Background
Inspiration (breathing in) is produced by the shortening (contraction) of various muscles that stretch the lungs to increase their size like rubber balloons. During this phase, oxygen enters the lungs. When these muscles stop their contractions, the lungs go back to their initial size. During this phase, carbon dioxide goes out of the lungs. When people are cared for under general anaesthesia, some of the drugs used will stop the movements of the muscles controlling lung size. Insufflation is the act of mechanically forcing air into a person's respiratory system. A machine is required to replace the effects of the muscles. A mixture of gas containing oxygen is blown into the lungs. It is actually not known whether it is better to blow small volumes of gas at a higher rate or bigger volumes at a lower rate. In this review, we tried to determine whether this volume should be lower or higher than 10 millilitres per kilogram of body weight.
Study characteristics
We searched medical databases up to 19 May 2017. We included 19 studies with 1548 adults of both sexes. The participants had had operations on the abdomen (tummy), heart, blood vessels of the lungs, back, lower limbs or various surgeries. Two studies mentioned financial support from the pharmaceutical industry or from medical equipment manufacturers. We do not think that this had an effect on the results as high or low volumes may be administered with any machine.
Key results
We did not find a difference in 0- to 30-day mortality (death within one month). We found that using a volume lower than 10 millilitres per kilogram of body weight reduced the risk of pneumonia (lung infection) and increased the chances that people would be able to get back to their normal respiratory status immediately after surgery. Low volumes should be used preferentially during surgery. For every 1000 people operated on, 84 would have pneumonia after the operation if high volumes were used during surgery. This number was reduced to 43 if low volumes were used instead. Likewise, the number of people needing additional non-invasive ventilatory support (through a mask applied to the face) would be reduced from 115 to 36 if volumes lower than 10 millilitres per kilogram of body weight were used during surgery and the need invasive ventilatory support (through a tube inserted in the person's windpipe) would be reduced from 39 to 13. Hospital length of stay may be slightly reduced (equivalent to almost one day). We identified no possible harmful effects of using low volumes.
Reliability of evidence
We judged the reliability of the evidence as moderate for pneumonia and reduced need for ventilatory support (non-invasive or invasive). Results on these three outcomes may be affected with additional data.
We found moderate-quality evidence that low tidal volumes (defined as less than 10 mL/kg) decreases pneumonia and the need for postoperative ventilatory support (invasive and non-invasive). We found no difference in the risk of barotrauma (pneumothorax), but the number of participants included does not allow us to make definitive statement on this. The four studies in 'Studies awaiting classification' may alter the conclusions of the review once assessed.
Since the 2000s, there has been a trend towards decreasing tidal volumes for positive pressure ventilation during surgery. This an update of a review first published in 2015, trying to determine if lower tidal volumes are beneficial or harmful for patients.
To assess the benefit of intraoperative use of low tidal volume ventilation (less than 10 mL/kg of predicted body weight) compared with high tidal volumes (10 mL/kg or greater) to decrease postoperative complications in adults without acute lung injury.
We searched the Cochrane Central Register of Controlled Trials (CENTRAL 2017, Issue 5), MEDLINE (OvidSP) (from 1946 to 19 May 2017), Embase (OvidSP) (from 1974 to 19 May 2017) and six trial registries. We screened the reference lists of all studies retained and of recent meta-analysis related to the topic during data extraction. We also screened conference proceedings of anaesthesiology societies, published in two major anaesthesiology journals. The search was rerun 3 January 2018.
We included all parallel randomized controlled trials (RCTs) that evaluated the effect of low tidal volumes (defined as less than 10 mL/kg) on any of our selected outcomes in adults undergoing any type of surgery. We did not retain studies with participants requiring one-lung ventilation.
Two authors independently assessed the quality of the retained studies with the Cochrane 'Risk of bias' tool. We analysed data with both fixed-effect (I2 statistic less than 25%) or random-effects (I2 statistic greater than 25%) models based on the degree of heterogeneity. When there was an effect, we calculated a number needed to treat for an additional beneficial outcome (NNTB) using the odds ratio. When there was no effect, we calculated the optimum information size.
We included seven new RCTs (536 participants) in the update.
In total, we included 19 studies in the review (776 participants in the low tidal volume group and 772 in the high volume group). There are four studies awaiting classification and three are ongoing. All included studies were at some risk of bias. Participants were scheduled for abdominal surgery, heart surgery, pulmonary thromboendarterectomy, spinal surgery and knee surgery. Low tidal volumes used in the studies varied from 6 mL/kg to 8.1 mL/kg while high tidal volumes varied from 10 mL/kg to 12 mL/kg.
Based on 12 studies including 1207 participants, the effects of low volume ventilation on 0- to 30-day mortality were uncertain (risk ratio (RR) 0.80, 95% confidence interval (CI) 0.42 to 1.53; I2 = 0%; low-quality evidence). Based on seven studies including 778 participants, lower tidal volumes probably reduced postoperative pneumonia (RR 0.45, 95% CI 0.25 to 0.82; I2 = 0%; moderate-quality evidence; NNTB 24, 95% CI 16 to 160), and it probably reduced the need for non-invasive postoperative ventilatory support based on three studies including 506 participants (RR 0.31, 95% CI 0.15 to 0.64; moderate-quality evidence; NNTB 13, 95% CI 11 to 24). Based on 11 studies including 957 participants, low tidal volumes during surgery probably decreased the need for postoperative invasive ventilatory support (RR 0.33, 95% CI 0.14 to 0.77; I2 = 0%; NNTB 39, 95% CI 30 to 166; moderate-quality evidence). Based on five studies including 898 participants, there may be little or no difference in the intensive care unit length of stay (standardized mean difference (SMD) –0.06, 95% CI –0.22 to 0.10; I2 = 33%; low-quality evidence). Based on 14 studies including 1297 participants, low tidal volumes may have reduced hospital length of stay by about 0.8 days (SMD –0.15, 95% CI –0.29 to 0.00; I2 = 27%; low-quality evidence). Based on five studies including 708 participants, the effects of low volume ventilation on barotrauma (pneumothorax) were uncertain (RR 1.77, 95% CI 0.52 to 5.99; I2 = 0%; very low-quality evidence).