Exercise Capacity
There has been an increased emphasis on preoperative evaluation
of exercise capacity before lung resection since the late 1990s. This assessment
is important because it considers cardiopulmonary interactions. Qualitative estimates
of cardiopulmonary reserve can be derived from the medical history, physical examination,
and arterial blood gas measurements. Additional qualitative estimates of exercise
capacity may be symptom limited (e.g., dyspnea). The best example of such testing
is stair climbing. The ability to climb three flights of stairs (one flight = 20
six-inch-high steps) at the patient's own pace without stopping is associated with
decreased morbidity and mortality. In contrast, an inability to climb two flights
identifies a high risk. Many such patients also exhibit a significant (>4%) decrease
in oxygen saturation when pulse oximetry is used.[28]
A more quantitative estimate of cardiopulmonary function can be
obtained by measuring the maximum oxygen uptake during exercise. Continued observations
have suggested that a patient's maximum oxygen uptake (V̇O2
max)
during exercise is an accurate preoperative means of identifying patients who are
likely to experience post-thoracotomy morbidity.[29]
The V̇O2
max is essentially a measure
of physical fitness and therefore reflects the ability to survive the stresses of
the perioperative period and beyond. During exercise, the lung must accommodate
the increased ventilation and blood flow, much as the remaining lung does after pneumonectomy.
Patients with V̇O2
max values of 20
mL/kg/min or more had minimal morbidity.[29]
Those
with a V̇O2
max of 15 mL/kg/min or
less had increased cardiopulmonary complications, whereas those whose V̇O2
max
was less than 10 mL/kg/min appeared to have an unacceptably high risk and a mortality
rate greater than 30% in the short term. Insight into these V̇O2
max
values is provided by evidence that a two-flight stair climb (20 steps/min) without
dyspnea approximates a V̇O2
max of
16 mL/kg/min. Standardized laboratory measurement of exercise V̇O2
max
has become the gold standard for assessing cardiorespiratory function and predicting
outcome in patients undergoing lung resection.
Another simple test that requires little equipment and correlates
well with V̇O2
max is the 6-minute
walk test. Its popularity has led to the establishment of standardization guidelines.
[30]
A patient who is able to walk 180 feet in
1
minute (2 mph) has a 6-minute walk distance of 1080 feet, which corresponds approximately
to a V̇O2
max of 12 mL/kg/min. Distances
of less than 2000 feet for the 6-minute walk test indicate a V̇O2
max
of less than 15 mL/kg/min.[28]
Increasing evidence suggests that resting pulmonary function,
as reflected by spirometric testing, does not accurately predict exercise performance
in patients with more severe lung disease.[31]
Cardiopulmonary exercise testing may be necessary to evaluate the degree of impairment.
Exercise testing has become attractive because it reflects gas exchange, ventilation,
tissue oxygenation, and cardiac output. If cardiac output is increased, blood flow
to the pulmonary vascular bed increases, as occurs when flow is diverted to the remaining
lung tissue after resection. Patients who otherwise might have been considered inoperable
on the basis of low FEV1
values may be considered to be operative candidates
because of their performance and high V̇O2
max
during exercise. Conversely, patients with marked reductions in exercise V̇O2
max
before surgery appear to be at high risk for postoperative morbidity, regardless
of how well they performed on routine and split-function pulmonary testing.
