The Circulation System — Page 26, Lesson 42

I want to walk you through the factors that determine your pulse rate and then look at what can go wrong with the circulation system. Let's start with the pulse rate itself. Your pulse rate isn't fixed — it goes up or down depending on how much oxygen your body needs. The main factors that drive this are exercise, altitude, and temperature. When you exercise, your muscles demand more oxygen, so your heart beats faster. At altitude, the air has less oxygen, so your heart has to work harder to deliver the same amount. Temperature changes also affect demand — heat or cold can stress the body and alter pulse rate. But there's another layer of control. Your heart rate is also influenced by the sympathetic and parasympathetic nervous systems — that's the automatic, involuntary control of the heart. We'll cover that in detail in Chapter 6, but for now, the key factors here are the fight or flight syndrome — also called the General Adaptation Syndrome, or GAS — shock, and emotion, specifically fear, anxiety, and anger. All of these can raise or lower your pulse rate through that nervous system control. Now, let's talk about what can go wrong. The efficient distribution of oxygen can be disturbed in two fundamental ways. First, the system itself — the heart, arteries, veins, and so on — can develop a fault. Second, the blood may simply be unable to carry enough oxygen for what the tissue cells need. We're going to focus on system failures first. The heart is a pump, and it's made of muscle. That heart muscle responds to electrical impulses that come from the brain, telling it to contract and relax in a coordinated, systematic rhythm. Sometimes that control system doesn't carry the impulses properly — they become uncoordinated. The result is that the heart beat may become irregular, or it may even fail altogether. In modern surgery, we can implant a mechanical device called a pacemaker. Its job is to produce regular electrical impulses that cause the heart to beat at a regular rate. We can measure those electrical impulses and see how they synchronise with the heart's pulse rate using an electrocardiogram — that's an ECG. An ECG test is a standard part of aircrew medical examinations, so you'll be familiar with it. Now, the heart muscle itself needs oxygen to keep working. That oxygen is delivered to the heart by the coronary arteries. If those arteries become narrowed, then insufficient blood may reach the heart muscle. This lack of oxygen becomes especially critical when the heart is beating faster due to exercise or stress. When that happens, you get the symptoms of angina — pain in the chest and sometimes the arms. If the blood supply is cut off completely, then a portion of the heart muscle may die. That dead tissue is called an infarct. Because that tissue is dead, it can no longer carry the electrical impulses, and the heart beat may become irregular or even fail completely. That's what we call a heart attack — medically, a myocardial infarction. Angina is often a precursor to a heart attack. About half of those who have a myocardial infarction die immediately. Of those who survive, a proportion go on to develop an associated abnormality of the heart rhythm — an abnormality that can completely interrupt the coordinated muscular contractions of the heart. That's called a cardiac arrest. The most common of these rhythm disturbances is called ventricular fibrillation. If it's treated promptly with a DC shock delivered by a defibrillator, the heart may revert to a normal rhythm. That's why you see defibrillators in airports, aircraft, and medical facilities — they can literally restart a coordinated heartbeat.