Diaphragmatic dysfunction is a potential cause of postcardiac surgery pulmonary complications, mainly induced by phrenic nerve injury, when intrapericardial ice is used.1 Its incidence has been reported to be about 20%1 and the left phrenic nerve is the one commonly involved due to its intrapericardial course.1 Ultrasound evaluation of diaphragmatic thickness has been reported to be particularly effective to assess muscle strength2; thickening fraction, in particular, correlates with respiratory effort signs.2–5
We designed the present prospective, observational study to evaluate effectiveness and feasibility of diaphragmatic thickness ultrasound evaluation to assess and monitor the diaphragmatic dysfunction in patients recovering from cardiac surgery. It was conducted at ‘Città della Salute e della Scienza’ Hospital in Turin, Italy (local ethic committee approval No. 0089149, 15 September 2016). Written informed consent was obtained from all the enrolled patients.
Consecutive patients scheduled for cardiac surgery from November 2016 to January 2017 were evaluated for enrolment. Exclusion criteria were age below 18 years and presence of sedation at the time of evaluation.
Diaphragmatic function was assessed bedside in the 24 h before elective surgery (TPRE) and within 1 week after surgery (TPOST) by two physicians (AM and MG) following proper training. All patients underwent a pain numeric rating scale evaluation before ultrasound examination. All the images were anonymously collected in a database and analysed separately and blindly by two other physicians (AC and EP) with expertise in ultrasound to assess interobserver reproducibility. Ultrasound evaluation of the diaphragm was performed bedside in the semirecumbent position,2,5 using a US machine (Mylab™-seven; Esaote S.p.A, Genova, Italy) equipped with a high-resolution (5 to 15 MHz) linear probe. The transducer was placed in the eighth or ninth right and left intercostal space, perpendicularly to the chest wall, between antero-axillary and mid-axillary line.
Diaphragmatic thickness at the end of a normal expiration (DT FRC) and at total lung capacity (DT TLC) were measured as the distance between pleural and peritoneal lines on both right and left hemidiaphragm in the zone of apposition. Thickening fraction (TF) was estimated as TF = (DT TLC − DT FRC)/(DT FRC) × 100. Right and left DT FRC, DT TLC and TF values were obtained at TPRE and at TPOST. All the patients were in spontaneous breathing at the time of ultrasound evaluation. We defined as normal a TF higher than 30%.3,6,7 Measurements were obtained as single values, evaluated posthoc on a static image (the best from several consecutive breaths at FRC and TLC). Data about indication for continuous positive airway pressure (CPAP) after extubation were collected. Descriptive data were tested for normal distribution by Shapiro–Wilk test and presented as mean (± SD) or median [with interquartile range] for continuous variables, and as numbers and percentages for categorical variables.
Data analysis was performed for continuous parametric variables with paired or unpaired t test, as appropriate. Categorical variables were analysed with Fisher's exact test. Inter-observer agreement between reviewers was evaluated using Cohen κ with associated 95% confidence interval. Statistical analyses were conducted using Stata 13.1/SE (Stata-Corp, College Station, Texas, USA).
Twenty-four consecutive patients were enrolled. The median age was 71 (66 to 76) years and the median BMI was 25 (24 to 28) kg m−2. Male : female ratio was 1 : 7. The median pre-operative left ventricle (LV)'s ejection fraction (EF) was 62% (55 to 65). Patients were evaluated at TPOST at a median of 2 (1 to 3) days after cardiac surgery. The median pain rating scale at TPOST was 1 (1 to 3).
Indications for surgery were valvulopathy, coronary artery disease, aortic disease or LV aneurysm in 21 (88%), four (17%), four (17%) and one (4%) of patients, respectively. The surgical approach was sternotomic, mini-sternotomic or right mini-thoracotomic in 11 (46%), four (17%) and nine (37%) of cases, respectively.
Mean diaphragmatic thickness at TLC significantly decreased from TPRE to TPOST for right and left hemidiaphragms (from 3.0 ± 1.0 to 2.3 ± 0.7 mm, P = 0.003 and from 3.0 ± 1.2 to 2.4 ± 1.1 mm, P = 0.047, respectively). Right and left TF significantly decreased at TPOST (from 78 ± 50 to 51 ± 26%, P = 0.040 and from 79 ± 44 to 60 ± 38%, P = 0.016, respectively). Thickening fraction at TPOST was below the pre-established threshold value for diaphragmatic dysfunction in four out of 19 patients (21%) and below the pre-established threshold value for left diaphragmatic dysfunction in six out of 24 patients (25%).
Interobserver agreement was measured on 189 static images (Cohen's κ for TPRE and TPOST were 0.83 and 0.80, respectively).
The sternotomic approach was significantly associated with a higher reduction in left TF at TPOST compared with mini-invasive techniques (78 ± 39 vs. 39 ± 26%, respectively; P = 0.0106). Four patients underwent CPAP after extubation: three of them showed unilateral left diaphragmatic dysfunction, whereas only one developed bilateral diaphragmatic dysfunction. Left TF at TPOST was significantly lower in patients who needed CPAP after extubation (67 ± 34 vs. 16 ± 8%, respectively; P = 0.007).
We found a more severe reduction of left diaphragmatic TF in patients undergoing surgical sternotomic approach, probably due to ice-cardioplegia, that can be a risk factor for intra-operative phrenic nerve injury.1 It follows that mini-invasive surgical approaches, might have a reduced impact on diaphragmatic function, leading for example to a reduced need for rescue therapies after extubation, such as CPAP.
In the current study, using the threshold value for TF of 30%,3,6,7 we identified 21 and 25% right and left diaphragmatic dysfunction, respectively. All the patients who underwent CPAP treatment had unilateral or bilateral diaphragmatic dysfunction. Our study has some limitations. First, we did not plan any follow-up to monitor diaphragmatic dysfunction in the long-term period. Secondly, in five patients, examination of the right hemidiaphragm was not feasable due to surgical dressings and chest tubes. Thirdly, patients were evaluated within a slightly broad time interval of 1 week after surgery. Lastly, sample size was too small to speculate either on possible surgical and clinical variables playing a role as risk factors for diaphragmatic dysfunction.
In conclusion, this study showed the feasibility of bedside point-of-care diaphragmatic ultrasound in the cardiac surgical field for the evaluation of postoperative diaphragmatic dysfunction.
Acknowledgements relating to this article
Assistance with the letter: we would like to thank Prof M Rinaldi, Prof M Boffini and Dr AC Trompeo for their clinical and academic support and Dr C Filippini for her support in data analysis and interpretation.
Financial support and sponsorship: none.
Conflicts of interest: none.
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* Daniela Pasero and Andrea Costamagna contributed equally to the letter.