Exercise- and hypoxia-induced blood flow through intrapulmonary arteriovenous anastomoses is reduced in older adults

H. Cameron Norris, Tyler S. Mangum, Joseph W. Duke, Taylor B. Straley, Jerold A. Hawn, Randy D. Goodman and Andrew T. Lovering J Appl Physiol 116:1324-1333, 2014. First published 13 March 2014; doi:10.1152/japplphysiol.01125.2013 You might find this additional info useful... This article cites 46 articles, 22 of which can be accessed free at: /content/116/10/1324.full.html#ref-list-1 Updated information and services including high resolution figures, can be found at: /content/116/10/1324.full.html Additional material and information about Journal of Applied Physiology can be found at: http://www.the-aps.org/publications/jappl

Journal of Applied Physiology publishes original papers that deal with diverse areas of research in applied physiology, especially those papers emphasizing adaptive and integrative mechanisms. It is published 12 times a year (monthly) by the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright © 2014 by the American Physiological Society. ISSN: 0363-6143, ESSN: 1522-1563. Visit our website at http://www.the-aps.org/.

Downloaded from on February 19, 2015

This information is current as of February 19, 2015.

J Appl Physiol 116: 1324–1333, 2014. First published March 13, 2014; doi:10.1152/japplphysiol.01125.2013.

Exercise- and hypoxia-induced blood flow through intrapulmonary arteriovenous anastomoses is reduced in older adults H. Cameron Norris,1 Tyler S. Mangum,1 Joseph W. Duke,1 Taylor B. Straley,1 Jerold A. Hawn,2 Randy D. Goodman,2 and Andrew T. Lovering1 1

Department of Human Physiology, University of Oregon, Eugene, Oregon; and 2Oregon Heart and Vascular Institute, RiverBend, Springfield, Oregon

Submitted 9 October 2013; accepted in final form 3 March 2014

aging; intrapulmonary arteriovenous anastomoses; saline contrast echocardiography BEYOND THE AGE OF 50 YR, mean pulmonary arterial pressure (Ppa) during exercise increases (17, 18). Although the reasons for this are not fully understood, age-related increases in exercise Ppa are believed to be partly attributable to the stiffening of pulmonary vascular and cardiac tissue, which increases pulmonary vascular resistance (PVR) and left atrial pressure (PLA), respectively (43). It has been hypothesized that PVR and Ppa during exercise may also partly depend on exercise-induced blood flow through large-diameter intrapulmonary arteriovenous anastomoses (IPAVA) (42), which is detectable using transthoracic saline contrast echocardiography in individuals who do not have a patent foramen ovale (PFO) (5). With a diameter ⱖ50 ␮m (30), IPAVA are notably larger

Address for reprint requests and other correspondence: A. T. Lovering, Dept. of Human Physiology, 122c Esslinger Hall, 1240 Univ. of Oregon, Eugene, OR 97403-1240 (e-mail: [email protected]). 1324

than 7- to 10-␮m pulmonary capillaries (10) and therefore may have the potential to act as low-resistance vascular conduits when recruited. Aligned with this idea, lower exercise-induced blood flow through IPAVA has been associated with high PVR and Ppa in humans during exercise (42). IPAVA may therefore act as potential pop-off valves (3) such that high blood flow through IPAVA during exercise mitigates excessive increases in PVR and Ppa. Alternatively, blood flow through IPAVA may not be entirely responsible for this observed association. IPAVA may be localized to the areas of the pulmonary circulation that are recruited under conditions of elevated pulmonary blood flow, such that when recruitment occurs, blood also flows through IPAVA. If true, then a reduction in blood flow through IPAVA may indicate a reduction in, or the loss of, these areas of the lung available for recruitment to accommodate increases in pulmonary blood flow. This association of blood flow through IPAVA and reduced pulmonary pressures may suggest that populations known to have high PVR and Ppa, such as older individuals, may also have less exercise-induced blood flow through IPAVA. Exercise-induced blood flow through IPAVA has not been studied in individuals ⱖ50 yr old, however. In support of this idea, no left heart contrast was detected at rest or during exercise in a study of subjects with chronic pulmonary diseases and pulmonary hypertension who did not have a PFO (12). In that study, right ventricular systolic pressure during peak exercise was 50 mmHg higher than in healthy controls. Therefore, the first purpose of this study was to investigate the presence and qualitative degree of exercise-induced blood flow through IPAVA in older individuals vs. younger controls who did not have a PFO (i.e., individuals ⱖ50 yr old vs. ⱕ41 yr old). We hypothesized that exercise-induced blood flow through IPAVA would be absent or qualitatively lower in older individuals compared with younger individuals, concomitant with high pulmonary arterial systolic pressure (PASP) and high total ˙ c), where Q ˙c pulmonary resistance (TPR) (TPR ⫽ PASP/Q represents cardiac output. Blood flow through IPAVA can also be detected in healthy young individuals acutely breathing hypoxic gas mixtures while at rest (23). It is unknown whether hypoxia-induced blood flow through IPAVA occurs through the same vessels as those observed during exercise, but presumably, IPAVA that accommodate blood flow during exercise are similar if not identical to IPAVA that accommodate blood flow in subjects breathing hypoxic gas mixtures. Similar to exercise-induced blood flow through IPAVA, hypoxia-induced blood flow through IPAVA at rest has not been studied in individuals ⱖ50 yr old. We reasoned that if there was also lower hypoxiainduced blood flow through IPAVA at rest in older individuals,

8750-7587/14 Copyright © 2014 the American Physiological Society

http://www.jappl.org

Downloaded from on February 19, 2015

Norris HC, Mangum TS, Duke JW, Straley TB, Hawn JA, Goodman RD, Lovering AT. Exercise- and hypoxia-induced blood flow through intrapulmonary arteriovenous anastomoses is reduced in older adults. J Appl Physiol 116: 1324 –1333, 2014. First published March 13, 2014; doi:10.1152/japplphysiol.01125.2013.—Mean pulmonary arterial pressure (Ppa) during exercise is significantly higher in individuals aged ⱖ50 yr compared with their younger counterparts, but the reasons for this are unknown. Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) can be detected during exercise or while breathing hypoxic gas mixtures using saline contrast echocardiography in almost all healthy young individuals. It has been previously hypothesized that a lower degree of exercise-induced blood flow through IPAVA is associated with high Ppa during exercise. This association may suggest that individuals who are known to have high Ppa during exercise, such as those ⱖ50 yr of age, may have lower blood flow through IPAVA, but the presence and degree of exercise-induced blood flow through IPAVA has not been specifically studied in older populations. Using transthoracic saline contrast echocardiography, we investigated the potential effects of age on exercise-induced blood flow through IPAVA in a cross-section of subjects aged 19 –72 yr. To verify our findings, we assessed the effects of age on hypoxia-induced blood flow through IPAVA. Age groups were ⱕ41 yr (younger, n ⫽ 16) and ⱖ50 yr (older, n ⫽ 14). Qualitatively measured exercise- and hypoxia-induced blood flow through IPAVA was significantly lower in older individuals compared with younger controls. Older individuals also had significantly higher pulmonary arterial systolic pressure and total pulmonary resistance (TPR) during exercise. Low blood flow through IPAVA was independently associated with high TPR. The reasons for the age-related decrease in blood flow through IPAVA are unknown.

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

then these findings would verify that blood flow through IPAVA is truly reduced in this population. Therefore, the second purpose of this study was to investigate the presence and qualitative degree of hypoxia-induced blood flow through IPAVA at rest in younger vs. older individuals who did not have a PFO. Because we reasoned that blood flow through IPAVA would be reduced in older individuals during exercise, we hypothesized that older subjects would not be able to recruit IPAVA under any condition such that hypoxia-induced blood flow through IPAVA would also be absent or qualitatively lower in older individuals compared with their younger counterparts. METHODS

Norris HC et al.

1325

22-gauge intravenous catheter with saline lock was placed into an antecubital vein with a three-way stopcock attached to the end. For exercise testing, a second stopcock and a 250-ml bag of sterile saline was connected in series with the first stopcock. Two 10-ml syringes, one containing 1 ml of air and the other containing 3 ml of sterile saline, were attached to open ports of the stopcock. Saline contrast bubbles were created manually by pushing the two syringe plungers back and forth for 10 s before a forced hand injection of the agitated saline (5). Saline contrast was imaged using a clear apical, fourchamber echocardiogram in the left lateral decubitus position for screenings (26). Subjects were considered positive for PFO if saline contrast appeared in the left ventricle in less than or equal to three cardiac cycles after saline contrast first opacified the right ventricle. In all subjects, PFO screenings were conducted with and without the release of a Valsalva maneuver (7). Forty-four subjects (younger, n ⫽ 18; older, n ⫽ 26) who had normal pulmonary function and no history of cardiopulmonary disease participated in the echocardiographic screening. Nine were excluded for the presence of PFO (21%). Among older subjects the prevalence of PFO was 8/26 (31%); among younger subjects it was 1/18 (6%). The prevalence of PFO among younger subjects was unusually low in this study because 15 of 18 subjects had been previously screened for ongoing studies in our laboratory and were known to be negative for PFO before enrolling in the present study. When blood does not flow through a PFO or IPAVA, saline contrast is visualized as a cloud of echoes in the right atrium and ventricle only. No contrast appears in the left atrium or ventricle because it is eliminated by the pulmonary microcirculation (4). If contrast passes through large-diameter IPAVA it will traverse the pulmonary circulation and appear in the left ventricle after a delay of more than three cardiac cycles. Accordingly, the appearance of saline contrast in the left ventricle in subjects who did not have a PFO was considered positive for blood flow through IPAVA. In two older subjects who were negative for PFO, saline contrast appeared in the left ventricle three cardiac cycles after right heart opacification, while they were in the left lateral decubitus position, at rest breathing room air, during the echocardiographic screening prior to any intervention (i.e., exercise and hypoxia). These two subjects were excluded from the study because subjects who demonstrate the transpulmonary passage of contrast prior to interventions do not provide a baseline of no left heart contrast on which we may form conclusions about the effects of interventions. Recently, we reported that 28% of healthy young individuals demonstrate left heart contrast at rest while breathing room air in the left lateral decubitus position and do not have a PFO (7). In this study, no younger subject without a PFO demonstrated transpulmonary passage of contrast while breathing room air in the left lateral decubitus position, which reflects that 15 of 18 younger subjects were previously screened for other studies. ˙ c in the left lateral decubitus position was measured using Resting Q ˙ c was calculated echocardiography in all subjects during screening. Q ˙ c ⫽ SV⫻HR, where SV ⫽ stroke volume and HR ⫽ using the equation Q heart rate. SV was calculated using the equation SV ⫽ EDLVV ⫺ ESLVV, where EDLVV ⫽ end diastolic left ventricular volume and ESLVV ⫽ end systolic left ventricular volume, both of which were measured from the apical, four-chamber view using the modified Simpson’s method for determining left ventricular volumes (21). HR was obtained from a three-lead electrocardiogram. PASP was calculated noninvasively using the simplified Bernoulli equation, PASP ⫽ 4v2 ⫹ PRA, where v ⫽ tricuspid regurgitation peak velocity as measured by continuous-wave Doppler ultrasound color flow imaging, and PRA ⫽ assigned right atrial pressure (26). Color flow was used to visualize and align the ultrasound probe with the tricuspid regurgitation jet. This calculation correlates well with direct measurements of PASP (12). Saline contrast injections (1 ml saline agitated with 0.2 ml air) were used to enhance the visualization of the v signal as requested by the ultrasonographer.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

The University of Oregon Institutional Review Board approved this study, and all subjects provided verbal and written informed consent before beginning study procedures. All studies were performed according to the Declaration of Helsinki. Subjects. A total of 74 subjects aged 19 –77 yr volunteered to participate in this cross-sectional study. Forty-four subjects were excluded for lower than normal pulmonary function values (n ⫽ 24; see criteria below under Pulmonary function, lung volume, and diffusion capacity screening), presence of PFO (n ⫽ 9), not completing the study (n ⫽ 4), disclosing a history of cardiopulmonary disease (n ⫽ 3), demonstrating transpulmonary passage of contrast at rest breathing room air in the left lateral decubitus position (n ⫽ 2), having a total pulmonary resistance greater than 3 SD from the mean (n ⫽ 1), or having a poor echocardiographic window during exercise (n ⫽1). Subjects were grouped on the basis of age in consideration of a recent metaanalysis of right heart catheter studies (17, 18) with a 9-year separation between groups: ⱕ41 yr (younger) and ⱖ50 yr (older). Thirty subjects aged 19 –72 yr with no history of cardiopulmonary disease were included in the final analysis of the study (younger, n ⫽ 16; older, n ⫽ 14). Pulmonary function, lung volume, and diffusion capacity screening. Comprehensive spirometry, including forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), forced midexpiratory flows (FEF25–75), and peak forced expiratory flow (FEFmax), was determined using a computerized spirometry system (MedGraphics Ultima PFX, Breeze v.6.3.006; MGC Diagnostics, St. Paul, MN) according to American Thoracic Society/European Respiratory Society (ATS/ERS) standards (34). Total lung capacity (TLC) was determined using whole body plethysmography (MedGraphics Elite Series Plethysmograph, Breeze v.7.1.0.34; MGC Diagnostics) according to ATS/ERS standards (45). Following this test, lung diffusion capacity for carbon monoxide (DLCO) was determined using the single-breath, breath-hold technique according to ATS/ERS standards (31). Predicted values for pulmonary function and DLCO were calculated as previously described (15, 16). Subjects were excluded from the study if FEV1, FVC, or DLCO per alveolar volume (VA) was ⬍80% of the predicted normal value (36). Ultrasound screening and saline contrast echocardiography. One of two registered diagnostic cardiac sonographers (R.D.G. and J.E.F.) with more than 44 years of combined stress echocardiography experience conducted all echocardiographic screenings (Philips Sonos 5500) on all subjects to assess cardiac abnormalities involving ventricular outflow, valvular function, ventricular function, the great vessels, and the pericardium. There were no signs of ischemia or heart disease in any subjects. Right atrial pressure, anterior right ventricular wall thickness, and pulmonary valve peak flow velocity were measured according to American Society of Echocardiography guidelines (40). Subjects were screened for PFO using saline contrast echocardiography. Saline contrast echocardiography was performed at rest and during exercise as detailed previously (6, 22, 23). Briefly, a 20- or

•

1326

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

Norris HC et al.

protocol. Because hypoxia-induced blood flow through IPAVA has been previously associated with FIO2 and SpO2 (23), subjects breathed a hypoxic gas mixture (FIO2 ⫽ 0.12, balance N2) for 15 to 30 min with the goal of achieving an SpO2 of between 80 – 85%. Only data from subjects achieving an SpO2 ⬍90% was considered for analysis (younger, n ⫽ 16; older, n ⫽ 11) to ensure that all subjects had a sufficient hypoxic stimulus, on the basis of our previous work (23). In three older subjects, an FIO2 of 0.12 was not sufficient to decrease SpO2 to ⬍90% after 25 min. Medical-grade nitrogen and air were mixed through a gas blender and stored in a nondiffusing Mylar bag to create the hypoxic gas mixture. Subjects breathed through a low-resistance two˙ c, PASP, and way nonrebreathing valve (2400; Hans Rudolph). Q bubble score (measured with saline contrast echocardiography) were obtained within the span of 1 min when SpO2 was closest to the targeted range after breathing hypoxic gas for 15 to 30 min. Bubbles scores. Blood flow through IPAVA for older and younger subjects was initially scored using our previously published scoring system where 0 ⫽ zero contrast bubbles; 1 ⫽ 1–3 bubbles; 2 ⫽ 4 –12 bubbles; 3 ⫽ ⬎12 bubble bolus; 4 ⫽ ⬎12 bubbles heterogeneously distributed; and 5 ⫽ ⬎12 bubbles homogenously distributed (6, 23, 29). Subsequently, we found the greatest separation between groups when the bubble scores were subclassified as either “low” or “high” using a modified version of our previously published scoring system, where low ⫽ 0 –3 contrast bubbles, and high ⫽ ⱖ4 contrast bubbles. In our previously published scoring system, this corresponded to scores of ⬍2 and ⱖ2, respectively. The bubble score of high or low was assigned by the echocardiographic technician using the single frame with the greatest number and spatial distribution of saline contrast bubbles in the left ventricle during the 20 cardiac cycles after saline contrast opacification of the right ventricle. The ultrasonographers assigned bubble scores on one occasion. A cardiologist (J.A.H.) who was blinded to subjects’ age and condition reassigned bubble scores on a separate occasion to validate the original observations. There was 92% agreement between observers that the bubble score was ⬍2 (low bubble score) or ⱖ2 (high bubble score), which was the statistical distinction made in this study for the ␹2-test analysis (see below). Data analysis. All statistical calculations were made using GraphPad Prism statistical software (v5.0d). Significance was set at P ⬍ 0.05. A ␹2-test analysis was used to compare bubble scores between age groups during exercise and while breathing hypoxic gas. Bubbles scores for the ␹2-test analysis were divided into two groups: low bubble score (⬍2) and high bubble score (ⱖ2) on the basis of our previous work (6, 22, 23). A linear regression analysis of exercise ˙ c data was performed individually for all subjects, and the PASP/Q slope of the line was used to assess total pulmonary resistance (TPR; ˙ c). Individual results were separated into groups TPR ⫽ PASP/Q (younger vs. older; high bubble score vs. low bubble score), and group comparisons of slope and intercept were made using a betweensubjects Student’s t-test with Welch’s correction. All other comparisons were analyzed using a between-subjects Student’s t-test. A Welch’s correction was used when the n between groups was unequal. RESULTS

Anthropometric, sex, age, and lung function. Anthropometric, sex, age, and pulmonary function data are presented in Table 1 and Table 2 for age and bubble groups, respectively. Between age groups, age, FEV1, FEV1% predicted, FVC, FEV1/FVC, DLCO% predicted, DLCO/VA, and DLCO/VA% predicted were significantly different. Between bubble score groups, age, FEV1/FVC, and DLCO/VA were significantly different. Echocardiographic screening at rest in the left lateral decubitus position. Echocardiographic data at rest breathing room air in the left lateral decubitus position is presented in Table 3

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

Arterial oxygen saturation. Peripheral estimates of arterial oxygen saturation (SpO2) were continuously measured on the exercise and the hypoxia study days using a pulse oximeter (Oximax N-600; Covidien Nellcor, Mansfield, MA) with forehead sensor (Oximax Max-Fast; Covidien). These data were continuously fed into the same data acquisition software (Breeze v.6.3.006) that collected breath-bybreath metabolic data from the subject. No direct measurements of arterial oxygen saturation (SaO2) were taken in this study, but we have found a strong correlation between SaO2 (OSM-3; Radiometer, Copenhagen, Denmark) and SpO2 measurements (n ⫽ 213) conducted in our own laboratory (23, 34) across a range of values (SaO2 ⫽ 63.5–99.5) that include those observed in this study (y ⫽ 1.04x ⫺ 5.83, where y ⫽ %SaO2 and x ⫽ %SaO2, r2 ⫽ 0.978). A Bland-Altman plot of these data revealed a mean difference of SpO2 ⫺ SaO2 of 2.15 ⫾ 1.31%; this indicates that our SpO2 measurements slightly overestimate the SaO2 measurements. The slope of the difference between SpO2 and SaO2 vs. the mean of SpO2 and SaO2 was ⫺0.0506 ⫾ 0.01, and was statistically significantly different from zero for the 213 pairs of measurements. Exercise while breathing room air in the forward leaning position. During a separate visit, subjects who passed the screening outlined above (younger, n ⫽ 16; older, n ⫽ 14) performed progressive incremental exercise while breathing room air in the forward leaning position (26) on a mechanically braked cycle ergometer (Lode Excaliber Sport) to 85% of age predicted maximum HR or volitional exhaustion. This forward leaning position facilitates obtaining a clear apical, four-chamber echocardiogram during exercise, by both allowing the heart to fall forward on the rib cage and subjects to stabilize their upper torso [see Fig. 3 in (26)]. Individualized protocols were developed with the goal for all subjects being the completion of three to five incremental stages of exercise that were each 3 min in length. All subjects completed a stage of exercise at 100 ⫾ 10 W, except for one 22-yr-old subject who started the protocol above 100 ⫾ 10 W, and one 69-yr-old subject who did not achieve a workload of 100 ⫾ 10 W. Subjects breathed through a low-resistance two-way nonrebreathing valve (2400; Hans Rudolph, Kansas City, MO) and pneumotachograph (PreVent; MGC Diagnostics). Breath-by-breath metabolic data were collected continuously with a computerized metabolic system (Ultima PFX and Breeze v.7.1.0.55; MGC Diagnostics). PASP was obtained at ⬃60 s into each exercise stage. Metabolic measurements ˙ c (see below) and bubble scores (using saline contrast echocarfor Q diography as described above) were obtained at ⬃90 s into each exercise stage. End-expiratory lung volume (EELV) was obtained after bubble score at rest and during exercise. EELV was determined from inspiratory capacity (IC) and used to determine whether subjects were hyperinflating during exercise (13). To obtain IC, all subjects were instructed how to perform an IC maneuver for the exercise experiment. No subject had difficulty reliably performing the IC. At rest and during each exercise stage, subjects performed an IC maneuver after a minimum of four respiratory cycles with a constant end expiratory volume were recorded. ˙ c was predicted from direct measureDuring the exercise study, Q ˙ O2 using one of two equations that correlate well with ments of V ˙ c when age and athletic status are direct Fick measurements of Q considered (8). For older subjects who exercised regularly (n ⫽ 11) and for all younger subjects (n ⫽ 16), the following equation was ˙ c l/min ⫽ (5.2⫻V ˙ O2 ml·kg⫺1·min⫺1 ⫹ 66)⫻mass (kg). For used: Q older subjects who did not exercise regularly (n ⫽ 4), the following ˙ c l/min ⫽ (5.9·V ˙ O2 ml·kg⫺1·min⫺1 ⫹ 49)·mass equation was used: Q ˙ I) was calculated using the equation Q ˙I ⫽ (kg). Cardiac index (Q ˙ c/BSA, where BSA ⫽ body surface area as calculated by the Q Mosteller formula (35). At rest while breathing hypoxic gas in the left lateral decubitus position. During a separate visit, subjects participated in the hypoxia protocol (younger, n ⫽ 16; older, n ⫽ 14). The order of visits for the exercise and hypoxia protocols was randomized, but all subjects who participated in the hypoxia protocol also participated in the exercise

•

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

Table 1. Anthropometric and pulmonary function data by age group Subjects, n Male, n Age range, yr Age, yr Height, cm Weight, kg BMI, kg/m2 BSA, m2 FEV1, liter FEV1, %p FVC, liter FVC, %p FEV1/FVC FEV1/FVC, %p FEF25–75, l/s FEF25–75, %p DLCO, ml·min⫺1·mmHg⫺1 DLCO, %p DLCO/VA DLCO/VA, %p

Younger

Older

16 12 50–77 24 ⫾ 6 177 ⫾ 10 75 ⫾ 11 23.8 ⫾ 2.3 1.9 ⫾ 0.2 4.3 ⫾ 0.9 98 ⫾ 9 5.2 ⫾ 1.1 99 ⫾ 7 83 ⫾ 5 99 ⫾ 5 4.3 ⫾ 1.2 95 ⫾ 20 37.7 ⫾ 9.7 119 ⫾ 19 5.6 ⫾ 0.8 118 ⫾ 18

14 9 19–41 63 ⫾ 9* 173 ⫾ 9 75 ⫾ 20 24.7 ⫾ 3.6 1.9 ⫾ 0.3 3.3 ⫾ 0.6* 108 ⫾ 0.15* 4.3 ⫾ 0.9* 105 ⫾ 13 77 ⫾ 6* 102 ⫾ 7 3.4 ⫾ 1.3 114 ⫾ 34 30.9 ⫾ 8.6 103 ⫾ 19* 4.8 ⫾ 0.5* 102 ⫾ 12*

and Table 4 for age and bubble groups, respectively. Older subjects had significantly lower left ventricular end systolic volume and significantly higher anterior right ventricular wall thickness than younger subjects. There were no significant differences between bubble groups. Exercise while breathing room air in the forward leaning position. During exercise, older subjects were significantly more likely than younger controls to achieve a low bubble score while exercising at both 100 ⫾ 10 W (␹2-test, P ⬍ 0.001) and 85% of predicted maximum HR (␹2-test, P ⬍ 0.0001) Table 2. Anthropometric and pulmonary function data by bubble group Subjects, n Male, n Age range, yr Age, yr Height, cm Weight, kg BMI, kg/m2 BSA, m2 FEV1, liter FEV1, %p FVC, liter FVC, %p FEV1/FVC FEV1/FVC, %p FEF25–75, l/s FEF25–75, %p DLCO, ml·min⫺1·mmHg⫺1 DLCO, %p DLCO/VA DLCO/VA, %p

High Bubble

Low Bubble

22 15 19–71 34 ⫾ 18 175 ⫾ 9 73 ⫾ 11 23.8 ⫾ 2.1 1.9 ⫾ 0.2 4.0 ⫾ 0.9 100 ⫾ 10 4.9 ⫾ 1.1 101 ⫾ 10 82 ⫾ 5 100 ⫾ 5 4.0 ⫾ 1.3 98 ⫾ 21 35.6 ⫾ 10.1 114 ⫾ 21 5.4 ⫾ 0.8 113 ⫾ 18

8 6 51–72 66 ⫾ 7* 174 ⫾ 11 79 ⫾ 25 25.6 ⫾ 4.4 1.9 ⫾ 0.4 3.4 ⫾ 0.7 108 ⫾ 19 4.4 ⫾ 0.9 105 ⫾ 13 76 ⫾ 6* 102 ⫾ 8 3.6 ⫾ 1.5 117 ⫾ 44 31.7 ⫾ 8.4 103 ⫾ 19 4.8 ⫾ 0.5* 103 ⫾ 12

Values are mean ⫾ SD. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

1327

Norris HC et al.

Table 3. Echocardiographic data at rest, left-lateral decubitus position by age group Subjects, n ˙ c, l/min Q ˙ I, l·min⫺1·m⫺2 Q HR, beats/min SV, ml LVEDV, ml LVESV, ml LVEF, % PASP, mmHg PRA, mmHg Anterior RV WT, mm

Younger

Older

16 4.2 ⫾ 1.3 2.2 ⫾ 0.6 60 ⫾ 13 71 ⫾ 20 120 ⫾ 33 49 ⫾ 16 60 ⫾ 6 25 ⫾ 7 3⫾0 3.9 ⫾ 0.6

14 4.0 ⫾ 1.5 2.1 ⫾ 0.7 60 ⫾ 9 67 ⫾ 24 103 ⫾ 32 36 ⫾ 13* 64 ⫾ 8 25 ⫾ 5 4⫾2 4.8 ⫾ 0.7*

Values are mean ⫾ SD. HR, heart rate; LVEDV, left ventricular end diastolic volume; LVEF, left ventricular ejection fraction; LVESV, left ventricular end systolic volume; PASP, pulmonary artery systolic pressure; Pra, ˙ c, cardiac output; Q ˙ I, cardiac index; RV WT, right right atrial pressure; Q ventricular wall thickness; SV, stroke volume. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

(Figs. 1 and 2). At 100 ⫾ 10 W, only one subject from each age group demonstrated hyperinflation, as defined by an increase in EELV from rest to exercise. Although EELV was significantly higher in older subjects, 28 of 30 (93%) subjects did not hyperinflate during exercise. No subjects demonstrated hyperinflation at 85% of predicted maximum HR. Older subjects had significantly higher TPR compared with young subjects (Fig. 3). Total pulmonary resistance index ˙ I) was also significantly higher in older (TPRI; TPRI ⫽ PASP/Q subjects (4.0 ⫾ 0.4 vs. 5.6 ⫾ 0.4; P ⬍ 0.01). During exercise at 100 ⫾ 10 W (Table 5), older subjects had significantly ˙ c were higher PASP than younger subjects. Workload and Q not different between groups. At 85% of predicted maximum HR (Table 6), PASP was not significantly different between age groups; however, younger subjects achieved significantly ˙ c, Q ˙ I, and HR. higher workloads, Q The subjects who achieved a bubble score ⬍2 (low bubble score group) had significantly higher TPR compared with subjects who achieved a bubble score ⱖ2 (high bubble score group) (Fig. 4). Additionally, TPRI was significantly higher in the low bubble score group (4.3 ⫾ 0.3 vs. 6.1 ⫾ 0.4; P ⬍ 0.01). During exercise at 100 ⫾ 10 W (Table 7), the low bubble score group also had significantly higher PASP than the high ˙ c were not different bubble score group. Workload and Q Table 4. Echocardiographic data at rest, in left-lateral decubitus position, by bubble group Subjects, n ˙ c, l/min Q ˙ I, L·min⫺1·m⫺2 Q HR, beats/min SV, ml LVEDV, ml LVESV, ml LVEF, % PASP, mmHg PRA, mmHg Anterior RV WT, mm

High Bubble

Low Bubble

22 4.1 ⫾ 1.4 2.2 ⫾ 0.6 60 ⫾ 11 69 ⫾ 22 113 ⫾ 34 44 ⫾ 16 61 ⫾ 7 25 ⫾ 6 3⫾1 4.1 ⫾ 0.7

8 4.0 ⫾ 1.3 2.1 ⫾ 0.7 60 ⫾ 11 68 ⫾ 23 107 ⫾ 33 39 ⫾ 16 64 ⫾ 8 26 ⫾ 5 4⫾2 4.8 ⫾ 0.9

Values are mean ⫾ SD. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

Values are mean ⫾ SD. BMI, body mass index; BSA, body surface area; DLCO, diffusion capacity for carbon monoxide; FEF25–75, forced midexpiratory flow; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; %p, percent of predicted value; VA, alveolar volume. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

•

1328

Effects of Age on Intrapulmonary Arteriovenous Anastomoses High Bubble Score

Norris HC et al. OLDER y = 2.9x + 10.2

90

Low Bubble Score *p = .0006

* p < .05

YOUNGER y = 2.1x + 14.4

80 70

PASP (mmHg)

Number of Subjects

15

•

10

5

60 50 40 30

0 Youger

Older

Fig. 1. Number of subjects achieving high or low bubble scores while exercising at 100 ⫾ 10 W in the forward leaning position. Data were analyzed using the ␹2-test method. Age categories were ⱕ41 yr (younger) and ⱖ50 yr (older). Bubble score categories were low and high. Compared with younger subjects, older subjects were more likely to achieve a low bubble score, P ⬍ 0.01.

DISCUSSION

To our knowledge, this is the first investigation of exerciseand hypoxia-induced blood flow through IPAVA in healthy humans ⱖ50 yr of age compared with healthy humans ⱕ41 yr High Bubble Score Low Bubble Score *p = .0001

Number of Subjects

20 15 10 5 0 Youger

Older

Fig. 2. Number of subjects achieving high or low bubble scores while exercising at 85% of predicted maximum heart rate in the forward leaning position. Data were analyzed using the ␹2-test method. Age categories were ⱕ41 yr (younger) and ⱖ50 yr (older). Bubble score categories were low and high. Compared with younger subjects, older subjects were more likely to achieve a low bubble score, P ⬍ 0.001.

5

10

15

20

25

30

QC (L/min) ˙ c during exercise in the forward leaning position, by age Fig. 3. PASP vs. Q group. Dotted lines represent individual regression of a single subject’s data during exercise. Thick solid lines represent group composite of individual regressions. Age categories were ⱕ 41 yr (younger) and ⱖ 50 yr (older). The slopes of regressions represent total pulmonary resistance (TPR). TPR was significantly different between groups (P ⬍ 0.05).

of age. Our cross-sectional analysis of subjects aged 19 –72 yr supported our primary hypothesis that exercise-induced blood flow through IPAVA was qualitatively lower in older individuals compared with younger controls, concomitant with higher TPR in older subjects. Additionally, hypoxia-induced blood flow through IPAVA was qualitatively lower in a subset of older individuals compared with younger controls. Effect of age on exercise-induced blood flow through IPAVA. During exercise, bubble scores were qualitatively lower in older subjects compared with younger controls at both 100 ⫾ 10 W (␹2-test, P ⬍ 0.001) and at 85% of predicted maximum HR (␹2-test, P ⬍ 0.0001) (Figs. 1 and 2). We Table 5. Exercise at 100 ⫾ 10 W, forward leaning position by age group Subjects, n Absolute workload, W Relative workload, W/kg PASP, mmHg ˙ c, l/min Q ˙ I, L·min⫺1·m⫺2 Q HR, beats/min HR reserve, beats/min ˙ O2, l/min V ˙ CO2, l/min V RER ˙ E BTPS, l/min V ˙ T BTPS, liter V f, breaths/min EELV, liter ⌬EELV, liter SpO2, %

Younger

Older

15 99 ⫾ 5 1.4 ⫾ 0.2 42 ⫾ 7 14.0 ⫾ 1.6 7.3 ⫾ 0.4 121 ⫾ 25 74 ⫾ 24 1.74 ⫾ 0.21 1.59 ⫾ 0.19 0.93 ⫾ 0.13 40.4 ⫾ 5.7 0.92 ⫾ 0.94 29 ⫾ 6 2.9 ⫾ 1.3 ⫺0.4 ⫾ 0.5 99 ⫾ 1

13 101 ⫾ 7 1.4 ⫾ 0.3 51 ⫾ 9* 13.7 ⫾ 3.1 7.2 ⫾ 0.8 119 ⫾ 20 38 ⫾ 20* 1.70 ⫾ 0.39 1.71 ⫾ 0.37 1.01 ⫾ 0.12 49.8 ⫾ 10.9* 1.98 ⫾ 0.40* 26 ⫾ 7 3.9 ⫾ 0.7* ⫺0.5 ⫾ 0.4 99 ⫾ 2

Values are mean ⫾ SD. EELV, end expiratory lung volume; ⌬EELV, change in EELV from rest; f, breathing frequency; RER, respiratory exchange ˙ E, ratio; SpO2, arterial oxygen saturation as measured using pulse oximetry; V ˙ CO2, carbon dioxide production; V ˙ O2, oxygen consumpminute ventilation; V ˙ T, tidal volume; BTPS, body, temperature, pressure saturated. Betweention; V subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

between groups. At 85% of predicted maximum HR, PASP was not significantly different between bubble score groups; however, the high bubble score group achieved significantly ˙ I, and HR (Table 8). higher workloads, Q At rest, breathing hypoxic gas in the left lateral decubitus position. All 16 younger subjects and 11 of the 15 older subjects who participated in the exercise portion of this study also participated in the hypoxia portion of the study. In the hypoxic condition, older subjects were significantly more likely than younger subjects to achieve a low bubble score (␹2-test, P ⬍ 0.05). Bubble scores in the hypoxic condition are presented in Fig. 5. Younger subjects breathed hypoxic gas for a longer time and also had lower SpO2 than the older subjects (Table 9).

20

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

Table 6. Exercise at 85% of predicted maximum heart rate, forward leaning position by age group Subjects, n Absolute workload, W Relative workload, W/kg PASP, mmHg ˙ c, l/min Q ˙ I, l·min⫺1·m⫺2 Q HR, beats/min HR reserve, beats/min ˙ O2, l/min V ˙ CO2, l/min V RER ˙ E BTPS l/min V ˙ T BTPS, liter V f, breaths/min EELV, liter ⌬EELV, liter SpO2, %

Younger

Older

16 217 ⫾ 81 2.9 ⫾ 1.0 55 ⫾ 8 20.2 ⫾ 5.2 10.5 ⫾ 2.1 164 ⫾ 6 32 ⫾ 5 2.93 ⫾ 0.91 3.25 ⫾ 1.00 1.11 ⫾ 0.08 83.5 ⫾ 24.2 1.23 ⫾ 1.21 39 ⫾ 10 2.8 ⫾ 1.2 ⫺0.5 ⫾ 0.4 97 ⫾ 2

14 129 ⫾ 43* 1.8 ⫾ 0.6* 54 ⫾ 8 15.9 ⫾ 5.2* 8.3 ⫾ 2.0* 133 ⫾ 6* 24 ⫾ 8* 2.13 ⫾ 0.84* 2.26 ⫾ 0.83* 1.08 ⫾ 0.09 66.2 ⫾ 25.8* 2.24 ⫾ 0.64* 30 ⫾ 8* 3.8 ⫾ 0.6* ⫺0.5 ⫾ 0.4 98 ⫾ 2

•

1329

Norris HC et al.

Table 7. Exercise at 100 ⫾ 10 W, in forward leaning position, by bubble group Subjects, n Absolute workload, W Relative workload, W/kg PASP, mmHg ˙ c, l/min Q ˙ I, l·min⫺1·m⫺2 V HR, beats/min HR reserve, beats/min ˙ O2, l/min V ˙ CO2, l/min V RER ˙ E BTPS l/min V ˙ T BTPS, liter V f, breaths/min EELV, liter ⌬EELV, liter SpO2, %

High Bubble

Low Bubble

20 100 ⫾ 5 1.4 ⫾ 0.2 43 ⫾ 7 13.8 ⫾ 1.8 7.3 ⫾ 0.6 120 ⫾ 25 67 ⫾ 27 1.73 ⫾ 0.26 1.60 ⫾ 0.23 0.94 ⫾ 0.13 42.0 ⫾ 7.5 1.15 ⫾ 0.92 28 ⫾ 6 3.2 ⫾ 1.3 ⫺0.3 ⫾ 0.4 99 ⫾ 1

8 102 ⫾ 7 1.4 ⫾ 0.3 55 ⫾ 7* 13.9 ⫾ 3.6 7.1 ⫾ 0.7 120 ⫾ 18 35 ⫾ 18* 1.70 ⫾ 0.40 1.75 ⫾ 0.38 1.04 ⫾ 0.12 51.8 ⫾ 11.1* 2.07 ⫾ 0.45* 26 ⫾ 8 3.7 ⫾ 0.8 ⫺0.6 ⫾ 0.4 99 ⫾ 2

Values are mean ⫾ SD. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

assessed EELV at rest and during exercise to determine whether the difference in bubble scores between age groups during exercise was the result of dynamic hyperinflation and therefore greater intrathoracic pressure swings in older subjects. The effect of large intrathoracic pressure swings on IPAVA has not been determined but could potentially reduce blood flow through IPAVA similar to reduced flow in alveolar capillaries during inspiration to high lung volumes. Furthermore, increases in lung volumes during exercise due to dynamic hyperinflation may attenuate ultrasound transmission and make contrast in the left ventricle difficult to visualize, which could potentially result in an underestimation of bubble score. However, 28 of 30 subjects did not hyperinflate, and of the 2 that did, neither demonstrated hyperinflation at rest or at 85% of predicted maximum HR. During exercise at both 100 ⫾

10 W and 85% of predicted maximum HR, mean EELV for both age groups was lower than values at rest, as represented by negative values for ⌬EELV (Table 5 and 6). These data suggest that dynamic hyperinflation was not common among subjects. Therefore, the differences in bubble scores during exercise are likely not the result of high intrathoracic pressure swings or attenuated transmission of the ultrasound signal but instead represent a true decrease in the transit of saline contrast bubbles across the lung. We have previously hypothesized that IPAVA are recruited ˙ c and/or PASP in healthy young subjects due to increases in Q ˙ c was not significantly different (22). At 100 ⫾ 10 W, Q between age groups, but older subjects had significantly higher PASP, suggesting a greater stimulus to induce blood flow through IPAVA. Despite the potentially greater stimulus, older subjects still had lower bubble scores than younger controls. Why this occurs is unclear. Although our view is speculative, exercise-induced blood flow through IPAVA may decrease

LOW BUBBLE y = 2.9x + 9.9

90

* p < .05

HIGH BUBBLE y = 2.3x + 13.4

80

Table 8. Exercise at 85% of predicted maximum heart rate, in forward leaning position

PASP (mmHg)

70 60 50 40 30 20 5

10

15

20

25

30

QC (L/min) ˙ c during exercise in the forward leaning position, by bubble Fig. 4. PASP vs. Q score group. Dotted lines represent individual regression of a single subject’s data during exercise. Thick solid lines represent group composite of individual regressions. Bubble score categories were low and high. The slopes of regression lines represent TPR. TPR was significantly different between groups (P ⬍ 0.05).

Subjects, n Absolute workload, W Relative workload, W/kg PASP, mmHg ˙ c, l/min Q ˙ I, l·min⫺1·m⫺2 Q HR, beats/min HR reserve, beats/min ˙ O2, l/min V ˙ CO2, l/min V RER ˙ E BTPS, l/min V ˙ T BTPS, liter V f, breaths/min EELV, liter ⌬EELV, liter SpO2, %

High Bubble

Low Bubble

22 196 ⫾ 82 2.6 ⫾ 1.0 54 ⫾ 9 19.3 ⫾ 5.6 10.1 ⫾ 2.4 156 ⫾ 14 30 ⫾ 7 2.77 ⫾ 0.98 3.05 ⫾ 1.06 1.10 ⫾ 0.08 80.5 ⫾ 27.5 1.50 ⫾ 1.17 37 ⫾ 10 3.1 ⫾ 1.1 ⫺0.5 ⫾ 0.3 98 ⫾ 2

8 120 ⫾ 24* 1.6 ⫾ 0.3* 57 ⫾ 4 15.3 ⫾ 4.7 7.7 ⫾ 0.9* 130 ⫾ 4* 24 ⫾ 8 1.95 ⫾ 0.58* 2.08 ⫾ 0.56* 1.08 ⫾ 0.11 61.6 ⫾ 15.6* 2.26 ⫾ 0.64* 29 ⫾ 9 3.8 ⫾ 0.7 ⫺0.5 ⫾ 0.4 98 ⫾ 3

Values are mean ⫾ SD. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

Values are mean ⫾ SD. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

1330

Effects of Age on Intrapulmonary Arteriovenous Anastomoses High Bubble Score Low Bubble Score *p = .014

Number of Subjects

15

10

5

0 Youger

Older

Fig. 5. Number of subjects achieving high or low bubble scores while breathing hypoxic gas at rest in the left lateral decubitus position. Data were analyzed using the ␹2-test method. Age categories were ⱕ41 yr (younger) and ⱖ50 yr (older). Bubble score categories were low and high. Compared with younger subjects, older subjects were more likely to achieve a low bubble score, P ⬍ 0.05.

Norris HC et al.

blood flow through IPAVA also had the highest PVR and Ppa among the subjects studied. More recently, La Gerche et al. ˙ c to show that used noninvasive measurements of PASP and Q low pulmonary transit of agitated succinylated gelatin contrast during exercise was associated with high TPR and PASP; however, this study did not screen subjects for PFO, so whether contrast traversed the lung or a PFO is uncertain (19) (see Comparisons with studies using agitated succinylated gelatin contrast, below). The present study also associates low blood flow through IPAVA with high TPR and PASP. Using nonin˙ c and PASP during exercise in vasive measurements of Q subjects who did not have a PFO, we found that older subjects had both lower bubble scores and significantly higher TPR than younger controls (Fig. 3). Additionally, when separated by bubble score group instead of age group, the low bubble score group demonstrated significantly higher TPR than the high bubble score group (Fig. 4). Further supporting these results, the significant differences between age groups and between ˙ I was substituted for Q ˙c bubble groups were maintained when Q in the calculation of TPR, suggesting that body size was not a confounding factor. Although we did not make invasive measurements of Ppa and PLA assigned left arterial pressure and were therefore were unable to calculate PVR, our data agree with findings by Stickland et al. that lower blood flow through IPAVA is associated with high pulmonary vascular pressures and resistance during exercise. Whether or not exercise-induced blood flow through IPAVA directly regulates PVR and Ppa or is just associated with these measurements is unclear. Blood flowing through IPAVA dur˙ c (27) ing exercise has been estimated to be ⬃2% of the total Q and has been measured to be 1–3% in healthy humans (28) and dogs (41), which would likely be insufficient to cause an appreciable decrease in PVR. Changes in PVR may be better explained by changes in vascular compliance, as described above. If blood flow through IPAVA depends on recruiting specific regions of the lung, then age-related changes in pulmonary vascular recruitment may play a role. Determining the precise roles that IPAVA play in regulating pulmonary hemodynamics requires further investigation. Effect of age on hypoxia-induced blood flow through IPAVA. Similar to exercise in the forward leaning position, hypoxiainduced blood flow through IPAVA at rest in the left lateral decubitus position was qualitatively lower in older subjects compared with younger subjects (␹2-test, P ⬍ 0.05) (Fig. 5). Table 9. Acute normobaric hypoxia data, left-lateral decubitus position Subjects, n Time breathing gas SpO2, % PASP, mmHg ˙ c, l/min Q ˙ I, L·min⫺1·m⫺2 Q HR, beats/min SV, ml LVEDV, ml LVESV, ml FIO2

Younger

Older

16 24 ⫾ 6 79 ⫾ 4 31 ⫾ 8 5.0 ⫾ 1.7 2.6 ⫾ 0.7 68 ⫾ 8 72 ⫾ 21 112 ⫾ 34 41 ⫾ 15 0.12 ⫾ 0.0

11 19 ⫾ 2* 83 ⫾ 4* 32 ⫾ 10 4.6 ⫾ 1.2 2.5 ⫾ 0.5 67 ⫾ 5 69 ⫾ 17 100 ⫾ 26 32 ⫾ 13 0.12 ⫾ 0.0

Values are mean ⫾ SD. FIO2, fraction of inspired oxygen. Between-subjects Student’s t-test with Welch’s correction, *P ⬍ 0.05.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

with age secondary to one or more age-related changes in the lung. These include vascular remodeling and decreased vessel compliance. Although reports have been inconsistent, elastin fiber concentration in the pulmonary circulation may decrease with age (43), which is believed to contribute to age-related decreases in the compliance of conventional pulmonary vessels. This may apply to IPAVA as well. If exercise-induced blood flow through IPAVA is determined by pulmonary vascular pressures and flows as we have previously hypothesized (22), a decrease in compliance of IPAVA may reduce the ability of IPAVA to accommodate blood flow during exercise. Implications for age-related decreases in exercise-induced blood flow through IPAVA. It has been suggested by others that large-diameter right-to-left connections in the lung may provide additional pathways for blood flow that decrease PVR (42, 46). For example, Whyte et al. demonstrated that patients with arteriovenous malformations (AVMs) ⱖ23 ␮m have lower than normal PVR, and that the patients with the largest AVMs have the lowest PVR (46). IPAVA are ⱖ50 ␮m in diameter (30), which is at least sixfold larger than the average pulmonary capillaries (7–10 ␮m) (10). Although the location of IPAVA along the pulmonary vascular tree is not definitively known, IPAVA are believed to branch from small pulmonary arteries to connect with the venous end of the pulmonary capillary bed, bypassing arterioles and capillaries in the lung (33, 39, 44). The recruitment of IPAVA during exercise may potentially increase cross-sectional area over a resistive portion of the vascular bed to decrease PVR, which has led some researchers to suggest that IPAVA may act as pop-off valves and decrease Ppa during exercise (3). Several reports, including the present investigation, have associated low blood flow through IPAVA with high pulmonary pressure and resistance during exercise. In seminal work, Stickland et al. made direct and invasive measurements of PVR and Ppa during exercise while observing blood flow through IPAVA in eight healthy young humans who did not have a PFO (42). They found that 7 of 8 subjects demonstrated exercise-induced blood flow through IPAVA during exercise. The one subject who did not demonstrate exercise-induced

•

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

Norris HC et al.

1331

after the first three heart beats. Of note, blood can flow through a PFO after three heart beats when blood is also flowing through IPAVA. Therefore, the potential exists in individuals who have a PFO for contrast to traverse only the PFO, only IPAVA, or both simultaneously. This makes it challenging to determine the source of left heart contrast in individuals who have a PFO. Additionally, the presence of a PFO must be verified and/or ruled out using the release of a Valsalva maneuver. As reported by our group and others, PFO is present in 25– 40% of the population and is therefore a common finding (7, 32, 47). To definitively conclude that left heart contrast traversed the lung via IPAVA, all subjects with a PFO must be identified and excluded from these studies. Saline contrast may have higher specificity for detecting blood flow through IPAVA than gelatin contrast. Pulmonary capillaries may distend to a maximum diameter of 13 ␮m (10), with corner vessels reaching 20 ␮m. By comparison, IPAVA are ⱖ50 ␮m in diameter (30). Gelatin contrast used by La Gerche et al. and Lalande et al. (19, 20) was measured at 9 –33 ␮m in diameter with an average diameter of 21 ⫾ 6 ␮m. Although these measurements were made ex vivo and may not represent the actual in vivo diameter in moving blood subjected to intravascular pressures, gelatin contrast could potentially pass through distended capillaries and corner vessels in addition to IPAVA. By comparison, the estimated size of saline contrast that can survive long enough to traverse the lung is 60 –90 ␮m (5). This is larger than the maximum diameter of pulmonary capillaries or corner vessels but potentially smaller than the largest IPAVA. Although this size estimate is theoretical, studies using saline contrast echocardiography to detect blood flow through IPAVA are consistent with and supported by work using gold-standard microsphere techniques (2, 30, 41, 44). Thus saline contrast may be a preferable method for detecting blood flow through IPAVA. Clinical relevance of age-related reductions in blood flow through IPAVA. We and others have postulated that IPAVA may be a facilitator of embolic stroke (1, 25). By compromising the lung as a biological filter, these vessels may permit microemboli to bypass the lung and cause neurological sequelae. Indeed, microemboli as small as 98 ␮m in diameter can cause severe cerebral infarction in rats (38). Recently, a study in a broad range of human subjects showed that the transpulmonary passage of contrast at rest is significantly associated with stroke and transient ischemic attack (TIA) and most strongly associated with cryptogenic stroke and TIA (1). Thus these findings may implicate IPAVA in the development of cryptogenic neurological sequelae. Up to 40% of cerebrovascular infarctions in hospitalized patients are cryptogenic (1), but the prevalence of cryptogenic stroke may vary with age. A study of 1,004 consecutive first-ever stroke patients found that cryptogenic stroke was significantly lower in individuals aged 45– 49 yr compared with those aged 15– 44 yr (37). The association between blood flow through IPAVA and cryptogenic stroke offers an interesting direction for future investigations. ACKNOWLEDGMENTS We thank J. E. Futral for invaluable experience obtaining echocardiographic images. We also thank J. Kern for technical assistance and J. Elliott for providing critical feedback.

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

Although the specific mechanisms for this are unknown, we have previously shown that hypoxia induces blood flow through IPAVA in a dose-dependent manner in healthy young subjects (23) such that blood flow through IPAVA increases as FIO2 and SpO2 decrease. In agreement with our previous studies, younger subjects in the present study had significant blood flow through IPAVA while breathing hypoxic gas. Older subjects, however, demonstrated lower blood flow through IPAVA despite breathing the same FIO2. This could potentially be explained by the fact that younger subjects had significantly lower SpO2 (i.e., greater stimulus for blood flow through IPAVA), but we believe this is unlikely because the difference in SpO2 was small and within a range in which significant blood flow through IPAVA would be expected (23). Alternatively, lower SpO2 in younger subjects may be a result, rather than a cause, of higher blood flow through IPAVA. IPAVA can act as a shunt to decrease gas exchange efficiency (42) such that increasing blood flow through IPAVA decreases gas exchange efficiency. Thus, lower SpO2 in younger subjects may result from higher blood flow through IPAVA. A third explanation may be that older individuals tend to have a more brisk hypoxic ventilatory response (24) and therefore would be more likely to have higher SpO2 than younger subjects when breathing the same FIO2. In either case, we interpret these findings to mean that older individuals have lower blood flow through IPAVA than younger controls when breathing an FIO2 of 0.12. Transpulmonary passage of contrast at rest is lower in older subjects. We have recently shown that 28% of healthy young subjects demonstrate left heart contrast at rest while breathing room air in the left lateral decubitus position and do not have a PFO, suggesting the transpulmonary passage of saline contrast (7). These individuals are appropriately screened out of our studies along with individuals who have a PFO because individuals who demonstrate left heart contrast at rest while breathing room air do not provide a baseline of no left heart contrast from which we may form conclusions about interventions. In the present study we found that only 8% (2/26) of older subjects who participated in the echocardiographic screening demonstrated the transpulmonary passage of contrast at rest while breathing room air in the left lateral decubitus position. In humans, this is consistent with AVMs and/or pathologically dilated capillaries, though these cases are considered rare (11). It remains unknown whether saline contrast in this condition traverses the lung via pathological AVMs and dilated capillaries or via normal physiologic IPAVA. Currently, we cannot distinguish between these vessels using saline contrast echocardiography. Interestingly, however, the lower prevalence of the transpulmonary passage of contrast at rest while breathing room air in older subjects is similar to our findings that exercise- and hypoxia-induced blood flow through IPAVA is lower in older individuals. This may suggest that contrast can traverse the lung via IPAVA at rest while breathing room air, but that the likelihood of this decreases with age. Comparisons with studies using agitated succinylated gelatin contrast. The investigations by La Gerche et al. (19) and Lalande et al. (20) have important methodological considerations. Primarily, subjects in these studies were not screened for the presence of PFO, which may lead to ambiguous results. Using transthoracic contrast echocardiography in subjects who have a PFO, it is difficult if not impossible to determine whether contrast has entered the left heart via the lung or PFO

•

1332

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

GRANTS Support for this study was provided by the American Lung Association in Oregon and administered through American Thoracic Society Grant C-10-014. Additional support was provided by the American Physiological Society’s Giles F. Filley Memorial Award for Excellence in Respiratory Physiology and Medicine to A.T.L. DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author(s). AUTHOR CONTRIBUTIONS Author contributions: H.C.N. and A.T.L. conception and design of research; H.C.N., T.S.M., J.W.D., T.B.S., and R.D.G. performed experiments; H.C.N., T.S.M., J.W.D., T.B.S., J.A.H., R.D.G., and A.T.L. analyzed data; H.C.N., J.W.D., T.B.S., J.A.H., R.D.G., and A.T.L. interpreted results of experiments; H.C.N. prepared figures; H.C.N. drafted manuscript; H.C.N., T.S.M., J.W.D., and A.T.L. edited and revised manuscript; H.C.N., T.S.M., J.W.D., T.B.S., J.A.H., R.D.G., and A.T.L. approved final version of manuscript. REFERENCES

Norris HC et al.

17. Kovacs G, Berghold A, Scheidl S, Olschewski H. Pulmonary arterial pressure during rest and exercise in healthy subjects: a systematic review. Eur Respir J 34: 888 –894, 2009. 18. Kovacs G, Olschewski A, Berghold A, Olschewski H. Pulmonary vascular resistances during exercise in normal subjects: a systematic review. Eur Respir J 39: 319 –328, 2012. 19. La Gerche A, MacIsaac AI, Burns AT, Mooney DJ, Inder WJ, Voigt JU, Heidbüchel H, Prior DL. Pulmonary transit of agitated contrast is associated with enhanced pulmonary vascular reserve and right ventricular function during exercise. J Appl Physiol 109: 1307–1317, 2010. 20. Lalande S, Yerly P, Faoro V, Naeije R. Pulmonary vascular distensibility predicts aerobic capacity in healthy individuals. J Physiol 590: 4279 –4288, 2012. 21. Lang RM, Bierig M, Devereux RB, Flachskampf FA, Foster E, Pellikka PA, Picard MH, Roman MJ, Seward J, Shanewise JS, Solomon SD, Spencer KT, Sutton MS, Stewart WJ. Recommendations for chamber quantification: a report from the American Society of Echocardiography’s Guidelines and Standards Committee and the Chamber Quantification Writing Group, developed in conjunction with the European Association of Echocardiography, a branch of the European Society of Cardiology. J Am Soc Echocardiogr 18: 1440 –1463, 2005. 22. Laurie SS, Elliott JE, Goodman RD, Lovering AT. Catecholamineinduced opening of intrapulmonary arteriovenous anastomoses in healthy humans at rest. J Appl Physiol 113: 1213–1222, 2012. 23. Laurie SS, Yang X, Elliott JE, Beasley KM, Lovering AT. Hypoxiainduced intrapulmonary arteriovenous shunting at rest in healthy humans. J Appl Physiol 109: 1072–1079, 2010. 24. Lhuissier FJ, Canouï-Poitrine F, Richalet JP. Ageing and cardiorespiratory response to hypoxia. J Physiol 590: 5461–5474, 2012. 25. Lovering AT, Elliott JE, Beasley KM, Laurie SS. Pulmonary pathways and mechanisms regulating transpulmonary shunting into the general circulation: an update. Injury 41, Suppl 2: S16 –S23, 2010. 26. Lovering AT, Goodman RD. Detection of intracardiac and intrapulmonary shunts at rest and during exercise using saline contrast echocardiography. In: Applied Aspects of Ultrasonography in Humans, edited by Ainslie PN. Rejeka, Croatia, InTech, 2012, p. 159 –174. 27. Lovering AT, Haverkamp HC, Eldridge MW. Responses and limitations of the respiratory system to exercise. Clin Chest Med 26: 439 –457, iv, 2005. 28. Lovering AT, Haverkamp HC, Romer LM, Hokanson JS, Eldridge MW. Transpulmonary passage of 99mTc macroaggregated albumin in healthy humans at rest and during maximal exercise. J Appl Physiol 106: 1986 –1992, 2009. 29. Lovering AT, Stickland MK, Amann M, Murphy JC, O’Brien MJ, Hokanson JS, Eldridge MW. Hyperoxia prevents exercise-induced intrapulmonary arteriovenous shunt in healthy humans. J Physiol 586: 4559 –4565, 2008. 30. Lovering AT, Stickland MK, Kelso AJ, Eldridge MW. Direct demonstration of 25- and 50-micron arteriovenous pathways in healthy human and baboon lungs. Am J Physiol Heart Circ Physiol 292: H1777–H1781, 2007. 31. MacIntyre N, Crapo RO, Viegi G, Johnson DC, van der Grinten CP, Brusasco V, Burgos F, Casaburi R, Coates A, Enright P, Gustafsson P, Hankinson J, Jensen R, McKay R, Miller MR, Navajas D, Pedersen OF, Pellegrino R, Wanger J. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J 26: 720 –735, 2005. 32. Mariott K, Manins V, Forshaw A, Wright J, Pascoe R. Detection of right-to-left atrial communication using agitated saline contrast imaging: experience with 1162 patients and recommendations for echocardiography. J Am Soc Echocardiogr 26: 96 –102, 2013. 33. McMullan DM, Hanley FL, Cohen GA, Portman MA, Riemer RK. Pulmonary arteriovenous shunting in the normal fetal lung. J Am Coll Cardiol 44: 1497–1500, 2004. 34. Miller MR, Hankinson J, Brusasco V, Burgos F, Casaburi R, Coates A, Crapo R, Enright P, van der Grinten CP, Gustafsson P, Jensen R, Johnson DC, MacIntyre N, McKay R, Navajas D, Pedersen OF, Pellegrino R, Viegi G, Wanger J, ATS Task Force/ERS. Standardisation of spirometry. Eur Respir J 26: 319 –338, 2005. 35. Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 317: 1098, 1987. 36. Pellegrino R, Viegi G, Brusasco V, Crapo RO, Burgos F, Casaburi R, Coates A, van der Grinten CP, Gustafsson P, Hankinson J, Jensen R, Johnson DC, MacIntyre N, McKay R, Miller MR, Navajas D, Peder-

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org

Downloaded from on February 19, 2015

1. Abushora MY, Bhatia N, Alnabki Z, Shenoy M, Alshaher M, Stoddard MF. Intrapulmonary shunt is a potentially unrecognized cause of ischemic stroke and transient ischemic attack. J Am Soc Echocardiogr 26: 683–690, 2013. 2. Bates ML, Fulmer BR, Farrell ET, Drezdon A, Pegelow DF, Conhaim RL, Eldridge MW. Hypoxia recruits intrapulmonary arteriovenous pathways in intact rats but not isolated rat lungs. J Appl Physiol 112: 1915–1920, 2012. 3. Berk JL, Hagen JF, Tong RK, Maly G. The use of dopamine to correct the reduced cardiac output resulting from positive end-expiratory pressure. A two-edged sword. Crit Care Med 5: 269, 1977. 4. Butler BD, Hills BA. The lung as a filter for microbubbles. J Appl Physiol 47: 537–543, 1979. 5. Eldridge MW, Dempsey JA, Haverkamp HC, Lovering AT, Hokanson JS. Exercise-induced intrapulmonary arteriovenous shunting in healthy humans. J Appl Physiol 97: 797–805, 2004. 6. Elliott JE, Choi Y, Laurie SS, Yang X, Gladstone IM, Lovering AT. Effect of initial gas bubble composition on detection of inducible intrapulmonary arteriovenous shunt during exercise in normoxia, hypoxia, or hyperoxia. J Appl Physiol 110: 35–45, 2011. 7. Elliott JE, Nigam SM, Laurie SS, Beasley KM, Goodman RD, Hawn JA, Gladstone IM, Chesnutt MS, Lovering AT. Prevalence of left heart contrast in healthy, young, asymptomatic humans at rest breathing room air. Respir Physiol Neurobiol 188: 71–78, 2013. 8. Faulkner JA, Heigenhauser GJ, Schork MA. The cardiac output– oxygen uptake relationship of men during graded bicycle ergometry. Med Sci Sports 9: 148, 1977. 9. Fischer CH, Campos O, Fernandes WB, Kondo M, Souza FL, De Andrade JL, Carvalho AC. Role of contrast-enhanced transesophageal echocardiography for detection of and scoring intrapulmonary vascular dilatation. Echocardiography 27: 1233–1237, 2010. 10. Glazier JB, Hughes JM, Maloney JE, West JB. Measurements of capillary dimensions and blood volume in rapidly frozen lungs. J Appl Physiol 26: 65–76, 1969. 11. Gossage JR, Kanj G. Pulmonary arteriovenous malformations. A state of the art review. Am J Respir Crit Care Med 158: 643–661, 1998. 12. Himelman RB, Stulbarg M, Kircher B, Lee E, Kee L, Dean NC, Golden J, Wolfe CL, Schiller NB. Noninvasive evaluation of pulmonary artery pressure during exercise by saline-enhanced Doppler echocardiography in chronic pulmonary disease. Circulation 79: 863–871, 1989. 13. Johnson BD, Weisman IM, Zeballos RJ, Beck KC. Emerging concepts in the evaluation of ventilatory limitation during exercise: the exercise tidal flow-volume loop. Chest 116: 488 –503, 1999. 14. Jones RS, Meade F. A theoretical and experimental analysis of anomalies in the estimation of pulmonary diffusing capacity by the single breath method. Q J Exp Physiol Cogn Med Sci 46: 131–143, 1961. 15. Knudson RJ, Kaltenborn WT, Knudson DE, Burrows B. The singlebreath carbon monoxide diffusing capacity. Reference equations derived from a healthy nonsmoking population and effects of hematocrit. Am Rev Respir Dis 135: 805–811, 1987. 16. Knudson RJ, Lebowitz MD, Holberg CJ, Burrows B. Changes in the normal maximal expiratory flow-volume curve with growth and aging. Am Rev Respir Dis 127: 725–734, 1983.

•

Effects of Age on Intrapulmonary Arteriovenous Anastomoses

37. 38. 39. 40.

41. 42.

sen OF, Wanger J. Interpretative strategies for lung function tests. Eur Respir J 26: 948 –968, 2005. Putaala J, Metso AJ, Metso TM, Konkola N, Kraemer Y, Haapaniemi E, Kaste M, Tatlisumak T. Analysis of 1008 consecutive patients aged 15 to 49 with first-ever ischemic stroke. Stroke 40: 1195–1203, 2009. Rapp JH, Pan XM, Yu B, Swanson RA, Higashida RT, Simpson P, Saloner D. Cerebral ischemia and infarction from atheroemboli. Stroke 34: 1976 –1980, 2003. Recavarren S. The preterminal arterioles in the pulmonary circulation of high-altitude natives. Circulation 33: 177–180, 1966. Rudski LG, Lai WW, Afilalo J, Hua L, Handschumacher MD, Chandrasekaran K, Solomon SD, Louie EK, Schiller NB. Guidelines for the echocardiographic assessment of the right heart in adults: a report from the American Society of Echocardiography. J Am Soc Echocardiogr 23: 685–713, 2010. Stickland MK, Lovering AT, Eldridge MW. Exercise-induced arteriovenous intrapulmonary shunting in dogs. Am J Respir Crit Care Med 176: 300–305, 2007. Stickland MK, Welsh RC, Haykowsky MJ, Petersen SR, Anderson WD, Taylor DA, Bouffard M, Jones RL. Intra-pulmonary shunt and

43. 44. 45.

46.

47.

•

Norris HC et al.

1333

pulmonary gas exchange during exercise in humans. J Physiol 561: 321–329, 2004. Taylor B, Johnson B. The pulmonary circulation and exercise responses in the elderly. Semin Respir Crit Care Med 31: 528 –538, 2010. Tobin CE, Zariquiey MO. Arteriovenous shunts in the human lung. Proc Soc Exp Biol Med 75: 827–829, 1950. Wanger J, Clausen JL, Coates A, Pedersen OF, Brusasco V, Burgos F, Casaburi R, Crapo R, Enright P, van der Grinten CP, Gustafsson P, Hankinson J, Jensen R, Johnson D, MacIntyre N, McKay R, Miller MR, Navajas D, Pellegrino R, Viegi G. Standardisation of the measurement of lung volumes. Eur Respir J 26: 511–522, 2005. Whyte MK, Hughes JM, Jackson JE, Peters AM, Hempleman SC, Moore DP, Jones HA. Cardiopulmonary response to exercise in patients with intrapulmonary vascular shunts. J Appl Physiol 75: 321–328, 1993. Woods TD, Patel A. A critical review of patent foramen ovale detection using saline contrast echocardiography: when bubbles lie. J Am Soc Echocardiogr 19: 215–222, 2006.

Downloaded from on February 19, 2015

J Appl Physiol • doi:10.1152/japplphysiol.01125.2013 • www.jappl.org