Cardiovascular Training in Spinal Cord Injury

Original Editor - Naomi O'Reilly

Top Contributors - Naomi O'Reilly, Ewa Jaraczewska, Admin, Tarina van der Stockt, Kim Jackson, Lucinda hampton, Jess Bell, Anas Mohamed, Stacy Schiurring and Olajumoke Ogunleye  

Introduction

Cardiovascular training involves the use of oxygen to meet the energy demands of the body’s muscles during exercise. It is associated with longer duration exercise during a given session of training, often at a consistent pace. Regular aerobic training will improve cardiovascular function. With improved cardiovascular function, individuals are more likely to be able to live independently, decrease secondary health complications, and improve muscular endurance.

Definition

According to the Oxford Dictionary of Sport Science and Medicine cardiovascular fitness is the "ability of the heart and blood vessels to supply nutrients and oxygen to tissues, including muscle, during sustained exercise".

Assessment of Cardiovascular Fitness

Assessment of cardiovascular fitness is essential for physiotherapists to directly determine training or conditioning intensities required to elicit improvments in cardiovascular and cardiometabolic health. Gold standard laboratory based assessment with ergomerter i.e. arn crank, wheelchair treadmill are becoming more common place, particulalry within competitive sport, although these results alone do not represent the full picture. Pushing proficiency, sport-specific skills, wheelchair propulsion technique, and individual adaptations to their wheelchair In order to develop appropriate exercise programs and monitor the response to training it is important to first assess cardiovascular fitness under reproducible test situations, ensuring factors such as the type of ergometer, constraints used, position of individual are standardized. Precautions should also be followed when conducting cardiovascular assessments as strenuous exercise can lead to a cardiovascular event.

Prior to completing any maximal exercise testing a detailed medical and surgical history is required to identify indications for an exercise test and determine any underlying conditions eg, cardiovascular, pulmonary, musculoskeletal, or neurological dysfunction or the presence of diabetes, hypertension or heart block requiring a pacemaker, anaemia, thyroid dysfunction, obesity, deformity, vertigo, or impaired cognitive function. It is also important to be aware of any medications that can influence the test procedures and the response to the exercise. 

Peak Oxygen Consumption Tests

The Peak Oxygen Consumption (VO_{2 Peak}) test, equivalent to the VO_{2 Max} test in able bodied individuals, measures the maximal capacity of the body to deliver oxygen from the lungs to the mitochondria of exercising muscles by expired gas collection, and is the most accurate way to assess cardiovascular fitness in spinal cord injury. The terminology is used to reflect the lower maximal rate of oxygen consumption with arm exercises v leg exercises due to both lower demand for oxygen from smaller muscle groups anand the circulatory implication of arm exercise.

In individuals with a spinal cord injury, the VO_{2 Peak} Test is typically performed using an arm cycle ergometer, but can also be completed with manual wheelchair propulsion or handcycle on an ergometer or treadmill with gradually increasing exercise intensities until exhaustion. Rest periods of 20 - 30 seconds are at times provided for between each increment. Starting points for arm ergometry vary depending on level of spinal cord injury and level of fitness. Power output can be adjusted by changing the cranking velocity and / or externally applied resistance. For example;

• Paraplegia; Start at 30 Watts and increase by 10 - 15 Watts every 2 minutes. Maximal power output is likely to be between 50 - 100 Watts.
• Tetraplegia; Start at 5 Watts and increase by 2.5 - 10 Watts every 2 minutes. Maximal power output is likely to be between 10 - 50 Watts.

While the VO_{2 Peak} is the gold standard method for assessing exercise reponse for an individual with a spinal cord injury, it is rarely used in spinal cord injury units due to the complex nature of the test.

Submaximal Exercise Tests

Submaximal exercise tests, typically involve measuring the responses to standardized physical activities that are typically encountered in everyday life and are more commonly used in individuals with a spinal cord injury to evaluate the adaptation of the oxygen transport system to exercise below maximal intensity, so that main energy system used is aerobic. While portable expired gases analysis systems can be utilised and are often used in high performance paralympic sport, heart rate measurement is more commonly used in spinal injury rehabilitation units. Use of heart rate measurements does not allow estiamtion of VO_{2 Peak}, but is used as a means to monitor the response of individuals with a spinal cord injury to training, with improvements in cardiovascuar fitness indicated by decreased heart rate at at the same power output with training or improvements in individials perception of exertion with the Borg Exertion Scale.

Performed in a similar way to the VO_{2 Peak} test with a different set of protocols and terminated prior to exhaustion. There are numerous submaximal protocols from which to choose, many of which have been developed to meet the needs of individuals with various functional limitations and impairments, including spinal cord injury. A commonly used protocol for individuals with a spinal cord injury include 3 x 7 Minute Exercise Bouts of exercise at 40%, 60% and 80% of predicted maximal exercise capacity.

• Paraplegia with High Level of Fitness; 7 mins each at 40 Watts, 60 Watts and 80 Watts
• Tetraplegia; 7 mins each at 20 Watts, 30 Watts and 40 Watts

Production of a sufficient level of exercise stress without physiologic or biomechanical strain is the key goal of submaximal testing. Factors that we believe should be considered in selecting the appropriate test include the person's primary and secondary pathologies and how these pathologies physically affect the person's daily life. Other factors include cognitive status, age, weight, nutritional status, mobility, use of walking aids or orthotic or prosthetic devices, independence, work situation, home situation, and the person's needs and wants. 

Submaximal exercise testing overcomes many of the limitations of maximal exercise testing, appears to have greater applicability to physiotherapists in their role as clinical exercise specialists compared with maximal exercise testing, and are much easier to implement within a spinal injury unit and rehabilitation setting. There is new evidence to suggest that in cervical level spinal cord injury peak heart rate and blood lactate concentration attained during maximal incremental laboratory-based wheelchair exercise on a treadmill were below those attained during maximal field-based exercise testing in highly trained wheelchair rugby athletes, suggesting that incremental exercise testing in the laboratory does not elicit true peak cardiometabolic responses in highly-trained wheelchair rugby athletes with a cervical spinal cord injury and field exercise tests msy give a better indication of maximal performance.

Field Exercise Tests

A field exercise test is usually a measurement of a physiological function produced, while an athlete is performing in a simulated sport situation. While often thought not to be as reliabe as lab based tests, they are considered to have more validity as a result of greater specificity.

Implications for Rehab

• Use of regular cardiovascular capcity testing during spinal cord injury rehabilitation allows us to monitor the impact of rehabilitation interventions on an individual level,
• Incremental arm ergonemetry with small increments per stage is the most relevant means of assessment for peak cardiovascular capacity for individuals with a spinal cord injury,
• Use of the submaximal wheelchair ergometer test is preferable to use for the assessment of daily life functioning,
• Systematic reporting on test termination, peak outcomes criteria and adverse events is key to enhance comparability of results.

Response to Cardiovascular Fitness Training

Response to cardiovascular fitness training is significantly influenced by type of spinal cord injury including neurological level, level of completeness and extent of the injury. Those with a incomplete level of injury, particularly those who can ambulate and have some lower limb use doing exercise, respond to exercise in a similar way to able bodied individuals. While those with a complete cervical level injury or upper thoracic level injury have a significantly differnt reponse as a result of reliance on upper limb exercise, lower limb paralysis and most importantly loss of supraspinal sympathetic nervous control, which adversley affect cardiac output and artery-venous oxygen; the two components of VO_{2 Peak.}

Cardiac Output

Cardiac Output (Q) is defined as the amount of blood pumped by the left ventricle of the heart per minute. It is expressed as litres/minute.

Cardiac Output = Heart Rate x Stroke Volume

Heart Rate

Heart rate is determined by the balance between sympathetic control to the heart via T1 - T4 nerve roots that increase heart rate and parasympathatic control via the vagal nerve which decrease heart rate. The heart will beat at between 70 - 80 beats per min, the intrinsic firing rate of the sinoatrial node in the heart, without input from either the sympathetic or parasympathetic systems.

Normally during exercise in able bodied individuals heart rate increases as a result of reduced vagal nerve activity and increased activity of the sympathetic nervous system, with maximal heart rates between 200 - 220bpm possible.

In spinal cord injury lesions between T1 - T4 there is partial loss of Supraspinal Sympathetic Control to the heart, with increases in heart rate occurring primarily as a result of withdrawal of excitatory input from the vagal nerve, resulting in lower maximal heart rates of between 110 - 130.

In spinal cord injury lesions T1 and above there is complete loss of Supraspinal Sympathetic Control to the heart, with increases in heart rate occurring primarily as a result of withdrawal of excitatory input from the vagal nerve. As a result in many individuals with tetraplegia they are unable to increase their heart rate beyond the natural rhythm of the heart, and as such, heart rate may not be considered the best indicator of training in tetraplegia.

Stroke Volume

Stroke volume is the volume of blood ejected at each stroke of the heart during systole, with typical stroke volume in able bodied individuals 70ml at rest increasing to a maximum of 120 ml during strenuous exercise. Typically stroke volume increases during exercise in able bodied individuals as an adaption to cardiovascular training.

In spinal cord injury maximal stroke volume is decreased due to loss of supraspinal sympathtic control and upper limb exercise, which have a negative effect on venous return or preload; as a result of venous pooling from paralysis with loss of lower limb and intra-thoracic muscle pumps, and contractility, the primary determinants of stroke volume.

Aterio-venous Oxygen Difference

The arteriovenous oxygen difference is a measure of the amount of oxygen taken up from the blood by the tissues. Cardiac output and arteriovenous oxygen difference are the determinants of overall oxygen uptake. During exercise blood flow increases to the tissues; haemoglobin dissociates quicker and easier. This results in a greater arteriovenous oxygen difference during exercise. In trained athletes, the arteriovenous oxygen difference is greater as a result of the tissues becoming more efficient in oxygen uptake with aerobic training.

Size Exercising Muscle Mass

Ability Muscle to Extract Oxygen

Exercise Prescription

Type

Examples of Cardiovascular Exercise include;

Arm Crank Ergometers

Cycling

Nordic Ski Erg

Swimming

Wheelchair Pushing

Walking

Intensity

Resources

References