In this article, I'll describe three innovations in respiratory care: a new way to measure cardiac output, new nasal cannulas for high-flow oxygen delivery, and heliox for treating respiratory distress.
Measuring cardiac output using CO2
As you know, blood pressure is directly affected by cardiac output and peripheral vascular resistance. Increased cardiac output reflects the body's effort to compensate for trauma, stress, fever, or increased activity; decreased cardiac output may indicate blood loss or heart failure.
The newest noninvasive technique for measuring cardiac output is partial carbon dioxide (CO2) rebreathing. The new noninvasive cardiac output (NICO) monitor by Respironics measures baseline and rebreathing end-tidal CO2 and calculates cardiac output.
You can easily perform this type of monitoring on patients receiving mechanical ventilation by using a special circuit connected between the patient's airway and the ventilator circuit. This special circuit has a loop of large-bore tubing (called the rebreathing loop, which adds dead space to the patient's circuit) and a valve that opens and closes to start and stop the period of rebreathing. The monitor cycles every 3 minutes to measure CO2 levels before, during, and after the 35-second rebreathing period.
A study has found that partial CO2-rebreathing monitoring is an acceptable alternative to using invasive cardiac output measurements; it also poses less infection risk because it's noninvasive. And because the system doesn't depend on operator technique for its measurements, it may be more accurate than other noninvasive methods.
High-flow oxygen delivery
Traditionally, if you wanted to administer supplemental oxygen to a patient at a flow rate greater than 6 liters/minute, you couldn't use a nasal cannula—you'd have to use a mask delivery system instead. But two high-flow nasal cannula systems now available may provide an option for various patients, including those who need high-flow oxygen but can't tolerate a mask.
Oxygen delivered by a standard nasal cannula provides a maximum fractional percentage of inspired oxygen (FIO2) of 44% at the maximum flow rate of 6 liters/minute, which may not be adequate to meet the patient's oxygen requirements. Humidification needs to be added for rates above 4 liters/minute, or the patient is at risk for drying of the nasal mucosa; nosebleeds; thickened, retained secretions; increased airway irritability; and decreased function of the mucociliary escalator.
In contrast, a high-flow oxygen delivery system can meet or exceed the patient's supplemental oxygen requirements. Let's look at the two new systems under study:
- Salter Labs, Arvin, Calif., has developed a high-flow nasal cannula designed to provide flow rates of 6 to 15 liters/minute. The cannula is used with a high-flow bubble humidifier that operates at ambient temperatures.
In a study, the high-flow cannula was tested in healthy volunteers to assess the FIO2 delivered at flows from 6 to 15 liters/minute, compared with a standard cannula with flow rates set from 1 to 6 liters/minute. Tested when patients were at rest, the standard cannula provided a mean delivered FIO2 of only 0.26 to 0.54, compared with 0.54 to 0.75 for the high-flow cannula. When patients were told to double their respiratory rate for 1 minute, the standard cannula's mean FIO2 range dropped to a range of 0.24 to 0.45, compared with 0.49 to 0.72 for the high-flow cannula.
- The Vapotherm 2000i, by Vapotherm, Inc., of Stevensville, Md., is designed to deliver flow rates between 5 and 40 liters/minute. This system uses a humidifier cartridge inside a case that enables the high-flow delivered gas to be heated to 95% or greater relative humidity (33° to 43° C [91.4° to 109.4° F]). The system also uses a special triple-lumen delivery tube connected to a shortened catheter. Two of the delivery tube lumens contain heated water; the third lumen carries the gas, which is kept warm and comfortable for the patient despite the high-flow rate.
How heliox eases respiratory distress
A mixture of helium and oxygen, heliox can be used to reduce the work of breathing, deliver aerosols (such as bronchodilators), and reduce fear and anxiety for patients in respiratory distress. Colorless and odorless, helium has no chemical, pharmacologic, or biologic action in the body. By itself, it has no effect on the usual components of respiratory distress: bronchospasm and inflammation. But this gas has two properties that make it useful for patients who are working hard to breathe. First, its very low density lets it flow easily into narrow or twisty air passages. Second, CO2 diffuses through helium at four to five times the rate it diffuses through room air, so it can exit the body faster and easier.
The two most common mixtures of heliox are 80:20 (80% helium, 20% oxygen, closest to room air) and 70:30 (70% helium, 30% oxygen). The latter mixture, which has a slightly higher density and slightly lower flow capability than the 80:20 mix, is used for patients who need additional oxygen to avoid hypoxemia.
Heliox isn't indicated for all patients in distress; most respond adequately to conventional therapy. It also has several drawbacks: it's expensive, requires the handling of large gas cylinders, and may require modification of delivery systems such as mechanical ventilators. But for certain severe conditions, such as status asthmaticus or severe COPD exacerbation, heliox may benefit patients by:
- reducing the severity of an asthma attack. When given to a patient early in the course of an asthma attack, heliox can act as a therapeutic bridge, reducing the need for intubation and buying time so corticosteroids can take effect. It can also cause measurable changes in pulsus paradoxus, improve peak flow rates, and improve patient-dyspnea scores.
- The patient should wear a mask (such as a non-rebreather mask) to reduce loss of heliox through diffusion into the atmosphere.
- getting medications into the lower airways. Studies have found that when heliox is used to nebulize bronchodilators, more medication is delivered into the lower airways. (Note that the flow rate must be increased to run the nebulizer. Increase the flow rate by a factor of 1.8 if you're using 80:20 heliox, or by a factor of 1.6 if you're using 70:30 heliox.)
Tell your patient that his voice will sound higher when he's breathing heliox. (The effect is the same as if he'd inhaled the gas from a helium balloon.)
Heliox can be used with mechanical ventilators and has been shown to decrease intrinsic auto-positive end expiratory pressure (auto-PEEP) in patients with COPD. These patients tend to have increased air trapping during exhalation due to premature airway closure. Heliox reduces auto-PEEP by increasing the patient's expiratory flow during exhalation. In a recent study, researchers found that heliox not only reduced auto-PEEP, but also improved cardiac function, as reflected in increased cardiac index and a reduction in systolic pressure variations related to mechanical ventilator breaths (pulsus paradoxus for mechanically ventilated patients).
When heliox is delivered by mechanical ventilators, the lower density of the gas may affect the accuracy of the inspiratory and expiratory valve, flow measurement, patient triggering, auto-PEEP levels, and gas mixtures using the built-in gas blender. Heliox is connected to the air inlet of the ventilator instead of compressed air, and the normal connection is made with oxygen running into the oxygen inlet. Researchers have tested various mechanical ventilators to see how heliox affected the machines and have found that each machine has particular changes in function that need to be considered when using heliox.
Staying on top of things
Like all of health care, respiratory care is constantly changing. By being aware of what's current and new, you can help your patient get the most appropriate care.
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