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A Randomized Crossover Study to Determine the Effect of a 30° Head-Up Versus a Supine Position on the Functional Residual Capacity of Term Parturients

Hignett, Rachel, FRCA*; Fernando, Roshan, FRCA; McGlennan, Alan, FRCA; McDonald, Sarah, FRCA§; Stewart, Adrienne, FRCA; Columb, Malachy, FRCA; Adamou, Tina, RCCP; Dilworth, Paul, FRCP**

doi: 10.1213/ANE.0b013e31822bf1d2
Obstetric Anesthesiology: Research Reports
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BACKGROUND: Airway management continues to pose challenges to the obstetric anesthesiologist. Functional residual capacity (FRC), which acts as an oxygen reservoir, is reduced from the second trimester onwards and is exacerbated in the supine position. Mechanisms to increase FRC may delay the onset of hypoxemia during periods of apnea. Values for changes in FRC in term parturients in semierect positions are unknown. We hypothesized that the FRC of healthy term parturients would increase significantly in the 30° head-up position in comparison with the supine position.

METHODS: Twenty-two healthy term parturients were recruited. Initial screening spirometry was performed to exclude undiagnosed respiratory disease. FRC was measured using the helium dilution technique in the supine, 30° head-up, and sitting erect positions. Subjects were randomized to sequence of position testing order. Noninvasive systolic blood pressure, heart rate, and oxygen saturation were measured twice in each testing position.

RESULTS: Results from 20 subjects were analyzed. The spirometry results for all subjects were within predicted normal reference intervals. FRC measurements differed significantly (P < 0.001) among all positions. FRC increased by a mean of 188 mL (95% confidence interval 18 to 358 mL) from the supine to the 30° head-up position (P = 0.03). There were no significant differences in vital signs among testing positions (P > 0.16).

CONCLUSIONS: We have demonstrated that the FRC of healthy term parturients increases significantly in the 30° head-up position in comparison with supine.

Published ahead of print September 14, 2011

From the *Department of Anesthesia, Royal Infirmary of Edinburgh, Edinburgh, United Kingdom; Department of Anesthesia, University College London Hospitals NHS Foundation Trust, London, United Kingdom; Department of Anesthesia, Royal Free Hampstead NHS Trust, London; §Department of Anesthesia, Guy's and St. Thomas' NHS Foundation Trust, London; Department of Anesthesia, University Hospital of South Manchester NHS Foundation Trust, Wythenshawe, Manchester, United Kingdom; Department of Lung Function, Royal Free Hampstead NHS Trust, London; and **Department of Thoracic Medicine, Royal Free Hampstead NHS Trust, London.

Funding: Smiths Medical, USA (unrestricted research grant to S.M.), Obstetric Anaesthetists' Association, UK (unrestricted research grant to A.S.), Royal Free Hampstead NHS Trust (unrestricted research grant to R.H.), and University College London Hospitals/University College London (UCLH/UCL) Comprehensive Biomedical Research Centre, which receives a proportion of funding from the United Kingdom Department of Health's National Institute of Health Research (NIHR) Biomedical Research Center's funding scheme (to R.F.).

The authors declare no conflict of interest.

Reprints will not be available from the authors.

Address correspondence to Roshan Fernando, FRCA, Department of Anesthesia, University College London Hospitals NHS Foundation Trust, London NW1 2BU, United Kingdom. Address e-mail to r.fernando@btinternet.com.

Accepted June 27, 2011

Published ahead of print September 14, 2011

The 2006 to 2008 Confidential Enquiry into Maternal Deaths, “Saving Mothers' Lives,” continues to document a dramatic improvement in the safety of anesthesia for cesarean delivery over the last half century.1 Both neuraxial and general anesthesia are >30 times safer than anesthesia during the triennia 1964 to1966.2 In the 1960s, most direct anesthetic deaths were attributed to failed airway management during general anesthesia. Most women now undergo neuraxial anesthesia for operative delivery; very few deaths have occurred because of neuraxial anesthesia since the 1960s.2 However, deaths due to failed airway management in women undergoing general anesthesia still feature in each triennial report.3

For many reasons, general anesthesia, and in particular the parturient's airway, poses challenges to the obstetric anesthesiologist. The incidence of failed intubation may be as high as 1:249.4 The functional residual capacity (FRC), which acts as an oxygen reservoir, decreases from the second trimester onward.5 Adopting the supine position rather than the erect position exacerbates this reduction.5 At the same time, oxygen consumption is substantially increased.6 These changes in FRC and oxygen consumption in pregnancy result in rapid desaturation during periods of apnea.7,8 Any technique to delay the onset of oxygen desaturation in parturients could be of great benefit.

When general anesthesia is performed, rapid sequence induction is usual because of the risk of acid regurgitation and aspiration. Integral to this technique is administration of oxygen, which denitrogenates the FRC of the lungs. The FRC acts as an oxygen reservoir: any maneuvers, which increase FRC, should therefore increase the time available to secure the airway before the onset of desaturation. Previous studies have demonstrated that the FRC in term parturients increases when the patient moves from the supine position to the erect position.9 However, there are very few data in the medical literature regarding changes of FRC in term parturients in intervening positions.

We hypothesized that the 30° head-up position in comparison with the supine position in term parturients would significantly increase FRC. This randomized crossover study aimed to quantify the increase in FRC from the supine to 30° head-up, and erect positions, as measured by helium dilution in healthy parturients at term.

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METHODS

After local ethics committee approval (Royal Free Hampstead NHS Trust, London, UK) and written informed consent, 22 healthy term parturients were enrolled. Exclusion criteria included gestation <37 weeks, multiple gestation, asthma, smoking history, cardiac disease, preeclampsia, nongestational diabetes mellitus, and body mass index (BMI) >35 kg/m2 at first prenatal visit. Demographic data including age, height, weight at first prenatal visit, current weight, gestation, and ethnicity were collected, and BMI was calculated.

A senior respiratory physiologist in a dedicated lung function laboratory performed all lung function tests using a MasterScreen PFT (Viasys Healthcare, Conshohocken, PA). The MasterScreen PFT incorporates a pneumotachograph to record spirometry and fast gas analyzers, which record helium, oxygen, and carbon monoxide concentrations. This system met the technical specification described by the American Thoracic Society and European Respiratory Society (ATS/ERS) guideline standards.10,11

Height, which enabled calculation of predicted spirometry values, and weight were measured and recorded. Initial spirometry was performed in the sitting position, as a screening test of general respiratory health, according to ATS/ERS guidelines.10 Measurements of forced expired volume in 1 minute (FEV1), forced vital capacity (FVC), and peak flow rate were recorded and the FEV1/FVC ratio calculated. Tests were repeated until there were 3 consistent sets of results with values within 5% of each other, produced from acceptable FVC maneuvers, as judged by respiratory physiologists against standard criteria.10 Noninvasive systolic blood pressure, heart rate, and oxygen saturation (SaO2) were measured twice in the supine (with left uterine displacement), 30° head-up, and erect sitting positions by an anesthesiologist.

Measurements of FRC were obtained by a helium dilution method in 3 positions: supine, 30° head-up, and erect sitting. A standard anesthesia gurney was used for the supine and 30° head-up positions. A standard wedge under the right hip was used to prevent aorto-caval compression. For the 30° head-up position, metal dividers set at 30° using a protractor were used to measure the correct angle of incline of the upper portion of the anesthesia gurney. Patients were assigned to a random order of 3 positions—supine, 30° head-up, and erect sitting— using a computer-generated random number table (Excel, Microsoft Corporation, Redmond, WA). Once randomized, the position orders were placed in 3 separate sequentially numbered opaque envelopes for each patient by an anesthesiologist not connected with the study. The envelopes containing the positions were not opened by the respiratory physiologist involved in the study until the time of position change to reduce bias and ensure that neither the patient nor the researcher knew the next allocated position in advance. Measurement of FRC was again standardized according to ATS/ERS guidelines.11 The procedure for measuring FRC was as follows: the subject was asked to breathe normally into a snorkel-type mouthpiece wearing a nose clip. After 30 seconds, the subject was automatically switched into the system at end-expiration, i.e., FRC. The test gas mixture contained air with additional oxygen 25%–30% and 6%–8% helium. CO2 was absorbed, and O2 added into the circuit to maintain a constant volume. The patient was disconnected when equilibrium was reached between the helium in the spirometer and in the lungs, i.e., when there was a <0.02% decrease in helium concentration over a 30-second interval. This usually took between 2 and 3 minutes. Subsequent measurements were timed to occur not less than 5 minutes after the previous measurement to prevent anomalous results due to helium accumulation. Two measurements were accepted if readings were within 10% of each other. If not, a third measurement was obtained.

Results are presented as mean (SD) or as mean difference with 95% confidence interval (CI) for difference. The D'Agostino omnibus test was used to test for Gaussian distribution and was not significant for any respiratory variables. FRC and hemodynamic data were analyzed using repeated-measures analysis of variance (RMANOVA) with Geisser–Greenhouse corrections.

Terms were included in the model to test for significant period and period–position interactions in the crossover analysis, and Tukey–Kramer multiple comparison tests were used to compare positions. Sample size calculations suggested that a minimum of n = 18 subjects would be required to find as significant a nominal 150 mL change (SD 150 mL) in FRC. Power was given at 90% and a Bonferroni correction was applied (P < 0.017) to keep the overall type I error rate to <0.05 for up to 3 comparisons. Analysis of data was performed using Number Crunching Statistical Systems (NCSS) 2007 (NCSS Inc., Kaysville, UT).

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RESULTS

Twenty-two subjects were approached. Two women were not enrolled into the study owing to exclusion criteria. As a result, complete data from 20 subjects were analyzed.

Mean (SD) patient age, height, and gestational age were 326 years, 1647 cm, and 401 weeks, respectively. The mean (SD) BMI at the first prenatal visit was 234 kg/m2, and the BMI at the time of the study was 284 kg/m2. Patients were from different ethnic backgrounds and included 11 Caucasian, 5 Asian, and 4 Afro-Caribbean women. There were no statistically significant differences in vital signs (data not shown) between testing positions (P > 0.16). The spirometry results for all subjects were within predicted normal ranges for age, height, sex, and pregnancy. Results for spirometry (mean [SD]) are summarized as follows: FEV1 2.99 L (0.63), FVC 3.61 L (0.81), and FEV1:FVC ratio 83.26 (6.18).

Measurements for mean FRC differed significantly (RMANOVA P < 0.001) among all positions (Fig. 1). There were significant increases in FRC of 188 mL (95% CI 18 to 358 mL; P = 0.03) from the supine position to the 30° head-up position, and of 862 mL (95% CI 692 to 1032 mL; P < 0.001) from the supine position to erect sitting position.

Figure 1

Figure 1

The randomized sequences of position testing (P = 0.70), period (P = 0.41), and position–period interaction (P = 0.17) did not significantly affect FRC measurements. (For example, the difference in FRC between the supine and 30° head-up positions, when supine measurements were taken before those in the 30° head-up position, and vice versa, did not differ). There were no significant differences in results among all tests performed first, in comparison with all those performed second and third (P = 0.41).

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DISCUSSION

Past research has consistently described a progressive decrease in FRC in parturients from the second trimester of pregnancy onwards.5,9,12 This reduction in FRC is enhanced in the supine position in comparison with the erect position. FRC reduction in pregnancy is probably caused by upward displacement of the diaphragm due to the enlarging uterus. In healthy male volunteers, FRC has been shown to increase gradually from the head-down position to the 90° head-up position.13 However, in other groups, the increase in FRC among these positions cannot be assumed to increase consistently. For example, in traumatic quadriplegics, no increase in FRC occurs between the horizontal and 35° head-up positions: maximal increase in FRC is achieved between the 35° and 60° head-up positions.13 There are few data regarding the change in FRC in term parturients in intermediate positions between supine and erect. We assumed that adopting a steeper head-up position would improve the chance of increasing the FRC. However, as the patient's head-up position increases, the ease of intubation will eventually become more unfamiliar to most anesthesiologists. We therefore chose the 30° head-up position as a compromise.

Our results demonstrate a significant increase in FRC from supine position to the 30° head-up position. Adopting this 30° head-up position was not associated with any cardiovascular or respiratory compromise: there were no changes in vital signs in any position. However, there was a tendency for the supine FRC values to be lower when the initial test was performed in the supine position, leading to a larger change in FRC. It is possible that the subjects were lying for a longer duration, when supine was the initial position, which may have led to dependent atelectasis. There were no differences in results among all tests performed first, in comparison with all those performed second and third; thus helium accumulation could be excluded.

What are the implications of our results for airway management in term parturients? The key question is whether time to desaturation during apnea is prolonged by an increase in FRC of 188 mL in the 30° head-up position in comparison with supine position. Onset of desaturation during apnea after oxygen administration in various head-up positions has been demonstrated to be prolonged in both nonpregnant and obese subjects. Lane et al.14 randomized 40 healthy nonpregnant patients undergoing general anesthesia for elective cholecystectomy into 2 groups. The 20° head-up position was associated with a significantly prolonged time for SaO2 to decrease to 95% during apnea in comparison with supine position. Similarly, Baraka et al.8 demonstrated a delay in the onset of desaturation in nonpregnant patients after oxygen administration and intubation at a 45° head-up position in comparison with supine position. Similar studies in obese subjects in the 20° head-up position15 and the sitting position16 have also shown a significantly prolonged time to desaturation. However, the single small study by Baraka et al.8 measured the time to desaturation in a pregnant group undergoing general anesthesia for cesarean delivery, and a nonpregnant group in a 45° head-up position in comparison with supine position. Surprisingly, unlike in the nonpregnant group, this head-up position did not result in a significantly longer time to desaturation in the pregnant group. This was a small study of 10 pregnant patients in each position group; it is not known whether these groups were matched, e.g., whether the indications for cesarean delivery were similar. Also, oxygen administration was timed, so it is possible that patients were being induced at differing degrees of oxygen administration.

Adopting a head-up position for oxygen administration and endotracheal intubation may have several consequences in addition to increasing FRC. Lee et al.,17 in a randomized crossover study of 40 ASA PS 1 and 2 nonpregnant patients undergoing elective surgery, demonstrated a 25% improvement in view of glottic opening at laryngoscopy in the 25° head-up position in comparison with supine position. Lung compliance is increased in a head-up position,18 which may make manual ventilation of patients' lungs easier in a failed intubation scenario. The 30° head-up position may be advantageous in reducing the risk of regurgitation of gastric contents due to an antigravity effect.19

A further consequence of an increased FRC is that time required to adequately administer oxygen would also be increased, although this is likely to be a clinically insignificant amount of time.2022

Obese parturients were a particularly high-risk group in the last 2 triennial reports.2,3 An increase in FRC and a prolongation of safe apnea time would be especially advantageous in this particular group.23 The BMI at first prenatal visit in our subjects ranged from 18 to 34 kg/m2. Given this wide range, we were interested in discovering what effect an increasing BMI has on change in FRC with position. An increasing BMI was associated with a smaller increase in FRC from the supine position to both the 30° head-up and the erect positions (Fig. 2). To investigate this further, we performed a post hoc analysis of the effect of BMI, when added as a covariate in a linear mixed-effects model analysis. The effect of position was still significant (P < 0.001) with BMI having a marginal effect overall (P = 0.19), again with significant (P < 0.001) changes between erect and other positions and a significant (P = 0.048) change between 30o tilt and supine. Therefore adopting a head-up position may be less advantageous in women with higher BMI in terms of increasing FRC. Eng et al. have also reported minimal FRC changes in obese parturients in a study comparing third-trimester changes with those 2 months postpartum, although the effect of position was not studied.24 Because evaluating the effect of BMI on FRC was not our primary aim, and because of the limited BMI range and number of subjects, it may be useful to repeat our study in obese women.

Figure 2

Figure 2

A limitation of our study is that measurement of FRC is more accurate using body plethysmography, because it measures lung volume behind closed airways. However, in healthy nonpregnant patients this difference is minimal.25 We chose the helium dilution technique because this is better tolerated by patients. Because we were interested in evaluating changes in lung volume, which participate in gas exchange and therefore denitrogenation (rather than behind closed airways), helium dilution was felt to be an appropriately accurate method of measurement. Another limitation of our study was that our subjects were from diverse ethnic backgrounds. Ethnicity influences FRC because of different body habitus,26 but the numbers we recruited in different ethnic groups are too small to assess whether ethnicity confounded our results. In future studies, it would be useful to select patients from a single ethnic group.

In conclusion, we have demonstrated a significant increase in the FRC of healthy term parturients in the 30° head-up position in comparison with supine. Further research needs to be undertaken to determine whether this increase in FRC prolongs the time to desaturation during the apnea phase of rapid sequence induction.

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DISCLOSURES

Name: Rachel Hignett, FRCA.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Rachel Hignett has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Roshan Fernando, FRCA.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Roshan Fernando has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Alan McGlennan, FRCA.

Contribution: This author helped design the study.

Attestation: Alan McGlennan has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Sarah McDonald, FRCA.

Contribution: This author helped design the study and conduct the study.

Attestation: Sarah McDonald has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Adrienne Stewart, FRCA.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Adrienne Stewart has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Malachy Columb, FRCA.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Malachy Columb has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Tina Adamou, RCCP.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Tina Adamou has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Paul Dilworth, FRCP.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Paul Dilworth has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by; Cynthia A. Wong, MD.

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