The most likely cause of the decrease in blood pressure is the cessation of the catecholamine infusion. In order for catecholamines to be administered without being affected by the rate of other infusions, they should be run through a separate lumen of a central venous catheter.

Regardless, raising a pressure infusion syringe over the bed of a lying patient leads to administration of a bolus. The hydrostatic pressure in the infusion system and the change in height influence the released volume. Conversely, lowering a syringe to a level significantly lower than the patient’s bed leads to a short but serious decrease in the infusion rate [4]. The hydrostatic pressure decreases, and the pump needs a bit of time to reestablish the internal force to increase the pressure. Figure 29.1 shows an example of such a pressure pump decrease when a pressure infusion pump is lowered.

Pressure infusion syringe pumps, which are used to administer cardiovascular medications, must always be secured at the level of the patient. This ensures that the rate of application remains the same, despite movements and repositioning. Fortunately, newer pumps now used in most institutions have safeguards to prevent positional changes in infusion rates.

>> “Why does this always have to happen,” commented nurse Sherry. “We get to the door to exit and the pressure hits rock bottom. I’ll give Mr. Peterson a bolus. Usually 0.2 ml is enough.”

Dr. Canute was relieved that he had such a knowledgeable and helpful nurse, and Mr. Peterson’s blood pressure increased promptly. As they were pushing the gurney down the long hallway, Dr. Canute realized that he forgot his emergency bag. He asked nurse Sherry to run get it. She rolled her eyes, “What for? What do you think is going to happen?” The CT scanner was only one floor below, and the obvious inexperience of Dr. Canute was beginning to get on her nerves. But she got the emergency bag for him.

They had to wait before they were allowed into the CT room. They knocked on the door a second time and the radiologist said it was going be about 10 more minutes. “Typical radiologist,” thought Dr. Canute, “no clue about the clinical condition of a patient. Why did I even bother to call them to ask when we could come down?”

“OK then, I’ll run back to the ward,” began nurse Sherry. “I have so much to do. I’ll be back here before Mr. Canute is called, I promise!” Dr. Canute hesitated, but Sherry was already on her way, so he called after her to bring a full oxygen tank with her on the way back. Due to the extended waiting period for the CT, there was a possibility that the patient’s oxygen might run out.

Nurse Sherry was just out of ear shot when the monitor sounded another alarm. The systolic was only 90 mmHg. Dr. Canute examined the blood pressure curve on the monitor (Fig. 29.2) and he knew what he had to do.

The blood pressure is 90/70 mmHg, which means the mean is about 75 mmHg. When examining the wave form, one sees that the amplitude is low and the incisures are missing. The most likely cause is partial obstruction of the system (see Sect. 29.1.6), for example, by a bend in the tube proximal to the pressure transducer or via obstruction of the tip of the catheter by a small blood clot. The decrease in blood pressure does not need medical therapy, only a catheter flush.

Adjustments and flushing are needed relatively often during a transport because the usual continual flushing of the system had been paused.

Every pressure system – consisting of a tube, connectors, fluids, and a pressure transducer – has its own frequency, which is termed resonance frequency. If the heart rate is close to this frequency, amplification of the measured signal can occur. As a result, the systolic value would be falsely high, and the diastolic falsely low. In order to prevent this signal amplification, pressure measurement systems have a resonance frequency which is at least 8 times larger than the frequency of the measured signal.

Here is a calculation example:

Heart rate 180 beats/min is equal to a signal frequency of 3 Hz. Eight times this value – also called the eighth overtone – is 24 Hz. The resonance frequency of the pressure system in this scenario must be >24 Hz.

Commercially available pressure measurement systems with standard short access lines have a resonance frequency which is significantly higher than the usual critical areas. Therefore, one can be assured even in extreme situations, realistic values are measured.

The clinical user can influence the resonance frequency of the system by placing exceptionally long access lines between the place of measurement and the pressure transducer. Such a change in the ratio of the oscillation of the mass to its resonance frequency can increase the amplitude.

In hospital use, underdamping as described is much less common than overdamping. Apart from the causes listed in Sect. 29.1.6, a common cause of damping is compressible air bubbles in the system. Therefore, the number of 3-way stop cocks must be kept to a minimum, due to the small air bubbles which tend to hide in them. Figure 29.3 shows how a simple test checks if the damping in a pressure system is correct.

>> Dr. Canute searched through Mr. Peterson’s bed for the arterial transducer pressure bag. After he found it, he briefly flushed the system. Immediate success: the blood pressure curve looked normal again, at 130/55 mmHg.

Dr. Canute waited some more. The parameters on the monitor were stable, and the infusion pumps were still full enough. As he waited, he received two phone calls from the ICU.

The waiting time seemed to be very, very long. Just as Dr. Canute thought to check again with the radiologists, the monitor’s alarm sounded again. The systolic blood pressure had dropped below the alarm level again. The curve form was normal this time. Dr. Canute cursed himself for not stopping nurse Sherry from leaving. The door to the radiologists was locked, and there was no one in the hallway except a patient in a wheelchair with head tilted back, closed eyes, and an open mouth emitting snoring noises. “I’d better do what the ICU nurse did, and give a bolus,” thought Dr. Canute.

He searched around Mr. Peterson’s feet for the norepinephrine pressure pump – which was hidden under the blanket. As he reached it, he saw that the battery had run out and it was off. “Dadgummit!” mumbled Dr. Canute who was really frustrated now. “How could that happen? I must not have heard the alarm when I was on the phone! Why was it under the blanket, anyway?”

… and the blood pressure dropped further to 50/30 mmHg.

The half-life of norepinephrine is very short, so that continual administration must be ensured immediately. In this case, the simplest solution would be to use one of the other infusion pumps. Midazolam and sufentanil can be given as a bolus.