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Size Matters; Can we improve intra-operative ventilation?


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Size matters; how can we IMPROVE intra-operative ventilation?

Dr Lavanya Raman & Dr Munita Grover, Northwick Park Hospital


High tidal volumes were traditionally used to ventilate patients in theatre in an attempt to prevent atelectasis and hypoxaemia.  We now know that high tidal volumes are associated with volutrauma, biotrauma and higher mortality.  Lung protective ventilation (LPV) using a tidal volume (VT) of 6-8mL/Kg ideal body weight (IBW) is recommended in the intensive care unit (ICU) and theatres to reduce the incidence of pulmonary complications.

The aim of this audit was to assess the extent to which LPV is used in theatres in a busy district general hospital and to identify whether certain patient groups are at higher risk of over ventilation.  This was then reassessed following the implementation of measures to promote adherence to the recommendations.    


• Anaesthetists completed questionnaires for all patients undergoing general anaesthesia at Northwick Park Hospital over  1 week.  
• Demographics, actual body weight (ABW), height, American Society of Anesthesiologists (ASA) score, and procedural information were recorded. 
• Ventilatory parameters included the ventilation mode, VT, and positive end expiratory pressure (PEEP). 
• The body mass index (BMI), IBW (Devine’s formula) and VT (expressed in mL/Kg of ABW and IBW) were calculated for each patient.  
•A Mann Whitney U test was used to compare IBW and PBW and a Chi squared test was used to identify an association between VT and other variables.
•The results were presented at a clinical governance meeting
• Aide memoirs in the form of IBW and tidal volume charts were constructed and displayed in each theatre. 
•Anaesthetic staff educated in lung protective ventilation. 
•The audit was repeated to ascertain  if there had been a change in practice.
•129 patients were included; 65 males and 64 females
• Mean age was 51
• Majority of patients ASA 2 (42%)
• 73 patients were overweight (BMI ≥25). 
• 88% patients received PEEP. 
• IBW was calculated in the 106 patients who had a documented height and was significantly lower than ABW (61Kg [54-71] vs 72Kg [62-85] p<0.05). 
• VT was significantly higher when calculated from IBW than ABW (8.7mL/Kg [7.1-9.3] vs 7.5mL/Kg [5.8-7.9] p<0.05).
• 500mL was the most commonly set VT 
• 52 patients (49%) received LPV with VT of <8mL/Kg IBW in accordance with the recommendations.  51% were over ventilated
Significantly more females (75%) received VT ≥8ml/kg than males (29%) (p<0.01) 
•96 patients;  47 males, 49 females
•Mean age 64
•Majority patients ASA 2
•58 patients were overweight
•91% received PEEP. 
•IBW was again significantly lower than ABW (65Kg [52-73] vs 78Kg [69-90] p<0.05) and
•VT was significantly higher when calculated from IBW than ABW (7.9mL/Kg vs 7.2mL/Kg p<0.05)
•500mL still the most commonly set VT 
•64 patients (67%) received LPV with VT of <8mL/Kg IBW.  33% were over ventilated
•This was significantly higher than in the initial audit (p<0.05). 
•The VT (7.2mL/Kg IBW) was significantly lower than in the initial audit (p<0.05)
Significantly more females (45%) received VT ≥8ml/kg than males (21%) (p<0.05) 
•Following the intervention, significantly more patients were ventilated using a LPV strategy (67% vs 49%) (p<0.05)
•VT was independent of age, ASA, BMI, speciality, position or ventilation mode both before and after the intervention
• Over half of the patients received VT ≥8ml/kg IBW. In the initial audit
• Females were more likely than males to be over ventilated with VT ≥8ml/kg
•Following staff education and the provision of VT/IBW charts the proportion of patients being ventilated using LPV improved,
•however 33% of patients were still receiving VT’s above the recommended volume
•Females still more likely to be overventilated than males
• A likely contributing factor is the disparity between ABW and IBW in this cohort
• Another possible explanation for this is  a lack of knowledge or support of the IMPROVE study amongst anaesthetists. 
• Whilst LPV is well established in the ICU, it is less consistently implemented in the theatre setting. 
• In patients undergoing laparoscopy high VT ≥8ml/kg  may be used to prevent hypercarbia
• 500ml is the default setting on the anaesthetic machine and was the most commonly used VT

Ongoing and repeated education about LPV, particularly targeting new doctors, perhaps in the form of online modules is necessary to promote awareness of and adherence to the recommendations.

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