Our results showed moderate correlation between MAC and arm conicity index in third trimester parturients. Furthermore, although MAC, weight, and BMI showed moderate correlation with arm conicity, BMI had the highest correlation coefficient with arm conicity, with every unit increase in BMI increasing the conicity index by 0.18. Out of 300 parturients, one had MAC below the recommended range for arm cuffs. However, all parturients were able to fit into one of the standard NexfinTM and CNAPTM finger cuffs. Compared to non-obese parturients, obese parturients experienced significantly more problems with BP measurement, including arm cuffs popping off, or requiring a change in BP cuffs.
Our finding that MAC is moderately correlated with conicity index is consistent with other studies of Caucasian parturients, suggesting that this correlation applies to our multi-ethnic Asian population. Bonso et al. were the first to mathematically quantify arm conicity using a conicity index , and despite using non-standard anthropometry techniques and arm length measurements, Bonso et al. and Palatini et al. demonstrated that MAC was correlated with arm conicity index [12, 13]. Subsequently, Eley et al. increased the robustness and reproducibility of arm length measurements by utilizing bony landmarks based on the Anthropometry Procedures Manual of the Center for Disease Control [3, 16], and reported a similar correlation between MAC and conicity index .
In addition to MAC, BMI and weight were also correlated with conicity index, and of the three variables, BMI was found to correlate the best with arm conicity. These results are similar to that of Eley et al., who also reported that BMI correlated best with arm conicity, accounting for 26% of the variation in conicity index . Furthermore, our finding that BMI, weight, and MAC were collinear was also reported by Eley et al. . This may be explained by the association between increasing BMI and higher MAC in both pregnant [3, 17, 18] and non-pregnant populations , and is likely due to an increase in arm fat mass as measured using bioelectrical impedance analysis . Additionally, obese parturients were more likely to experience challenges during BP measurement, with an increased incidence of the arm cuffs popping off and the need to change to a different cuff, compared to non-obese parturients. These findings were echoed by Eley et al. .
Our findings that obesity is associated with increasing MAC and arm conicity is concerning. Although the highest MAC measured in our study did not exceed the recommended MAC range of the largest cuff, these non-standard cuff sizes may not be readily available, and shifting the point of measurement from the upper arm to lower arm is unacceptable as it has been shown to overestimate BP [20-22]. Furthermore, the increase in conicity index may cause irregular expansion of the cylindrical cuff during inflation, leading to overestimation of both systolic and diastolic BP . Although the use of cone-shaped BP cuffs may improve the accuracy of BP measurements compared to cylindrical cuffs , the former is not available at our institution. Nonetheless, the increased risk of overestimating BP in obese parturients is clinically significant given that obese parturients are already at elevated baseline risk for hypertensive disorders [2,13], with the prevalence of obesity in Singapore likely to increase further .
The potential solution to the increased MAC and conicity index of obese parturients may lie with the use of finger cuff devices like the NexfinTM and CNAPTM. Our results demonstrated that all parturients were able to fit into the standard finger cuffs provided by NexfinTM and CNAPTM, which may provide an alternative means of measuring BP in parturients who do not fit the available arm cuffs. NexfinTM has been validated against sphygmomanometry in pregnant patients, and passed both Association for the Advancement of Medical Instrumentation (AAMI) standards and European Society of Hypertension International Protocol Phases 1 and 2.1 . Similarly, CNAPTM has been validated against intra-arterial BP in general surgery patients under anesthesia and met AAMI standards . In contrast, the accuracy of finger cuff-based BP measurements were unacceptable by AAMI standards in critically ill patients receiving vasopressor infusions or with finger edema , and should not be used as an alternative to arm cuffs in this patient population. However, our standard practice for such patients is intra-arterial BP monitoring.
We acknowledge several limitations to this study. Since BP measurements using arm or finger cuff-based systems were not recorded and compared against another standard such as intra-arterial BP, we are unable to assess the implications of increasing MAC or conicity index on BP measurement accuracy, and this should be determined in future studies. This information will help determine if finger cuff-based BP measurements should be used in parturients with high arm conicity or who do not fit standard arm cuffs. In addition, we enrolled parturients who were scheduled for elective cesarean delivery, which may not be representative of the general obstetric population since parturients with higher BMI are more likely to undergo cesarean delivery [3, 28]. Finally, we enrolled parturients in their third trimester, raising concerns that gestational weight gain during the interval prior to cesarean delivery will lead to increased BMI and MAC. However, Hogan et al. studied MAC in parturients across all three trimesters and found that the mean MAC did not vary significantly with different trimesters of pregnancy .