The bedside monitor is the most prominent device garnering the most attention for both medical staff and patients and their families. Bedside monitoring is designed to display, store, and trend the patient's heart rate, respiratory rate, blood pressure reading (noninvasive and invasive), oxygen saturation, electrocardiographic (ECG) tracing, and pressure waveforms generated from arterial, venous, or bladder catheters. The individual bedside monitor is connected to a computerized central monitoring system outside of the patient's room that will sound an alarm when abnormal measurements are recorded prompting immediate notification of the ICU staff and evaluation of the patient.
Alarm limits and scales are set by the nursing staff depending on the patient's clinical status (Figure 9–1). For instance, the ECG scale may be set to amplify the tracing 2- to 4-fold in a low-voltage state. Pressure waveform scales can vary based on the patient's pressure range; generally, the right atrial pressure is set at 20 mm Hg, pulmonary artery systolic pressure is set at 40 mm Hg, and systolic arterial blood pressure is set at 180 mm Hg.1
Cardiac monitoring is indicated for all critically ill patients and is used for the assessment of hemodynamics, rhythm diagnosis, and detection of ischemic changes. Monitors can employ 3-lead or 5-lead wire systems to depict the electrical activity of the heart; however, 5-lead systems are most common for continuous monitoring and provide a readout of 2 or more leads simultaneously.1
Lead placement must be standardized in order to maintain precise and accurate results and interpretation (Table 9–1). In order for the monitor's heart rate counter to detect the correct heart rate, the height of the R wave should be twice the height of the other electrocardiographic waves. The alarm mechanism relies on R-wave height for proper detection; false alarms can occur if T-wave height is equivalent to R-wave height, which causes double counting.1 Most monitors have the capability of taking a 12-lead ECG. In terms of lead placement, when only monitoring, the lower limb leads should be placed on the abdomen to pick up the breaths, when taking a 12-lead ECG they should be placed on the thigh.
Table 9–1Electrode placement for 5-lead system. ||Download (.pdf) Table 9–1 Electrode placement for 5-lead system.
|Electrode ||Placement |
|Right arm (RA) ||Apply to the right shoulder near the junction of the right arm and torso |
|Left arm (LA) ||Apply to the left shoulder near the junction of the left arm and torso |
|Right leg (RL) ||Apply at the level of the lowest right rib, on the right abdominal region, or right hip |
|Left leg (LL) ||Apply at the level of the lowest left rib, on the left abdominal region, or left hip |
|Precordial (choose either V1 or V6) || |
V1 Apply to fourth intercostal space on right sternal border
V6 Apply to fifth intercostal space on midaxillary line
Electrode resistance changes as the gel dries; if a problem occurs with one electrode, it is recommended that all electrodes be changed to prevent discordance in resistance between electrodes.1 Other key elements needed to ensure proper cardiac monitoring include adequately preparing the skin by cleaning and drying the sites of lead placement, testing the center of the pregelled electrode to make sure it is not dry, and reducing tension on wires. If these measures are not performed, interference and incorrect recordings can result.
Hemoglobin oxygen saturation is measured by pulse oximetry and is expressed as the percentage of oxygen (O2) that hemoglobin carries relative to the total amount of hemoglobin that is capable of carrying oxygen and is noted as “Spo2” to differentiate it from “Sao2” obtained from arterial blood gas analysis. Pulse oximetry employs the technologies of spectrophotometry and optical plethysmography. Spectrophotometry estimates the hemoglobin oxygen saturation by using a light source with 2 light-emitting diodes (LEDs) that emit light at red (660 nm) and infrared (940 nm) wavelengths. Deoxyhemoglobin absorbs red light and oxyhemoglobin absorbs infrared light; absorbed light is transmitted to a photodetector and converted to a digital value. Optical plethysmography detects pulsatile arterial changes at the pulse oximeter sensor site as the path length of light through the sensor site alternately increases or decreases with each pulsation.2 The plethysmography component of the pulse oximeter differentiates light absorption by hemoglobin from light absorption by surrounding tissue.
Pulse oximeter sensor probes should be placed on the best pulsatile vascular bed available; potential sites include fingers, great toe, and earlobe. Sensor probes should not be used on sites that are near indwelling arterial catheters, blood pressure cuffs, or areas of venous engorgement such as arteriovenous fistulas and blood transfusions. Standard pulse oximeters use transmittance spectrophotometry, that is, the light source and photodetector regions of the sensor should be directly opposite each other on the vascular bed and should be positioned so that all light from the sensor makes contact with perfused tissue beds. This minimizes ambient light interference and optical shunting which occurs when light bypasses the vascular bed.1 If the these probes are placed on the forehead or nasal bridge (as is sometimes erroneously done if there is difficulty detecting an adequate pulse waveform on the digits or earlobe) or in any configuration that does not allow the transmitter and receiver to be opposite each other, the Spo2 is completely unreliable. Specialized pulse oximeters that use reflectance spectrophotometry can be placed on the forehead and give reliable Spo2 readings. Make sure you know the capabilities of the device used in your ICU.
Two-wavelength pulse oximetry as described above is standard, but is inaccurate in the presence of dyshemoglobins such as carboxyhemoglobin and methemoglobin. Other causes of spurious readings (usually erroneously decreased Spo2) include low perfusion states, clinical motion, venous pulsation at sensor site, dark skin, intense ambient light, nail polish, artificial nails, and intravenously administered dyes such as methylene blue, indigo carmine, and indocyanine green.2
The function of the transducer system is to convert a biophysical event into an electrical signal that is transmitted as a waveform to the bedside monitor. The transducer system can be a single-pressure unit for arterial or right atrial monitoring or a multiple-pressure unit for arterial, right atrial, and pulmonary artery monitoring (Figure 9–2). The system consists of a catheter, pressure tubing, stopcocks, and a flushing device.
Multiple-pressure transducer system.
To ensure accuracy, the system must be zeroed against atmospheric pressure and then leveled with the phlebostatic axis which approximates the right atrium. The phlebostatic axis is located at the intersection of an imaginary line from the fourth intercostal space at the sternal border extending laterally to the right chest and the midaxillary line with the patient in supine position with the head of the bed between 0° to 45°s.
The pressure bag (Figure 9–3) should encase a bag of normal saline and be inflated to 300 mm Hg. If blood is noted to be flowing back into the system, the pressure is inadequate. Inadequate pressure may be due to an empty bag of normal saline, initially inadequate inflation of the pressure bag or later deflation of the pressure bag from leaks; or the tubing connected to the catheter might be disconnected from the circuit.
Flowmeters (Figure 9–4) measure the flow rate of the gas (usually medical air or oxygen) attached to the meter. Medical air or compressed air is filtered atmospheric air (ie, 21% oxygen, yellow-coded outlet) and its clinical uses include nebulizer treatments, providing clean air in ventilators, and powering air-driven medical equipment. Oxygen is 100% oxygen (green-coded outlet).
Flowmeters. Compressed air (color-coded yellow) on the left; oxygen (color-coded green) on the right.
Nasal cannulae deliver oxygen at flow rates of 0.5 to 6 L/min and are generally more comfortable for patients than face masks, making talking and eating easier. Although it can be approximated that the fraction of inspired oxygen (Fio2) increases by 0.03 to 0.04 per increase of 1 L/min in oxygen flow rate, this estimate is usually inaccurate as Fio2 also depends on the patient's tidal volume, inspiratory flow rate, respiratory rate, and the volume of the nasopharynx.3 Nasal cannulae are effective in the setting of mouth breathing since inspiratory air flow occurs while breathing through the mouth which causes entrainment of oxygen from the nose via the posterior pharynx.3 A bubble canister (Figure 9–5) for humidification should be used when patients require more than 4 L of oxygen to minimize irritation of nasal and oropharyngeal mucosa.
Bubble canister for delivery of humidified oxygen therapy or nebulizer treatments.
A simple face mask or variable performance mask delivers oxygen at flow rates of 5 to 8 L/min with a corresponding Fio2 of 0.40 and 0.60, respectively.4 The mask has holes on either side for entrainment of air and venting of exhaled gas but rebreathing may occur if the expiratory pause is absent. The Fio2 delivered is variable according to flow rate, pattern and rate of ventilation, inspiratory flow rate, and fit of mask and should not be used in patients who require a fixed oxygen concentration.3
The air-entrainment or Venturi mask (Figure 9–6A) delivers a predetermined and fixed oxygen concentration mechanistically by the Bernoulli principle. The mask comes with color-coded concentration dials or dilator jets labeled with the corresponding oxygen flow rate required for the desired fixed delivery of oxygen concentration ranging from 24% to 60%. The differently colored dilator nozzles (Figure 9–6B) have apertures of varying size that control the amount of atmospheric air entrained (a smaller nozzle entrains more room air and delivers lower Fio2) and subsequently the amount of inspired oxygen delivered to the patient. Expired gas rapidly exits the mask due to high flow rates and rebreathing does not occur.3
Color-coded concentration nozzles.
A non-rebreather mask (Figure 9–7) has 3 one-way valves in order to provide an inspired oxygen concentration of 100% and to allow venting of exhaled gas and prohibit entrainment of room air.4 However they usually deliver less than 100% (eg, ~60%-80%) Fio2 due to entrainment of room air since a completely sealed face mask could result in asphyxiation. Non-rebreather masks should not be used for extended periods of time given risk of absorption atelectasis and oxygen toxicity.
Non-rebreather face mask.
The bag valve mask (BVM) (often called an Ambu bag) is a portable device that provides intermittent positive-pressure ventilation with a self-inflating bag and one-way valve (Figure 9–8). The valve has 3 distinct ports: an inspiratory inlet which permits the entry of fresh gas during inspiration, an expiratory outlet for the exit of exhaled gas, and a connection to the face mask or endotracheal tube.3
Bag valve mask (Ambu bag).
While specifications may vary according to the manufacturer, the volume of an adult-sized BVM is 2100 mL. The average stroke volume delivered with one hand is 600 mL and using two hands yields 900 mL. A reservoir bag or corrugated tube is attached to the tubing and should be fully extended to its maximum length for maximum concentration of oxygen (90% for reservoir bag and 100% for corrugated tube). The recommended oxygen flow rate is 15 L/min. Patients that need PEEP to maintain oxygenation (eg, adult respiratory distress syndrome [ARDS]) and require BVM ventilation during disconnection from mechanical ventilation (eg, for transport, ventilator troubleshooting) should be ventilated with a BVM containing a built-in PEEP valve or with a PEEP valve added to the exhalation port of the BVM. This PEEP valve is adjusted to provide the current level of PEEP set on the ventilator and maintain oxygenation during BVM.