Clinical Review by Bruce E. Becker, MD, MS
The pulmonary system is profoundly affected by immersion of the body to the level of the thorax. Part of the effect is due to shifting of blood into the chest cavity, and part is due to compression of the chest wall itself by water.
The combined effect is to alter pulmonary function, increase the work of breathing, and change respiratory dynamics.
Vital capacity decreases by 6%–9% when comparing neck submersion to controls submerged to the xiphoid with about half of this reduction due to increased thoracic blood volume, and half due to hydrostatic forces counteracting the inspiratory musculature[32,33].
The combined effect of all these changes is to increase the total work of breathing when submerged to the neck. The total work of breathing at rest for a tidal volume of 1 liter increases by 60% during submersion to the neck.
Of this increased effort three-fourths is attributable to redistribution of blood from the thorax, and the rest to increased airway resistance and increased hydrostatic force on the thorax [32,34-36].
Most of the increased work occurs during inspiration. Because fluid dynamics enter into both the elastic workload component as well as the dynamic component of breathing effort, as respiratory rate increases turbulence enters into the equation.
Consequently there must be an exponential workload increase with more rapid breathing, as during high level exercise with rapid respiratory rates.
Inspiratory muscle weakness is an important component of many chronic diseases, including congestive heart failure and chronic obstructive lung disease .
Because the combination of respiratory changes makes for a significantly challenging respiratory environment, especially because respiratory rates increase during exercise, immersion may be used for respiratory training and rehabilitation.
For an athlete used to land-based conditioning exercises, a program of water-based exercise results in a significant workload demand on the respiratory apparatus, primarily in the muscles of inspiration .
Because inspiratory muscle fatigue seems to be a rate and performance limiting factor even in highly trained athletes, inspiratory muscle strengthening exercises have proven to be effective in improving athletic performance in elite cyclists and rowers [38-59] .
The challenge of inspiratory resistance posed during neck-depth immersion could theoretically raise the respiratory muscular strength and endurance if the time spent in aquatic conditioning is sufficient in intensity and duration to achieve respiratory apparatus strength gains.
This theory is supported by research finding that competitive women swimmers adding inspiratory training to conventional swim training realized no improvement in inspiratory endurance compared to the conventional swim trained controls, as these aquatic athletes had already achieved a ceiling effect in respiratory training .
These results have been confirmed by more recent studies at the University of Indiana and the University of Toronto [61,62].
The author has had a number of elite athletes comment on this phenomenon when returning to land-based competition after a period of intense water-based aquatic rehabilitation sufficient to strengthen the respiratory musculature.
The common response is a perception of easier breathing at peak exercise levels, effects similar to the studies quoted in elite cyclists and cyclists.
This is not surprising in view of the data existing on competitive swimmers who routinely train in the aquatic environment [60-68].
Comparative studies of young swimmers have consistently shown a larger lung capacity (both vital capacity and total lung capacity) and improved forced expiratory capacity, and a number of studies have also shown improvement in inspiratory capacity [60-62,64,66,68-73].
Respiratory strengthening may be a very important aspect of high level athletic performance, as demonstrated in some of the studies above.
When an athlete begins to experience respiratory fatigue, a cascade of physiologic changes follows. The production of metabolites, plus neurologic signaling through the sympathetic nervous system, sends a message to the peripheral arterial tree to shunt blood from the locomotor musculature [38,74-76].
With a decline in perfusion of the muscles of locomotion, the rate of fatigue increases quite dramatically [39,75].
A considerable body of literature supports the plasticity of the respiratory musculature to strengthening with appropriately designed exercise in various disease conditions, although not specifically through aquatic activity [41,55,57,58,62,77-82].
Respiratory muscle weakness, especially in the musculature of inspiration, has been found in chronic heart failure patients and this weakness is correlated closely with cardiac function and may be a significant factor in the impaired exercise capacity seen in individuals with chronic heart failure [83-87].
Because the added work of respiration during immersion occurs almost entirely during the inspiratory phase, it is intriguing to speculate that a period of inspiratory muscle strengthening through immersed activity might improve exercise capacity in these individuals, but this has not been studied to date.
Aquatic therapy may be very useful in the management of patients with neuromuscular impairment of the respiratory system, such as is seen in spinal cord injury and muscular dystrophy [88-91].
A lengthy study of swimming training on cardiorespiratory fitness in individuals with spinal cord injuries was done in the late 1970s in Poland.
The authors found a 442% increase in fitness levels, as contrasted with a 77% increase seen in patients with spinal cord injury in a standard land-based training program over the same period .
A review in 2006 concluded that respiratory muscle training tended to improve expiratory muscle strength, vital capacity, and residual volume in individuals with spinal cord injury, but that insufficient data were available to make conclusions concerning the effects on inspiratory muscle strength, respiratory muscle endurance, quality of life, exercise performance and respiratory complications .
Programs typically used include chest-depth aerobic activity for general rehabilitation populations usually at therapy pool temperatures. For chronic obstructive pulmonary disease patients, depth should start at waist level, and progress into deeper water as strength and respiratory tolerance improves.
A simple technique for expiratory muscular exercise uses a 1”PVC tube 16” in length, with the patient blowing out into the water with the end of the tube submerged, beginning with the tube end 2-3 feet below water surface and progressing deeper as strength builds.
This can be quantified as a measure of expiratory force increase, both by measuring depth of the tube end and number of full exhalations completed.