decreases and minute ventilation (VE)Increases Initially and then stabilizes. ... the role of thoracoabdomlnal mechanics, control of breathing, and Inspiratory ...
Ventilatory Drive and Respiratory Muscle Function in Pregnancy1-3
GUSTAVO CONTRERAS, M6NICA GUTIERREZ, TERESA BEROizA, ALOO FANTiN, HERNAN 0006, WIS VILLARROEL, EOGAROO CRUZ, and CARMEN LISBOA
Introduction
The effects of pregnancy on lung volumes have been extensivelyinvestigated (1-7), demonstrating a moderate progressive fall in FRe and expiratory reserve volume (ERV). These changes are attributed to a rise in abdominal pressure secondary to the enlargement of the uterus, but this pressure has not been measured. Hyperventilation is also a feature of pregnancy that has been associated with the rise in progesterone blood levels. However, there is disagreement concerning its time course during gestation (2,4,8-11). The majority of the investigators have found that minute ventilation (VE) increasesearly in pregnancy, but it remains unchanged thereafter despite a continuous rise in progesterone blood levels (8-11). In order to obtain additional information about the mechanisms involved in the reduction of ERV and the stabilization of VE, we studied respiratory muscle strength, thoracic and abdominal pressures, and ventilatory drive during and after pregnancy in normal women. Methods Thirty-two nonsmoking, healthy pregnant women 25 to 35 yr of age (mean ± SD: 28.3 ± 3.3; mean height, 158.5 ± 4.7 ern; mean body weight, 58.6 ± 7.8 kg), with no previous history of cardiovascular or respiratory diseases,wererecruited for the protocol. Study sessions were planned for Weeks 12, 20, 30, and 38 of pregnancy and after the first menstrual cycle after childbirth. In each session we measured body weight, chest wall configuration, lung volumes, VE, mouth occlusion pressure (P O• I ) , and respiratory pressures. Eighteen of the 32 women completed the four planned sessions during pregnancy, and 10 of them werestudied between 1and 16months (mean,6.7 ± 5.0) after delivery. Ten women abandoned the study after the second session, and in four the study was stopped because of associated pregnancy disease. Of the 18 women who completed the prepartum studies, 12 had had no previous pregnancies, four had had one, one had had two, and one had had three. In an additional group of seven healthy pregnant women (28.7 ± 4.6 yr of age), we
SUMMARY It has been demonstrated that during pregnancy expiratory reserve volume (ERV) decreases and minute ventilation (VE) Increases Initially and then stabilizes. In order to determine the role of thoracoabdomlnal mechanics, control of breathing, and Inspiratory muscle function In these alterations, we studied Inspiratory pressures, lung volumes, thoracic configuration, and respiratory drive In 18 normal pregnant women at Weeks 13, 21, 30, and 37 of pregnancy. Tenof them were studied 6 months after delivery. ll'ansdlaphragmatlc pressure (Pdl) wes measured at Week 37 and 3 months after delivery In an additional group of seven women. VEas well as VTlTllncraased early during gestltlon and remained unchanged therelfter. In contrast, mouth occlusion pressure (p•.,) Increased progressively during pregnlncy, from 1.53 ± 0.16 (meln ± SE) to 2.02 ± 0.18cm H2 0 , Ind fell significantly to 1.1 ± 0.15cm H2 0 after delivery, Indlcltlng thlt effective respiratory Impedance Increlses during pregnlncy. Meln P•., correllted with progesterone plasmllevels (r = 0.918P < 0.05). No chlnges In Plmlx, PEmax, Ind Pdl,nax, were observed. End-expiratory gastric pressure (Pga) Increlses significantly during pregnancy: 11.8 ± 0.8 versus 8.4 ± 1.12cm H2 0 Ifter delivery (p < 0.012).This Increment wes correlated with the fill In ERVobserved In lIte pregnancy (r = 0.74P < 0.05). Our results demonstrate thlt during pregnancy ventllltory drive and respiratory impedlnce Increase with the consequent stablllzstion of VE,but our data do not permit us to dlfferentllte whether the Increment In p•., Is secondlry to the Increase In Impedlnce or to the rise In progesterone. Resplrltory muscle function remains normll despite the alterltlon of thorlclc conflgurltlon. AM REV RESPIR DIS 1991; 144:837-841
measured the transdiaphragmatic pressure (Pdi) at Week 37 of pregnancy and 3 months after delivery. Twoof them had had two previous pregnancies, one had had one, and the remaining four were primigravid. In order to determine if the supine position affects P O• I in pregnancy, in another group of seven women (Week 20 to Week 34 of pregnancy), we measured this index in the sitting and the supine postures. Informed consent was obtained from all subjects, and the protocol was approved by the Human Studies Committee of our School of Medicine.
Chest Wall Configuration Changes in thoracic configuration during pregnancy were assessed in the supine position by measuring the abdominal perimeter at umbilical level, the rib cage perimeter at submammary level, and the subcostal angle formed by the rib margins at the xyphoid process.For the measurement of this angle, costal margins were marked with a dermatographic pencil and traced on a sheet of paper. Lung Volumes VCand ERV weremeasured in the sitting posture with a 9-L water-sealed spirometer (W.E. Collins, Braintree, MA). The best of three technically satisfactory maneuvers is reported. FRC and TLC were determined in a volume-displacement, pressure-compensated body plethysmograph.
Control of Breathing This aspect was assessed by measuring P O• I , tidal volume (VT), duration of the respiratory cycle (Ttot), respiratory rate, and inspiratory time (TI). Mean inspiratory flow was calculated as VT/TI and duty cycle as 'Il/Ttot (12). Measurements were performed in the supine posture after a 20-min period of rest. Flow was measured with a Fleisch no. 3 pneumotachograph, connected to a MP 45 Validyne ± 2 ern H 2 0 presssure transducer (Validyne Engineering, Northridge, CA). VT was obtained by integration from flow. Values from 10cycleswere averaged for the analysis of results. P O• I was measured according to the method of Whitelaw and coworkers (13) by occluding, during expiration, the inspiratory limb of the circuit with a silent solenoid valve, which was reopened 0.5 s after the onset of inspiration. Pressure was measured through
(Received in original form October 31, 1989 and in revised form April J.fi. 1991) 1 From the Departments of Respiratory Disease, Gynecology and Obstetrics, and Statistics, School of Medicine, Universidad Cat6lica de Chile, Santiago, Chile. 2 Supported by Direccion Investigacion Universidad Catolica de Chile and Fundacion Gildemeister. 3 Correspondence and requests for reprints should be addressed to Carmen Lisboa, M.D., Escuela de Medicina, Casilia 114-0, Santiago, Chile.
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a lateral port in the mouthpiece connected to a MP45 Validyne pressure transducer. To avoid subject awareness of the maneuvers, the occlusions were performed at irregular intervals every 1 or 2 min, with the subjects visually and acoustically isolated. The signals wereregistered in an eight-channel strip chart recorder (Hewlett-Packard, Palo Alto, CA). The mean of at least sevenmeasurements is reported. Effective respiratory impedance was calculated as proposed by Milic-Emili and coworkers (12) by dividing P O• I by VTl1t In each study session a venous blood sample was obtained for the measurement of progesterone plasma levels.Analysis was performed by radioimmunoassay.
Respiratory Pressures Maximal inspiratory and expiratory mouth pressures (Prmax and Psmax) were measured in the sitting posture with a Statham P23D300 pressure transducer (Statham Instruments, Hato Rey, PR). For the measurement of Prmax, subjects were encouraged to perform maximal efforts at FRC against an occluded mouthpiece provided with a 1.5-mm orifice in order to keep the glottis open and avoid the use of buccal muscles. Maximal expiratory mouth pressure was measured at TLC as proposed by Black and Hyatt (14). During both maneuvers, the subjects were provided visual feedback of the generated pressure through an oscilloscope. At least five technically satisfactory measurements of maximal inspiratory and expiratory pressures were recorded, and the largest values were considered for analysis. Pdi and its components, gastric (Pga) and esophageal (Pes) pressures, were measured in the sitting posture during quiet breathing as well as during maximal inspiratory efforts at FRC against an occluded airway (Pdi max) (15). For the measurement of Pdi max , visual feedback was provided with an oscilloscope. No instructions were given regarding the relative use of rib cage or abdominal muscles during measurements. Pes and Pga were obtained with the two-balloon method (Hyatt type balloons; AE Medical Corp., Farmingdale, NJ). Both balloons were attached to pressure transducers (Statham P23D3(0) calibrated over the appropriate pressure ranges (15). Frequency response of the balloon-catheter systems was linear to 8 Hertz, and the time response to a sudden pressure change of 30 em of H 2 0 was less than 20 ms. The distance between the tip of the balloon and the nostril was registered in each subject in order to place the balloon at the same distance in the postpartum study. Pdi was obtained by subtraction of Pes from Pga with a third differential pressure transducer (Statham PM131). Flow rate and VT were simultaneously recorded so that Pes and Pga during quiet breathing could be calculated at points of zero flow at the end of inspiration and expiration. All signals were recorded on an eight-channel strip chart recorder (DR8; Electronics for Medicine, White Plains NY). Reported Pdi during tidal breathing corresponds to the mean of 10 re-
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