Having attended numerous MET calls and code blues over the years, there is a trend that becomes quite noticeable. Despite the blood pressure, heart rate and oxygen saturations trending up or down; the respiratory rate predominantly stays the same right up to the point just before the MET call or code blue is called.
If I asked you to pick one number, not a range, but one number that you identify with as a normal respiratory rate…what would you pick? For instance, my number would be 12. Now imagine you walk into your patient’s room to do their observations. You put on the blood pressure cuff and press start, put on the oxygen saturation probe awaiting the results for the oxygen levels and heart rate, and check their temperature with the thermometer.
While doing all of this, you notice the patient appears to have a “normal respiratory rate”. So you write down your “normal respiratory rate number” which might be 18. They next shift, I come in and write down 12. The following shift, another nurse comes in and writes down 16. Despite 3 different numbers, the patient has not actually altered their respiratory rate! Likewise, a patient’s respiratory rate may slowly be trending up. However, writing in a respiratory rate similar to the nurse before does not identify this trend. In an article called Respiratory rate records: the repeated rate, 83% of the 484 respiratory rates reviewed were either recorded as 16 or 18. This identifies an important issue, respiratory rates are just not being counted! Despite many saying that respiratory rate is not accurately counted due to time constraints in a busy nursing shift, I believe nurses would spend the extra time counting a respiratory rate if they understood the importance of this simple assessment.
In order to understand why respiratory rate is so important, we need to understand what makes us take a breath? If I put on an oxygen saturation probe with a starting value of 99% and held my breath for as long as I could, what do you think my oxygen saturations are going to be right before I have to take a breath again?
Did anyone say 99%…? If you did, you would be correct! Don’t believe me, try it out for yourself the next time you have access to an oxygen saturation probe! In a normal adult, what makes us take a breath is the increased levels of carbon dioxide in our blood. When I hold my breath, my carbon dioxide levels start to rise. Carbon dioxide in the body converts into carbonic acid which in turn breaks down to release hydrogen ions. As hydrogen ions are acidic in nature, the increase in hydrogen ions, caused by the increase in carbon dioxide, lowers the pH in the body. The body likes to maintain a pH between 7.35 – 7.45 and the carbon dioxide levels between 35 – 45 mmHg. As the respiratory centre in the brain becomes aware of an increasing carbon dioxide level and therefore decreasing pH, it stimulates the diaphragm and external intercostal muscles in order to make the person take a breath and “blow out” some carbon dioxide.
Ventilation is defined as the physical movement of air in and out of the lungs. Respiratory rate is one element of ventilation, but it goes hand in hand with tidal volume. Tidal volume is the amount of air that the lungs breath in and out in one breath. The average tidal volume is approximately 500 mL or 7 mL/kg of ideal body weight. When tidal volume is multiplied by respiratory rate, we get a minute volume defined as the amount of air that is breath in and out over the course of a full minute.
Minute volume = respiratory rate x tidal volume
The body establishes the most optimal minute volume in order to maintain the carbon dioxide levels and pH of the body within their normal limits. Let’s say Mr Smith is happily maintaining his carbon dioxide and pH while breathing at a respiratory rate of 10 and a tidal volume of 500 mL. His minute volume would therefore be 10 x 500 = 5000 mL (5 litres). Now let’s say that Mr Smith’s entire left lung collapses for the purposes of this example…
As he is only ventilating with one lung, half of his tidal volume capacity has been reduced and he can now only take in 250 mL per breath. In order to maintain a minute volume of 5 litres to keep the pH of his body happy, Mr Smith now has to increase his respiratory rate to 20. At this point, his oxygen saturations are probably still ok as he is still getting air down into his lung to facilitate the movement of oxygen from the lung into the bloodstream. Let’s say now that half of Mr Smith’s remaining right lung has collapsed for the purposes of this example…
Now Mr Smith can only take in 125 mL per breath and in order to maintain a minute volume of 5 litres to keep the pH of his body happy, Mr Smith now has to increase his respiratory rate to 40. At this point, his oxygen saturation levels may still be ok as he is still getting air down into his lung to facilitate the movement of oxygen from the lung into the bloodstream. However, how long do you think Mr Smith will be able to keep chugging away with respiratory rate of 40 before he tires out all his respiratory muscles and cannot keep up the work of breathing anymore? As much as he wants to breathe, he is not going to be able to. At this point, the point of decompensation, his oxygen saturations are no longer normal! And sadly, this is the point when patient deterioration is often picked up and escalated.
As the MET call/code blue team are en route, what is happening with his carbon dioxide levels? What is happening with his pH? An inadequate minute volume is going to result in a quick rise of carbon dioxide and subsequent drop in pH which is going to result in an unconscious Mr Smith. An unconscious Mr Smith that is not breathing is defined as a patient with a respiratory arrest!
To sum it all up: a ventilation problem is a problem with the inability to clear carbon dioxide. The body will attempt to achieve a minute volume that maintains a normal carbon dioxide level and pH by increasing respiratory rate and/or tidal volume. This is known as compensation. There will be a point that the body will no longer be physically able to maintain the work of breathing required to maintain the required minute volume, leading to decompensation. At this point, the patient is not ventilating and getting air into their lungs and therefore oxygen cannot move from the lungs into the bloodstream. Oxygen saturations only become affected at this point. Failure to ventilate is also known as type 2 respiratory failure and is defined as an elevated carbon dioxide level with concurrent hypoxemia (low oxygen level in the blood)
Remember the following:
A failure to ventilate causes an increase in carbon dioxide levels
Carbon dioxide is 2 words
Therefore, a failure to ventilate is type 2 respiratory failure
If there is one thing that you take away from reading this blog, it should be this: respiratory rate and oxygen saturations tell us two DIFFERENT stories, so don’t depend only on oxygen saturations for a respiratory assessment of your patient! Be vigilant about counting the respiratory rate and assessing work of breathing as well! A low oxygen saturation in a patient with a ventilation problem is a late, late sign! If respiratory rates that are trending up can be identified early, we can intervene before it becomes a medical emergency!
Have you ever observed a trend of a similar respiratory rate right up to the point the MET call occurs? Does 16 and 18 tend to be the main numbers that are written in the observation charts at your facility? Do you count respiratory rate for 15 seconds, 30 seconds or the full minute? Please feel free to leave your comments or questions in the section below…
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