Principles of Critical Care
Basics of Mechanical Ventilation and Hemodynamic Support in the ICU
Management of a patient in the intensive care unit involves two treatment modalities that are unique to this clinical setting.
The intensive care unit (ICU) can be a frightening place for patients and families. With so many things happening and a need for continuous monitoring, it is natural to be concerned. Knowing the basic aspects of critical care can help one understand and appreciate the care given in the ICU.
Indications for ICU Care
People get hospitalized when they are so sick that they cannot be treated in the clinic or at home. Among hospitalized patients, degree of illness can range from stable to life-threatening. Obviously, the sickest go to the ICU.
ICU patients have a variety of conditions, all with a significant degree of severity. They include, but are not limited to, brain injuries, infections, bleeding, and unstable postoperative course. What separates these patients from non-critical hospitalized patients is the need for one or both of the following:
- Mechanical ventilation to assist in breathing and respiration
- Hemodynamic support to ensure adequate circulation throughout the body
Mechanical Ventilation
A patient who needs ventilatory assistance may have conditions like pneumonia, pulmonary embolism, high levels of blood carbon dioxide (hypercapnia), low levels of blood oxygen (hypoxia), traumatic brain injury, or stroke. Basically, the patient cannot get enough oxygen to the blood or breathe on his or her own. Either way, the patient needs to have an endotracheal (ET) tube inserted and hooked up to a ventilator.
The ET tube serves as an artificial airway, particularly when the patient is not awake enough to protect his or her own airway. The ventilator has settings for breaths per minute, volume of air per breath, and other parameters. The machine also has modes of breathing, depending on whether the critical care physician feels the machine must do all the work of breathing for the patient or some of the work as backup for the patient's own efforts to breathe.
The ventilator is kept in place as the underlying cause of the breathing problem is treated, if it is reversible. At this point, the patient may become more awake and closer to breathing on his or her own. This leads to the process of weaning the patient off the ventilator. For successful weaning, the patient must demonstrate several things, including a sufficient level of consciousness, a good likelihood that he or she can protect his or her upper airway, and the ability to breathe while maintaining normalized blood gas levels.
To transition from mechanical ventilation with an ET tube to normal voluntary breathing with one's own airway, a breathing trial is performed. The ventilator is set to a spontaneous mode, allowing the patient to breathe entirely on his or her own while the ET tube is still in place. If the patient appears safe to breathe with little chance for going back to the ventilator, the ET tube can be removed.
Hemodynamic Support
A patient who requires hemodynamic support may have conditions involving life-threatening bleeding, an infection associated with a large drop in blood pressure (sepsis), or an unstable heart. Intravenous fluids, or blood if there is hemorrhage, are one option to correct low blood pressure. Another important treatment involves medications.
Medications for hemodynamic support, such as epinephrine and dobutamine, function by stimulating the heart, clamping down on blood vessels, or both. The physician picks the appropriate medications according to their function and the particular problem in the patient. The medication is given in a continuous infusion, constantly titrated to keep the heart rate and/or blood pressure within certain parameters.
These medications are not given through an intravenous line in a peripheral vein. They are given in central lines, catheters inserted into one of three large veins: internal jugular vein in the neck, subclavian vein in the upper chest, or femoral vein in the upper thigh. The purpose of a central line is to provide access to the bloodstream that is much closer to the heart, a very important benefit for these hemodynamic support medications.
As with mechanical ventilation, hemodynamic support is given until the underlying cause of the hemodynamic instability is addressed, after which the infused medications are no longer needed.
References
- American Thoracic Society - A Primer on Critical Care for Patients and Their Families
- American College of Chest Physicians - Evidence-Based Guidelines for Weaning and Discontinuing Ventilatory Support
