BY DAN SENN
Many misinformed individuals argue ABOUT the role and importance of certain modes of physical conditioning for the fire service. Some say that cardiovascular, or aerobic, conditioning is most important; others say that resistance, or strength, training is most important. The truth is, both are equally important not only in firefighting but also in regular daily activities. It is well known that many deaths and physically limiting conditions are directly heart related, hence the argument for aerobic conditioning. The bottom line is, your heart has the vital role of keeping every cell in the body adequately perfused 24 hours a day, seven days a week. This also means that the heart must meet perfusion requirement to each cell throughout every mode of physical activity and nonactivity. Therefore, the heart’s work capacity must be as dynamic as the range of physical activity in which we perform. Because of our dynamic range of activity, a viable argument is made for both aerobic and resistance training and every mode of training in between. This article will examine the similarities between our cardiovascular system and the apparatus and equipment we use to fight fires. Through this you will better understand your heart’s response to different modes of exercise and recognize the need for appropriate conditioning.
PUMP PERFORMANCE
As firefighters, we’re all familiar with the importance of our apparatus pumps and their diverse functions. The pump itself is our lifeline when we’re on the end of the hoseline in an untenable environment. Apparatus pumps may serve as relay pumps where they’re performing flow work by receiving a large volume of flowing water from a source and simply boosting pressure; we can pull water into our pump by creating a negative pressure; and we can perform high-pressure work by supplying a hoseline, standpipe, or sprinkler system on the top floor of a high-rise. Which mode of pump operations do you think places the most stress on the apparatus?
Have you ever thought twice about your safety as a firefighter on days when you’re riding a backup engine, dreading that you may have to rely on a questionable pump to perform any of these functions? Saturday pump tests on the backup apparatus can be a scary task. I remember testing one standby truck; it was a banana yellow-colored relic that had a crew cab with a tall narrow door and a high window that made it look like a phone booth. As I pushed the revolutions per minute (rpm) up, there was a corresponding increase in the intensity of the leaking streams of water and the rattling sounds coming from the pump. I was hoping the pump wouldn’t suddenly explode like a round of antiaircraft flak, pelting me with pieces of cast shrapnel before I had a chance to turn and run.
What’s the purpose of performing these tests? We push the pump through a near maximal workout to determine if it meets a minimum peak performance—in other words, we would rather have the pump explode when it’s not needed as opposed to on a fire scene. If something breaks or doesn’t meet performance requirements, it’s taken out of service and sent to the shop for repairs.
HEART’S FUNCTIONAL CAPACITY
Have you ever considered your own heart in this context? How much confidence do you have in your own heart’s functional capacity when going into your next fire? Those who lack physical conditioning probably unknowingly run their hearts through this same near-max test at every working fire. However, our heart’s performance measures aren’t as objective as those for our apparatus pumps. We don’t have the luxury of gauges and controls that allow us to monitor the function and performance of our heart at all times.
Our cardiovascular system—which consists of the heart, system of vessels, and blood—performs the same functions as our apparatus pump, water, and hoses and is governed by the same laws of physics. Our apparatus pumps function to eject the water that is inside the pump through the outlet, thus pressurizing and directing the flow of water through a distribution system of hose. As long as the pump receives a supply of water and the pump operates within the capacity of that supply, we have a constant pressurized flow of water through the distribution system. Think of our heart in the same context.
The heart relies on a constant supply of returning blood flowing in by the vena cava. This flow converges between the superior and inferior vena cava to supply the heart with blood to pump. This returning blood flow subsequently collects in the right atrium during atrial relaxation. This filling of the right atrium is referred to as preload. The amount of preload is determined by the volume and pressure of returning blood and the volume the right atrium accepts during its filling phase. Preload plays a significant factor in cardiac output, since it ultimately determines the volume of blood pumped into the left ventricle and that is available for distribution during ventricular contraction. Therefore, the heart receives blood supply during relaxation and subsequently contracts against this blood volume, creating pressure within the chambers and thus directing flow through valves and maintaining a pulsating systemic blood pressure by maintaining a pressurized volume of blood within the arteries.
With any pump, the amount and pressure of flow entering the pump from its supply source will largely determine the effort and efficiency of the pump. This is evident when we switch from tank to hydrant supply. The water entering the pump from a hydrant has its own source of pressure and, therefore, allows the truck to reduce its workload while still maintaining outlet volume and pressure. The same is true for the heart. The workload and efficiency of the heart are largely determined by the flow of returning blood (hydrant supply) and the subsequent required output (gallons per minute) and systemic resistance to flow (hose diameter). The condition of valves and interior wall surfaces and the strength of the power unit (engine or muscle tissue) are factors that limit the functional capacity of our apparatus pump or heart. These factors that reduce functional capacity greatly challenge the heart to meet the perfusion requirements. In addition, the mode of physical activity and subsequent stress on the body also provide different challenges to the heart and affect its efficiency.
During a continuous aerobic activity, such as running, our heart performs flow work much like our apparatus pump would operate in a relay pumping operation: A pressurized high-volume flow is entering the pump through a large intake, and a high volume of flow is circulated through and subsequently ejected from the pump through a large-diameter hose at an increased pressure. A whole-body aerobic activity requires a large volume of blood flow because of the mass of musculature involved and the vasodilation of the arteries supplying those muscles. Think of it as switching from multiple 1¾-inch hoselines to multiple 2½-inch hoselines that accommodate the need for increased volume of blood flow and also reduce the workload of the heart by reducing resistance to flow. So, increased activity increases the required volume flow, but friction loss is minimized by vasodilation. Therefore, we can reduce the workload of the heart as long as the flow returning to the heart is maintained. The heart is simply providing an extra boost of pressure to maintain a consistent flow.
Blood flow during an aerobic activity assumes a consistent flow through the arteries because of a lower resistance and the venous flow returning to the heart, aided by a “milking” or “muscle pump” action created by the repetitious action of the musculature, thus enhancing blood return and providing adequate supply to the heart so it can meet output demands. In addition, the right atrium tends to create its own negative pressure following its contraction phase as it relaxes and expands its volumetric capacity during atrial filling. This also aids in maintaining a constant return of blood flow through the vena cava. These processes are just as important to the heart as it is for an apparatus pump to be connected to a large water main with the ability to supply an adequate volume and residual pressure to operate multiple large-diameter hoselines or master streams. The efficiency and output volume of pumping operations rely on a good hydrant supply. As a general rule of thumb, we can step on the supply hose to make a quick assessment of our ability to increase output volume. If the supply hose maintains an unforgiving firm pressure when stepped on, we know our supply is adequate for our current output volume. However, our body has a limited capacity to maintain this high-volume flow operation.
As dehydration mounts, blood volume declines. This is a significant concern for firefighters becoming dehydrated as a result of sweat loss from working or training in full turnout gear in heated environments. This lower blood volume causes a reduction in returning blood volume and, therefore, reduces preload. Consequently, this reduces stroke volume and creates a greater stress on the heart to maintain cardiac output and systemic blood pressure. Relate this to apparatus pump operations, and you can make the connection. In this situation, residual hydrant supply is being exceeded, as evidenced by a fluttering supply hose approaching the brink of collapsing and cavitating the pump.
Now consider a situation where we have to supply the standpipe of a high-rise. If using only tank water, this task would resemble the task performed by our heart during resistance training. In this situation, we are pumping at a much greater resistance with no relief in workload provided by the augmenting pressure from a hydrant. Remember, a pump can only distribute an output that is limited by the condition of the pump and the supply entering the pump. Leaky valves prevent the ability to draft and compromise output volume and pressure capacity. In addition, if the residual from our hydrant supply drops to a critical point, the supply hose will collapse, and the pump will cavitate, reducing its output and efficiency. In this case, we are most likely limited to pumping through small-diameter hoselines and are having to pump against high resistance and are limited to a lower volume of flow. In other words, we have a high workload stress on our heart, having to pump at high rpm with a limited output volume: high stress/low efficiency. During higher-intensity activity, such as weight lifting, our heart faces these challenges by pumping blood through a distribution system restricted by muscle tension, which restricts the return flow supplying the heart and reduces cardiac output.
Measuring Cardiac Performance and Efficiency
Consider how we measure cardiac performance and efficiency. Cardiac output is measured by the product of heart rate and stroke volume (HR × SV = CO). Stroke volume is determined by preload, as explained earlier, and the percentage of blood volume ejected from the left ventricle by each contraction, also known as the ejection fraction. It is important to note that the left ventricle is unable to eject its entire contents during each contraction because the ventricle is unable to contract to the point of completely collapsing its volumetric capacity. Therefore, a residual volume remains in the ventricle following its systolic phase. The fraction of blood ejected during each contraction is determined by two factors: afterload and contractility. Afterload refers to the backpressure the left ventricle has to overcome to move blood. During ventricular contraction, pressure builds until a level exceeding the afterload pressure before the aortic valve is forced open, allowing flow to pass through. Contractility refers to the strength of each ventricular contraction. Greater contractility allows the ventricle to more easily overcome afterload and produces a more powerful contraction, which allows more blood to be expelled over a shorter time and increases the ejection fraction. Afterload and contractility become significant factors as work or exercise intensity increase beyond the demands of an aerobic activity. With this in mind, let’s consider hemodynamics during resistance training.
HEMODYNAMICS DURING RESISTANCE TRAINING
High-intensity, intermittent exercise involves greater muscle tension over longer sustained durations when compared with the rhythmic nature of aerobic exercise. For example, during a bicep curl, the muscle tightens and squeezes down on the vasculature, pinching off the flow of blood, much like having a kink in a hoseline. This creates a greater resistance to flow and, therefore, a greater resistance to be overcome by the heart to move blood through those working muscles. The heart has to compensate by increasing the strength of its contraction, described earlier as contractility, to overcome systemic resistance in an attempt to sustain an efficient ejection fraction per stroke volume. This is the reason resistance training is considered pressure work for the heart, as opposed to flow work during aerobic activity.
Additionally, this resistance to flow also prevents blood from returning to the heart. Although the veins have one-way valves preventing backflow, the compliant structure of the veins do little to augment the movement and return of blood back to the heart against resistance caused by muscle tension. This, of course, in a sense means that the heart is connected to a dead-end hydrant of a small water main. The decrease in returning blood and residual pressure means less blood entering the right side of the heart during diastole and subsequently a lower stroke volume, which results in a lower output pressure (stroke volume is the most significant factor influencing blood pressure). We can increase the rpm of the truck or the rate of the heart all we want, but our output volume and subsequent pressure are governed by the supply feeding into the pump.
During resistance activities, the heart is greatly challenged to maintain adequate systemic blood pressure. Cardiac efficiency relies on the return blood flow and the capacity of the heart to produce forceful contractions to move blood against high-resistance pressures. This is different from aerobic work in which cardiac efficiency relies on the heart’s ventricular volume capacity and the responsiveness of the ventricular musculature to repetitive stretch-contract cycles.
Man and machine both have their distinct advantages and disadvantages. If our apparatus pump is in bad condition, we can fix or replace it, but it cannot fix itself with continued further use. Our heart, on the other hand, can adapt and improve its function with proper training; however, it is not a quick fix. Adaptations occur over time with training, and these adaptations must be maintained with continued training stimulus. As explained earlier, the stress on our hearts depends on the mode of physical activity. Continuous aerobic activity presents a flow stress on the heart, and resistance activity presents a pressure stress.
Although most perceive aerobic training as “the way” to train your heart, many overlook the stress placed on the heart when performing pressure work and thus fail to condition the heart for this mode of stress. For example, in northern climates, this danger is evident by the high incidence of heart attacks following a heavy, wet snowfall. Shoveling heavy snow is not strictly an aerobic activity. For people of low physical work capacity, shoveling wet snow would fall into the high-intensity intermittent activity category. The heart’s workload is also exacerbated by high intrathoracic pressure caused by tensioning the core muscles for stabilization. It is no surprise that heart attacks occur under these stressful conditions. The heart itself has a high demand for blood under a high workload. However, an underconditioned individual may have a weak heart that has limited contraction strength or hardened or clogged arteries, limiting the ability to “go to a larger-diameter hose” and reduce friction loss and pump pressure. Couple this with a low-volume “dead-end hydrant,” and you have a heart that is pumping a maximum rpm but unable to adequately supply the cardiac muscle tissue itself or the rest of the body with the required volume of blood during physical stress.
WORKING IN FULL TURNOUT GEAR
There are additional concerns to consider about our cardiovascular health when training or working in full turnout gear. The added weight and resistance to movement our turnout gear provides consume a portion of our total work capacity. In other words, for any given task, that task will require a higher heart rate with higher metabolic demand when performed in turnout gear compared with performing without turnout gear. Therefore, additional workload is placed on the heart when performing on-scene tasks. Additionally, our turnout gear restricts body heat dissipation. As our core temperature increases, two cooling response mechanisms increase in activity: sweat response in an attempt to enhance evaporation and triage of blood flow to the skin in an attempt to exchange heat through convection. As mentioned earlier, dehydration reduces preload. The triage of blood flow creates competition between the working muscle and the skin, subsequently reducing the residual pressure and volume of returning blood flow and causing the heart rate to increase further to maintain blood pressure and perfusion.
Perfusion demands throughout the body are not met by simply varying the heart rate. Our cardiovascular system engages in dynamic adjustments in the size of the vessels to maintain blood flow and makes adjustments according to the returning blood supply and the systemic resistance to flow. The type and intensity of activity determine the challenges the heart faces. Both our hearts and our apparatus pumps are limited by the capacity of the supply source and pump in modes ranging from low-pressure/high-volume to high-pressure/low-volume conditions. Our physical fitness training must prepare our heart for all modes of operations. Continuous aerobic exercise prepares our heart to perform low-pressure/high-volume flow work. However, many overlook the importance of resistance training to prepare our hearts for high-pressure/ low-volume modes of operation. Therefore, it is important to participate in a fitness program that incorporates a variety of modes of training including low-intensity, continuous, aerobic work and high-intensity, intermittent, anaerobic work. A good fitness trainer or advisor should prioritize your training based on individual needs but still incorporate a variety of training methods to prepare your heart for the full spectrum of activity you encounter on the job and in your daily life. With the proper conditioning prescription, the heart develops adaptations that enhance its ability to endure stress and allows us a greater work capacity during all modes of physical stress.
References
Brooks, G. A.; Fahey, T. D.; and Baldwin, K. M. (2005). Exercise physiology: Human bioenergetics and its applications (4th ed.). New York: McGraw Hill.
Guyton, A. C. and Hall, J. E. (2006). Textbook of medical physiology (11th ed.). Philadelphia: Elsevier Saunders.
DAN SENN, MS, CSCS, NSCA-CPT, is a firefighter/EMT for the Fargo (ND) Fire Department. He coaches and trains athletes in sports performance and works with firefighters on job performance and injury rehabilitation. He teaches firefighter physical fitness and safety and contributes to his department’s monthly fitness newsletter. His education has focused on exercise physiology, motor learning, biomechanics, and sports medicine. He is working on a second master’s degree and assists with organizing and training a firefighter combat challenge team.
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