Alden H. Harken, MD and Brian C. George, MD
Ms. O’Sullivan is a 65-year-old woman who arrived in the PACU 30 minutes ago following a right colectomy. She was extubated in the operating room. Her vitals are: BP 140/90, pulse 120 (regular), respiratory rate 30, temperature 37.5°C, and finger oximetry 80%. She is anxious and agitated and says that she wants to “leave right now and go home.”
1. Until proven otherwise, what is the cause of agitation and/or disorientation in the postoperative recovery unit?
2. Explain several causes of postoperative hypoxemia.
3. What are the indications to intubate a patient?
If you are sitting comfortably reading this chapter, you are using about 3% of your energy in the work of breathing. Interestingly, the “driver” of this lung work is blood acidosis (actually CSF hydrogen ion concentration)—not oxygen. You are exquisitely sensitive to PCO2/pH. If you hold your breath for a minute, at the end of that minute your only wish is for another breath—breathing is high on everyone’s list of fun things to do. However, at the end of a minute of apnea, your PCO2 has risen from 40 mm Hg to only about 48 mm Hg. Therefore, a relatively tiny decrease in arterial pH (or increase in PCO2) translates into a profound stimulus to breathe.
Mrs. O’Sullivan’s respiratory rate has increased to 30. We could attribute this to her “agitation,” but were we to check her arterial blood gas at this time, we would find:
PO2: 55 mm Hg
PCO2: 30 mm Hg
O2 sat: 80%
She has a respiratory alkalosis and is actually “overbreathing.” Her only abnormality is the drop in her hemoglobin saturation (O2 sat) that confers a 20% (100% minus 80%) decrease in arterial oxygen content, which can be completely compensated by a 20% increase in her cardiac output, resulting in a rock stable systemic oxygen delivery. The patient’s agitation has unquestionably pumped up her cardiac output at least the necessary 20%—so, what’s the problem? You decide that “everything is fine,” so you reassure the patient and the nurses and leave to check on another patient.
1. Fifteen minutes later you get a call that Mrs. O’Sullivan is climbing out of bed, making a lot of noise, and bothering the other patients (and the nurses)—so, “Can we sedate her?”
Warning: if you depress this patient’s respiratory drive with a sedative now, you will receive a follow-up call in 30 minutes that she just suffered a cardiac arrest. Agitation/irritability/confusion in the PACU/ICU is hypoxemia until proven otherwise, and should be cause for alarm! In most of us, the symptoms of hypoxemia are not an alteration in breathing volume/rate/pattern. Acute hypoxemia just makes a patient feel anxious and restless. Hypoxemia makes a patient want to get up, go out, and travel to someplace safer—and hypoxemic patients are very likely to announce this desire to everyone.
2. There are lots of reasons for hypoxemia in the early postoperative period. Atelectasis, aspiration, a touch of pneumonia, and even a pulmonary embolus are the most obvious reasons that ventilation doesn’t match up with perfusion. More fundamental is that most inhalational anesthetics block hypoxic pulmonary vasoconstriction. Remember, this is the unique capability of the pulmonary arterioles to divert perfusion away from hypoventilated alveoli. When poorly ventilated lung is perfused, a shunt occurs that results in hypoxemia.
3. When you walk into any surgical ICU today, you have access to machines that can support almost all failing organ systems. The oldest, and arguably the best, of these devices is the ventilator. There are both hard and soft rules relating to when you breathe for your patient and when you can safely cut the patient loose (and the indices for starting and stopping are really the same criteria):
A. Most importantly, the patient’s mental status must be adequate to protect his/her own airway.
B. Respiratory rate below 25 to 30. Again, breathing is a high priority for all of us. Although you and I are currently expending only 3% of our energy on “breathing,” a big burn can exert 25% of energy expenditure on the “work of breathing” and leave little residual energy for “getting better.”
C. Your patient’s lungs have got to be “working.” Carbon dioxide excretion is a linear function of alveolar ventilation, so if patients can “breathe harder,” they can rid themselves of CO2. Oxygenation is not so simple. By the time hemoglobin crosses halfway across a ventilated alveolus, it is already fully saturated with oxygen. So, increasing the inhaled oxygen (in a patient with a shunt) won’t help at all.
D. Strength is important, but it is hard to measure unless the patient is intubated. Negative inspiratory force (NIF) is an extubation parameter only. You and I can comfortably generate 100 mm Hg NIF. A patient who can only pull −25 mm Hg is solidly in the gray zone.
Putting It All Together
For the surgical intensivist, the following guidelines apply:
Intubation: After examining the aforementioned parameters, when you get frightened, intubate.
Extubation: After examining the aforementioned extubation criteria, when you are comfortable, extubate.
Tracheostomy: After living with your patient for 10 days, if you don’t “feel” that he can fly, trach him.
Does anyone think this is science? It is art!
TIPS TO REMEMBER
“Agitation” in the SICU/PACU represents a hypoxemic patient until proven otherwise.
The “driver” of minute ventilation in the vast majority of patients is CSF pH—not oxygen.
“Dyspnea” is a feeling—not a blood gas.
Most inhalational anesthetics inhibit healthy hypoxic pulmonary arteriolar vasoconstriction.
Increasing the inspired oxygen concentration in a patient with a shunt doesn’t help.
1. You are called about an elderly patient who is in the PACU. He is extremely agitated and is trying to hit the staff. Which of the following is the correct answer(s)?
A. Sedate him with a benzodiazepine (eg, lorazepam).
B. Check his vitals.
C. Check an ABG.
D. Check an EKG.
1. B, C, and D. The important point is that answer A is wrong. In the acute postoperative period this patient’s agitation most likely represents cerebral hypoxia. This could be due to several factors, including inadequate cardiac output or inadequate respiration. The vital signs are quick and easy to obtain and can help guide further workup. If they reflect inadequate oxygenation or are normal, then an ABG is indicated. An EKG is indicated if you suspect inadequate cardiac output, since this could be caused by a myocardial infarction or arrhythmia.