National Health and Nutrition Examination Survey

The cardiovascular (CV) fitness component was implemented in the NHANES in 1999. The goals of this component are to provide:

1. nationally representative data on cardiovascular fitness;
2. prevalence estimates of persons at risk due to poor physical fitness; and
3. data to study the association between cardiovascular fitness and other health conditions and risk factors, such as obesity, cardiovascular disease, diabetes, hypertension, and activity and dietary patterns.

Survey participants aged 12-49 years old are eligible for the CV fitness component.

Participants are excluded from this component based on medical conditions, medications, physical limitations, limits on heart rate and blood pressure, and irregular heart rates. The screening is done prior to the treadmill test using questions in the household interview, questions administered by the physician in the NHANES Mobile Examination Center (MEC) and aspects of the physician examination such as measurements of heart rate and blood pressure. The list of exclusion criteria used for the CV fitness component is attached as Appendix A.

The CV fitness exam was performed by trained health technicians. The protocol is a submaximal exercise test. Based on gender, age, body mass index, and self-reported level of physical activity, participants are assigned to one of eight treadmill test protocols. The goal of each protocol is to elicit a heart rate that is approximately 75 percent of the age-predicted maximum (220-age) by the end of the test. Each protocol includes a 2-minute warm-up, two 3-minute exercise stages, and a 2-minute cool down period.

Heart rate was monitored continuously using an automated monitor with four electrodes connected to thorax and abdomen of the participant and was recorded at the end of warm-up, each exercise stage, and each minute of recovery. Blood pressure was measured at the end of each stage. At the end of warm-up and each exercise stage, participants were asked to rate their perceived exertion using the Borg scale.

The main outcome of this component is the estimated maximal oxygen uptake (VO2max). Based on gender and age specific criteria, the estimated VO2max is also categorized and reported as “low”, “moderate” or “high” level of cardiovascular fitness in the present dataset.

Detailed descriptions of the protocol are provided in the NHANES Cardiovascular Fitness Procedure Manual (National Center for Health Statistics, 2004).

Health technicians are regularly monitored by NHANES supervisory staff and expert consultants from the Cooper Institute using a structured performance evaluation and an annual recertification process. Retraining sessions are conducted periodically with the technicians to reinforce the proper protocols and technique. Inspection, calibration, and maintenance of the equipment and supplies are also performed on a regular basis.

As part of on-going quality control practice, all the data are reviewed systematically for logical inconsistencies and technician errors.

Chapter 6 of the NHANES Cardiovascular Fitness Procedure Manual details the quality control procedures.

During the data processing, edits were made to ensure the logical consistency and analytic usefulness of the data. Extreme values were reviewed and cross-checked with other available data. When there was insufficient information to conclude that the values were invalid, they were retained in the data set.

Variable specific editing:

CVDEXSTS (CV fitness exam status)

This derived variable indicates the following for each participants:

1. VO2max estimated.
2. Tested but VO2max estimate missing - data obtained during the treadmill test were insufficient in calculating VO2max.
3. Did not participate in the treadmill test.

CVDEXCMT (Comment code for the exam status)

This derived variable denotes the reason the treadmill test was not completed or data are missing.

“0” – None: the exam was completed with no missing data point present.
“1” – Met exclusion criteria: exam was not done due to the participant met the exclusion criteria of the component other than pregnancy. Responses in variables CVDEXCL1 - CVDEXCL6 identify specific exclusion criteria the participant met.
“2” – Pregnant more than 12 weeks: pregnant women are excluded from the component because physiologic changes with pregnancy affect heart rate, and therefore the interpretation of the data.
“3” – Refusal: participant refused to undergo or continue the test.
“4” – No time: the exam was not done because there was not enough time.
“5” – Technical problem: problems that occurred during the test with equipment, software applications, or technician error.
“6” – Met priority 1 stopping criteria: participant discomfort or distress observed by the health technician or reported by the participant that warranted an emergency stop of the protocol. See Cardiovascular Fitness Procedure Manual, section 3.13.1 for further details.
“7” – Met priority 2 stopping criteria: safety concerns arose during the test that warranted an early stop of the protocol. Responses in variables CVQ220a-CVQ220m identify specific reason(s) called for the priority 2 stop. See Cardiovascular Fitness Procedure Manual, section 3.13.2 for further details.
“8” – Not able to calculate VO2max: heart rate obtained at the end of stage 2 lower than what obtained at the end of stage 1, or the difference between stages 1 and 2 heart rates is 5 beat/minute or less.
“90” – Other.

Please note that these codes are descriptive and that incomplete treadmill test or missing data does not necessarily denote that VO2max was not estimated. For example, if the monitor indicated that the participant’s systolic blood pressure (SBP) suddently dropped more than 20 mmHg at 2 minutes 54 seconds of the stage 2, the technician would terminate the stage 2 testing based on the priority 2 stopping criteria. The exam status comment, CVDEXCMT, will be coded as “7” (Met priority 2 stopping criteria) for this record. It will also be shown in the record that the amount of time spent in stage 2 is less than 3 minutes (CVDS2TIM=2.9 minutes) and the reason for priority 2 stop is a significant drop in SBP (CVQ220e=1). On the other hand, since the end of stage 2 heart rate measurment was usually captured at approximately 2:40 to 2:50 minutes into stage 2, enough data were collected from this participant to calculate his/her estimated VO2max, even with an incomplete stage 2 test.

CVDEXCL1 – CVDEXCL6 (Reasons for exclusion)

The exclusion criteria used for the CV fitness component include more than 50 questions or measurements collected during the household interview and the examinations in the MEC. This set of variables summarizes these exclusion criteria into six categories: physical functioning limitations, cardiovascular conditions/ symptoms, lung/breathing conditions/symptoms, asthma symptoms, medication exclusions, and others. Appendix A documents the classification frames for these categories. These six variables are not exclusive to each other to accommodate participants who met multiple criteria in different categories.

CVXPARC (Physical activity level)

This variable summarizes the responses from a series of questions and describes the participant’s typical physical activity level using the NASA/JSC physical activity scale (Ross, 1990). See Cardiovascular Fitness Procedure Manual, Appendix E for further details.

CVDVOMAX (Predicted VO2max)

This derived variable designates predicted VO2max value calculated with the equation developed by Jackson et al (Jackson, 1990). The prediction model is based on participant’s gender, age, body mass index, and self-reported level of physical activity. Predicted VO2max value is used in the determination of the exercise protocol for the treadmill test. See Cardiovascular Fitness Procedure Manual, Appendix F for further details.

CVAPROT and CVDPROT (Assigned exercise protocol and the actual protocol used in stages 1 and 2)

Each participant was assigned to one of 8 protocols based on the predicted VO2max. According to the procedure, if the heart rate in warm-up is greater than 60% of the predicted maximal heart rate (PMHR), the assigned exercise protocol is decreased by 1 for the remainder of the treadmill test. When the heart rate in warm-up is less than 50% of the PMHR, the assigned protocol is increased by 1 for the remainder of the test. The variable CVAPROT denotes the exercise protocol that was originally assigned to the case while variable CVDPROT reflects the actual exercise protocol used in stage 1 and stage 2 after the adjustment during warm-up. See CV Fitness Procedures Manual, Sections 3.12 and 4.5 for further details.

CVDESVO2 (Estimated maximal oxygen uptake)

This derived variable designates the main outcome of the component: VO2max (ml/kg/min). It is estimated by extrapolation using measured heart rate responses to known levels of exercise workloads, assuming the relation between heart rate and oxygen consumption is linear during exercise (American College of Sports Medicine, 1995). See the CV Fitness Procedures Manual, Appendix G for further details.

CVDFITLV (Cardiovascular fitness level)

The level of cardiovascular fitness is categorized based on gender-age specific cut-points of estimated VO2max. The reference cut-points used for adults 20-49 years are based on data from the Aerobics Center Longitudinal Study (ACLS) (American College of Sports Medicine, 1995, Blair, 1989). Low level of CV fitness is defined as an estimated VO2max below the 20th percentile of the ACLS data of the same gender and age group; moderate fitness is defined as a value between the 20th and 59th percentile, and high fitness level is defined as at or above the 60th percentile. The reference standards used for adolescents and young adults 12-19 years are based on the criteria used in the FITNESSGRAM program (Cureton, 1990, Cureton, 1999). See CV Fitness Procedures Manual, Appendix I for further details.

The present dataset contains the NHANES CV fitness data collected during 2003-2004. It is strongly recommended that data users merge this file with the CV fitness data collected in NHANES 1999-2000 and 2001-2002 to ensure an adequate sample size for analysis. NHANES CV fitness data collected in these previous years are publicly available on the NHANES website.

The NHANES examination sample weights should be used for analyses with the CV fitness data. When combining two or more 2-year cycles of the continuous NHANES, the user must calculate new sample weights before beginning any analysis of the data. Please refer to the NHANES Analytic and Reporting Guidelines for further details on the use of sample weights, percedures of calculating weights for combining multiple cycles, and other analytic issues. The Analytic Guidelines are available on the NHANES website.
To ensure the safety and validity of the test, a series of exclusion criteria were developed for the NHANES CV fitness component to preclude participants with conditions that might endanger them during the testing or affect the estimation of the VO2max from the exam. As a result, approximately 70% of the eligible NHANES participants were tested in this component during 1999-2002. The exclusion rate increases with age. In the present dataset, women have a much higher exclusion rate than men. It is largely due to the pregnancy exclusion which accounts for about 23% of the exclusions among women, and the fact that pregnant women are one of the subpopulations oversampled in the NHANES 2003-2004. Data users need to be aware of the exclusion rate and be cautious when making inferences for the analyses.

Approximately 9% of the participants in 2003-2004 underwent the treadmill test but did not have their VO2max estimated. Most of these cases resulted from a prematurely terminated test due to meeting the priority 2 stopping criteria. Analysts need to take the incomplete tests into account when interpreting the data.

It is not feasible to utilize a maximal treadmill test to directly measure the cardiovascular fitness level in a population-based study setting such as NHANES. However, due to the nature of the submaximal test, some estimates obtained in this component may appear extreme compared to data obtained from more direct measures. Analysts need to carefully examine the data distribution and consider whether or not it is appropriate to include or exclude extreme values in a given analysis.

During the course of data collection in 2003-2004, several protocol modifications were implemented to help improve the completion rate. These modifications do not affect the comparability of the data and are described below:

1. Add participant’s perceived exertion factor to the existing stopping criteria related to pre-determined heart rate limits. For example, if a participant’s heart rate exceeded 80% of predicted maximal heart rate during stage 1, the test would be stopped only if the participant also reported perceived exertion > 17 and confirmed that he/she was unable to continue the test.
2. Eliminate exercise protocol 1 and protocol 8 for all participants and protocol 2 for participants 12-19 years. Accordingly, persons who used to be assigned to exercise protocol 1 were re-assigned to protocol 2, and persons who used to be assigned to exercise protocol 8 were re-assigned to protocol 7.