.79) in sheep (P-values<0.0001). The correlation of atelectasis with lnPaO2 (R2 = 0.63) and shunt (R2 = 0.34) was weaker in pigs, but R2 increased to 0.71 for lnPaO2 and 0.72 for shunt 12 hours after induction of ARDS. In both, sheep and pigs, changes in atelectasis correlated strongly with corresponding changes in lnPaO2 and shunt. Discussion and Conclusion: In lung-healthy sheep, atelectasis correlates closely with lnPaO2 and shunt, when blood gases are measured during ventilation with pure oxygen. In lung-healthy pigs, these correlations were significantly weaker, likely because pigs have stronger hypoxic pulmonary vasoconstriction (HPV) than sheep and humans. Nevertheless, correlations improved also in pigs after blunting of HPV during ARDS. In humans, the observed relationships may aid in assessing anaesthesia-related atelectasis.

Background: The use of pure oxygen during preoxygenation and induction of general anaesthesia is a major cause of atelectasis. The interaction between reduced lung volume, resulting in airway closure, and varying inspiratory fractions of oxygen (FIO2) in determining the risk of developing atelectasis is still obscure. Methods: In this thesis, computed tomography (in studies I and II during anaesthesia, in studies III and IV postoperatively) was used to investigate the area of atelectasis in relation to FIO2 and varying levels of continuous positive airway pressure (CPAP) or positive end-expiratory pressure (PEEP). Study I investigated the short-term influence of reducing FIO2 during preoxygenation and induction of general anaesthesia, and the time to hypoxia during apnoea. Study II focused on the long-term effect of an FIO2 of 0.8 for preoxygenation. Study III applied CPAP/PEEP with an FIO2 of 1.0 or 0.8 for pre- and postoxygenation until extubation. After extubation, CPAP with an FIO2 of 0.3 was applied before the end of mask ventilation. Study IV compared two groups given CPAP/PEEP during anaesthesia and an FIO2 of 1.0 or 0.3 during postoxygenation, but without CPAP after extubation. Results: Study I showed a reduction in atelectasis with an FIO2 of 0.8 or 0.6, compared with 1.0, but the time to hypoxia decreased. In study II, atelectasis evolved gradually after preoxygenation. In study III, atelectasis was reduced with an FIO2 of 1.0 and CPAP/PEEP compared with an FIO2 of 1.0 without CPAP/PEEP. The intervention failed in the group given an FIO2 of 0.8, this group had more smokers. Atelectasis and age were correlated. In study IV, no difference was found between the groups. Post hoc analysis showed that smoking and ASA class increased the risk for atelectasis. Conclusion, the effect of reducing FIO2 during preoxygenation to prevent atelectasis might be short-lived. A lower FIO2 shortened the time to the appearance of hypoxia. Increasing lung volume by using CPAP/PEEP also decreased the risk of atelectasis, but the method might fail; for example in patients who are heavy smokers. In older patients care must be taken to reduce a high FIO2 before ending CPAP.