ABSTRACT
Background and Aims:
The prone position is one of the common surgical positions used in clinical practice. Manoeuvring patients from supine to a prone position can impact respiratory dynamics and result in haemodynamic variations.
Methods:
This study included 64 patients and was conducted after obtaining approval from the ethics committee and registration of the trial. The primary objective was to evaluate the changes in peak inspiratory pressure (PIP), plateau pressure (Pplat) and mean airway pressure (MAP) in patients undergoing surgery under general anaesthesia in the prone position with (Group S) and without (Group P) spine frame. The secondary objective was to evaluate and compare the variations in heart rate and blood pressure.
Results:
On turning the patient prone, there was statistically significant increase in median PIP (Group S 4 cmH2O vs. Group P 0.5 cmH2O, P < 0.001), Pplat (Group S 3.5 cmH2O vs. Group P 1 cmH2O, P = 0.004) and dynamic compliance (Group S −5.513 vs. Group P −2.78, P < 0.004).
Conclusions:
Our study found that prone positioning with a spine frame led to a significantly greater increase in airway pressures and a decrease in dynamic compliance when compared to patients positioned prone without the spine frame.
Keywords: Lung compliance, nephrolithotomy, patient positioning, prone position, respiratory dynamics, spine surgery, spine frame
INTRODUCTION
Prone positioning is one of the common positions used to facilitate surgical access, but changes in body positioning can have a marked impact on the patient’s physiology. In patients undergoing surgery under general anaesthesia using neuromuscular blocking drugs with controlled ventilation, changing the posture from a supine to a prone position alters the patient’s respiratory dynamics and haemodynamics. There are many studies which have found that prone position with general anaesthesia increases the airway pressure and reduces pulmonary and thoracic compliance.[1-6] In the prone position, abdominal contents are displaced upwards, and as the abdominal pressure increases, part of it can be transmitted to the chest, impairing compliance. This reduction in airway compliance is characterised by high airway pressures to accomplish adequate ventilation. The high airway pressures may, in turn, impair venous return to the heart, decreasing the cardiac output and increasing systemic venous pressure.[7]
To overcome these variations, conventionally, a bolster is placed below the chest and pelvis to reduce the abdominal pressure and its allied alterations.[8] Spine surgeons favour using spine frames to accomplish it and allow suitable positioning for spinal surgeries. But in percutaneous nephrolithotomy (PCNL) surgeries, the urologists do not opt for any support as it hinders surgical access.
The primary objective of this study was to measure and compare the changes in peak inspiratory pressure (PIP), plateau pressure (Pplat) and mean airway pressure (MAP) in the prone position with and without a spine frame. The secondary objective was to compare the changes in heart rate (HR) and blood pressure in the prone position with and without a spine frame.
METHODS
This prospective observational study was carried out from December 2019 to September 2020 after getting clearance from the institutional ethical committee (vide approval number ECR/215/Inst/KA/2013/RR-16 dated 27 October 2018). The study was registered with Central Trial Registry-India (vide registration number CTRI/2018/12/016592 available at https://ctri.nic.in). This study was carried out following the principles of the Declaration of Helsinki, 2013. The study included American Society of Anesthesiologists (ASA) physical status I and II patients of either gender aged between 18 and 65 scheduled for elective surgery in the prone position. Patients with severe debilitating pulmonary disease or having significant spinal deformities, scoliosis, lordosis, kyphosis, ankylosing spondylitis, pregnant and lactating mothers, multiple attempts at intubation and post-intubation bronchospasm were excluded from the study.
A thorough pre-anaesthetic evaluation was done. The patient’s consent was obtained for participation in the study and the use of the patient data for research and educational purposes after explaining the study protocol.
The recruited participants were allotted to two groups: Group S – patients undergoing spine surgeries with spine frame in the prone position and Group P – patients undergoing urological surgeries without spine frame in the prone position.
In the operating theatre, the patient monitoring included an electrocardiogram, non-invasive blood pressure (NIBP), HR and oxygen saturation (SpO2). After pre-oxygenation for 3 min, anaesthesia was induced with intravenous fentanyl 2 μg/kg and propofol 1.5–2 mg/kg until the loss of verbal response. Vecuronium of 0.1 mg/kg was used for neuromuscular blockade; mask ventilation was continued for 3 min. Subsequently, the trachea was intubated with a cuffed endotracheal tube of 8/8.5 mm internal diameter (ID) for males and 7/7.5 mm for females. After confirming by auscultation that the breath sounds of both lungs were identical, the tracheal tube was fixed. The lungs were ventilated mechanically with a tidal volume of 8 ml/kg and a respiratory rate of 14/min. Anaesthesia was maintained with isoflurane 1%, air 2 l/min and oxygen 2 l/min.
The PIP, Pplat, MAP, HR and blood pressure were measured at 5 min after tracheal intubation and just before turning the patient prone. The patients were then turned prone and positioned with and without a spine frame as per the group [Figures 1 and 2]. After confirming that the fixed position of the tube of the patient did not change in the prone position, the breath sounds of both lungs were confirmed by auscultation to be the same. These parameters were measured again immediately after turning the patient prone and 5 min later.
Figure 1.
(a and b) Spine frame used in the study. (c) Volunteer positioned prone with spine frame for spine surgeries (Group S)
Figure 2.
Volunteer positioned prone without spine frame for PCNL surgeries (Group P). PCNL = percutaneous nephrolithotomy
Nam et al.,[2] in their study, observed that with the spine frame, the peak pressure significantly increased (from 15.3 ± 2.8 to 16.7 ± 2.7 mmHg, P < 0.05). Therefore, with 80% power and 95% confidence level, considering the minimal detectable difference between supine and prone of 1.8 mmHg, the study required a minimum of 32 patients in each group. Sixty-four patients were enrolled after obtaining written informed consent.
IBM Statistical Package for the Social Sciences (SPSS; version 22) was used for statistical analysis. Categorical data were presented as numbers with percentages, and significant differences between the groups were assessed with a Chi-square test. Descriptive statistics were provided in terms of mean, standard deviation (SD) and median for all the relevant parameters in the study. Since the data was skewed in the mean and SD, the median was used to compare respiratory parameters. Mann–Whitney U test was used to compare quantitative variables between the two groups. Paired t-test was used as the test of significance for the comparison of paired data in the same group. The Chi-square test was used for the comparison of qualitative data. A P value of < 0.05 was considered statistically significant after assuming all the rules of statistical tests.
RESULTS
We enrolled 64 patients in the study and there were no dropouts [Figure 3]. The demographic data [Table 1] were comparable between both groups. The airway pressures obtained for 5 min after turning prone were higher than when supine in both groups. The increase in PIP, Pplat and MAP was significantly higher and the reduction in dynamic airway compliance was significantly lower in patients with a spine frame.
Figure 3.
Particpant flow diagram
Table 1.
Demographic data
| Group S (n=30) | Group P (n=30) | P | |
|---|---|---|---|
| Age (Mean±SD) (years) | 43.69±13.39 | 45.81±13.19 | 0.525 |
| BMI (Mean±SD) (Kg/m2) | 23.7±4.55 | 24.16±4.46 | 0.683 |
| Gender (male:femlae) (numbers) | 25:7 | 24:8 | 0.768 |
BMI=body mass index, SD=standard deviation
On positioning the patients prone, we found a significant rise in PIP (Group S 4 cmH2O vs. Group P 0.5 cmH2O, P < 0.001), Pplat (Group S 3.5 cmH2O vs. Group P 1 cmH2O, P = 0.004) and dynamic compliance (Group S- 5.513 cmH2O vs. Group P -2.78 cm H2O, P < 0.001) [Table 2].
Table 2.
The study parameters of PIP, Pplat, MAP, static and dynamic compliance
| Group S (n=30) | GroupP (n=30) | P | |||
|---|---|---|---|---|---|
|
|
|
||||
| Median (IQR) [95% CI] | Δ median | Median (IQR) [95% CI] | Δ median | ||
| PIP 5 min after intubation (cmH2O) | 16 (13.25–19) [15.15–18.20] | 4 | 17 (15–19) [15.86–18.76] | 0.5 | <0.001 |
| PIP 5 min after turning prone (cmH2O) | 20 (17.25–23) [18.91–22.04] | 17.5 (16–22) [17.5–20.55] | |||
| Pplat 5 min after intubation (cmH2O) | 13 (11–16) [12.10–14.69] | 3.5 | 14 (12–15) [12.67–14.88] | 1 | 0.004 |
| Pplat 5 min after turning prone (cmH2O) | (14.25–19) [14.44–17.31] | 15 (13.25–19) [14.27–16.78] | |||
| MAP 5 min after intubation (cmH2O) | 7 (6–8) [6–8.87] | 1 | 6 (5.25–7.75) [6.17–7.32] | 1 | 0.004 |
| MAP 5 min after turning prone (cmH2O) | 8 (7–10) [7.31–10.92] | 7 (6–8.75) [6.74–8] | |||
| Dynamic compliance 5 min after intubation (ml/cmH2O) | 28.12 (25–33.99) [26.20–31.16] | −5.51 | 27.78 (23.68–32.14) [25.99–30.28] | −2.78 | <0.001 |
| Dynamic compliance 5 min after turning prone (ml/cmH2O) | 22.61 (19.57–26.1) [21.59–24.27] | 25 (20.45–29.41) [23.66–27.35] | |||
CI=confidence interval, IQR=interquartile range, MAP=mean airway pressure, PIP=peak inspiratory pressure, Pplat=plateau pressure. Δmedian – difference in median values with position change from supine to prone
There was no significant difference in mean HR, systolic, and diastolic blood pressure between the two groups [Table 3].
Table 3.
The haemodynamic variables between the two groups
| Group S (n=30) Mean±SD (95% CI) | Group P (n=30) Mean±SD (95% CI) | P | |
|---|---|---|---|
| Heart rate 5 min after intubation (beats/minute) | 81.56±11.34 (77.47–85.64) | 83.31±14.58 (78.05–88.56) | 0.594 |
| Heart rate 5 min after turning prone (beats/minute) | 81.25±12.04 (76.90–85.59) | 80.75±13.17 (76–85.49) | 0.875 |
| SBP 5 min after intubation (mmHg) | 121.34±17.75 (114.94–127.74) | 126.59±20.01 119.38–133.80) | 0.271 |
| SBP 5 min after turning prone (mmHg) | 123.56±15.43 (118–129.12) | 129.91±16.15 (124.08–135.73) | 0.113 |
| DBP 5 min after intubation (mmHg) | 75.63±11.39 (71.51–79.73) | 78.75±13.46 (73.89–83.60) | 0.32 |
| DBP 5 min after turning prone (mmHg) | 78±12.47 (73.50–79.73) | 82.47±9.99 (78.86–86.07) | 0.119 |
CI=confidence interval, DBP=diastolic blood pressure, SBP=systolic blood pressure, SD=standard deviation
DISCUSSION
Our study found a significant rise in PIP, Pplat and MAP and a fall in dynamic compliance in both groups when changed from a supine to a prone position. This increase in airway pressures, airway resistance, and decrease in dynamic compliance were significantly greater when a spine frame was used to position the patient prone.
Many studies have found that there is a significant increase in PIP when the patient is positioned prone from the supine position.[1-3,5] Prone positioning increases the PIP by reducing compliance and we found that the spine frame further increases the PIP.
Pplat can be affected by changes in tidal volume and respiratory system compliance, but not by changes in airflow and airway resistance. As the tidal volume was constant in our study, any change in Pplat was due to a change in compliance. We found that the Pplat increased significantly more due to a reduction in compliance when a spine frame was used. MAP also increased in our study, but other studies observed that there were no significant changes in MAP.[5,6]
Lung compliance decreases when the patient is positioned prone from the supine position.[1-3,5,6] A decrease in dynamic compliance occurs in cases of an increase in airway resistance or a decrease in pulmonary and thoracic elasticity. In our study, the increase in PIP, Pplat and MAP can be attributed to a decrease in dynamic compliance and patients placed prone on a spine frame had significantly greater reduction in dynamic compliance. This finding is contrary to the traditional beliefs that the abdomen movements are free in the spine frame.
Studies have reported a 17%–18% decrease in dynamic compliance when patients were turned to the prone position on the Wilson frame during volume-controlled ventilation.[3,6] Lynch et al.[9] observed a 30%–35% decrease in respiratory compliance and an increase in peak airway pressures when positioned prone; however, they used parallel, hard rubber rolls to support shoulders and hips. The longitudinal parallel supports used by them would mimic the effect of a spine frame, and thus could have impaired chest and abdominal movements with consequent alterations in respiratory mechanics.
In a study done by Palmon et al.,[3] patients had a greater compromise in ventilatory function when placed on a Wilson frame or chest rolls than when positioned on a Jackson table. They concluded that, unlike the Jackson table, Wilson frame and chest rolls do not allow the abdomen to hang completely free; therefore, these devices result in increased peak airway pressures and decreased compliance. The spine frame used in our study is similar to the Wilson spine frame, and the movement of the abdomen is splinted laterally by the bars of the frame, thus reducing the dynamic compliance.
Backofen and Schauble[10] found that when non-obese patients were placed in prone position, HR, mean arterial, venous, and pulmonary occlusion pressures were not altered. There were no significant changes in haemodynamics in the prone position with and without spine frame in our study.
The traditional belief is that in the prone position, the abdomen is compressed and internal organs push the diaphragm cephalad, thus reducing the respiratory system compliance. The reduced compliance leads to elevated respiratory pressures resulting in lung oedema and postoperative acute lung injury risk.[11-13]
The major limitation in our study was that the blinding of the study groups was not possible as we compared two different groups of surgical patients.
CONCLUSION
The use of a spine frame resulted in a much greater increase in airway pressures, a decrease in dynamic compliance and an increase in airway resistance compared to positioning the patient without spine frame. We conclude that if a patient must be turned prone during general anaesthesia, no support is required to keep the abdomen free unless it is required for surgical access.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
REFERENCES
- 1.Kumaresan A, Robert G, Ariel M, Stephen HL, Daniel T. Effects of prone positioning on transpulmonary pressures and end-expiratory volumes in patients without lung disease. Anaesthesiology. 2018;128:1187–92. doi: 10.1097/ALN.0000000000002159. [DOI] [PubMed] [Google Scholar]
- 2.Nam Y, Yoon AM, Kim YH, Yoon SH. The effect on respiratory mechanics when using a Jackson surgical table in the prone position during spinal surgery. Korean J Anaesthesiol. 2010;5:323–8. doi: 10.4097/kjae.2010.59.5.323. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Palmon SC, Kirsch JR, Depper JA, Toung TJK. The effect of the prone position on pulmonary mechanics is frame dependent. Anesth Analg. 1998;87:1175–80. doi: 10.1097/00000539-199811000-00037. [DOI] [PubMed] [Google Scholar]
- 4.Cho SY, Noh GJ, Yeom JH, Shin WJ, Kim YC, Kim KH, et al. The effect of prone position on pulmonary compliances by anesthesia duration. Korean J Anesthesiol. 2000;38:997–1001. [Google Scholar]
- 5.Lee JM, Lee SK, Kim KM, Kim YJ, Park EY. Comparison of volume-controlled ventilation mode and pressure-controlled ventilation with volume-guaranteed mode in the prone position during lumbar spine surgery. BMC Anaesthesiol. 2019;19:133. doi: 10.1186/s12871-019-0806-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Jo YY, Kim JY, Kwak YL, Kim YB, Kwak HJ. The effect of pressure-controlled ventilation on pulmonary mechanics in the prone position during posterior lumbar spine surgery: A comparison with volume-controlled ventilation. J Neurosurg Anesthesiol. 2012;24:14–8. doi: 10.1097/ANA.0b013e31822c6523. [DOI] [PubMed] [Google Scholar]
- 7.Pelosi P, Massimo C, Emiliana C, Marco C, Davide M, Vicardi P, et al. The prone positioning during general anaesthesia minimally affects respiratory mechanics while improving functional residual capacity and increasing oxygen tension. Anesth Analg. 1995;80:955–60. doi: 10.1097/00000539-199505000-00017. [DOI] [PubMed] [Google Scholar]
- 8.Kwee MM, Ho YH, Rozen WM. The prone position during surgery and its complications: A systematic review and evidence-based guidelines. Int Surg. 2015;100:292–303. doi: 10.9738/INTSURG-D-13-00256.1. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Lynch S, Brand L, Levy A. Changes in lung-thorax compliance during orthopaedic surgery. Anesthesiology. 1959;20:278–82. doi: 10.1097/00000542-195905000-00004. [DOI] [PubMed] [Google Scholar]
- 10.Backofen JE, Schauble JF. Hemodynamic changes with prone position during general anesthesia. Anesth Analg. 1985;64:194. [Google Scholar]
- 11.Licker M, de Perrot M, Spiliopoulos A. Risk factors for acute lung injury after thoracic surgery for lung cancer. Anesth Analg. 2003;97:1558–65. doi: 10.1213/01.ANE.0000087799.85495.8A. [DOI] [PubMed] [Google Scholar]
- 12.Van der Werff YD, Van der Houwen HK, Heijmans PJ, Duurkens VA, Leusink HA, van Heesewijk HP, et al. Postpneumonectomy pulmonary edema. A retrospective analysis of incidence and possible risk factors. Chest. 1997;111:1278–84. doi: 10.1378/chest.111.5.1278. [DOI] [PubMed] [Google Scholar]
- 13.Fernández-Pérez ER, Sprung J, Afessa B, Warner DO, Vachon CM, Schroeder DR, et al. Intraoperative ventilator setting and acute lung injury after elective surgery: A nested case control study. Thorax. 2009;64:121–7. doi: 10.1136/thx.2008.102228. [DOI] [PubMed] [Google Scholar]