Introduction to Patient Monitoring Case File

Introduction to Patient Monitoring Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 7

The surgeons have requested a brief general anesthetic for a dressing change to an open infected wound. They suggest that the procedure can be performed in the patient’s bed on the hospital floor.

➤ Does this patient need special monitoring for this procedure?

ANSWER TO CASE 7:

Introduction to Patient Monitoring

Summary: A patient undergoes a dressing change on the floor, and a brief anesthesia is required.

➤ This patient’s monitoring must adhere to the standards for basic monitoring published by the American Society of Anesthesiologists. These standards are described in greater detail in the following discussion.

ANALYSIS

Objectives

1. Be familiar with the basic anesthetic monitoring parameters.

2. Be aware that monitoring the key parameters allows the anesthesiologist to assess clinical status and changes to patient condition.

APPROACH TO

Patient Monitoring

Monitoring is an essential duty of an anesthesiologist. During administration of an anesthetic the anesthesiologist needs to be able to continuously monitor a patient’s clinical status, rapidly detect changes in a patient’s condition, assess responses to therapeutic interventions, and ensure proper equipment function at all times. The American Society of Anesthesiologists first established standards for basic monitoring in 1986, and these standards have formed the cornerstone of safe anesthetic care since that time.

Depending on the acuity and magnitude of the surgery additional monitoring may also be required; the decision to use more invasive monitoring is made by the anesthesiologist performing the case. In all instances, vigilance is the most important action required by the anesthesiologist and the provider should never consider monitors an adequate substitute for personal vigilance and participation.

CLINICAL APPROACH

Standards for Basic Anesthetic Monitoring

The first standard states that “anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics, and monitored anesthesia care.” This standard emphasizes the vital role of the anesthesiologist in the safe care of the patient.

The second standard states that “the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated.” These will be considered individually.

Oxygenation

During all anesthetics an assessment of blood oxygenation is required to ensure adequate oxygenation at all times. In addition, during a general anesthetic the oxygen concentration in the inspired gas must be measured, and there must be a low-oxygen concentration alarm in place.

The patient’s blood oxygen saturation is determined by pulse oximetry. Pulse oximeters should be equipped with an audible pulse tone, and the pitch should change when the oxygenation drops, warning the anesthesiologist of a change in patient status. The pulse oximeter works by illuminating a tissue sample with two wavelengths of light: 660 nm red light and 940 nm infrared light. Plethysmography is used to differentiate the pulsatile arterial waveform from background tissue. As the pulse oximeter depends on both pulsatile flow and arterial color to determine saturation, any condition that affects detection of pulsatile flow (arrhythmias, patient movement, shivering, hypotension) or changes light absorption (intravascular dyes, dysfunctional hemoglobin) may falsely affect the oxygen saturation reading.

Oxygen sensors should be on the inspiratory limb of the anesthesia circuit to ensure recognition of a hypoxic mixture prior to the patient’s inhalation. Polarographic analyzers are commonly used to determine oxygen concentration. Oxygen diffuses through a polymeric membrane and reacts with water to form hydroxide; this reaction produces a current change in proportion to the number of oxygen molecules present. Low-limit oxygen alarms alert the anesthesiologist when the inspired oxygen level falls below 21% or alternative parameters chosen for the case.

Ventilation

Ventilation must be monitored during all anesthetics. This may be accomplished using qualitative clinical signs such as chest excursion and auscultation of breath sounds or quantitatively through continuous carbon dioxide monitoring of expired gas.

Whenever an endotracheal tube/laryngeal mask airway is inserted, the placement must be confirmed by sustained presence of CO2 for greater than three breaths. Although CO2 production is in general associated with an endotracheal intubation, CO2 can also be produced by the gastric bubble. Thus, confirming CO2 for three breaths, as well as auscultating the stomach and chest are required to ensure tracheal intubation. The end-tidal CO2 (ETCO2) must also be monitored continuously until the removal of the endotracheal tube or LMA. During mechanical ventilation of a patient, an alarm must be present that will create a characteristic audible alert if the ETCO2 is above or below a certain range and in the event of a circuit disconnect, which is determined by a drop in peak inspiratory pressure below a critical value.

Figure 7–1. Normal capnograph where each phase represents the following: I-inspiratory baseline, II-expiratory upstroke, III-alveolar plateau, IV-inspiratory downstroke.

Monitors display a continuous waveform of inspiratory and expiratory carbon dioxide concentration over time; this is known as capnography (Please see Figure 7–1). This data can provide invaluable information such as a change in respiratory rate, dead space/perfusion, anesthesia circuit disconnect, and an estimate of arterial carbon dioxide concentration, PaCO2. In healthy individuals undergoing general anesthesia, the difference between PaCO2 and ETCO2 is 5 to 7 mm Hg.

Circulation

The anesthetist continuously assesses circulation through observation of the patient, pulse, blood pressure, and continuous electrocardiography (ECG).

At least one lead of an ECG is displayed; this can provide valuable information about the patient’s underlying cardiac rate and rhythm. If at least two leads are available, ST-segment analysis can detect myocardial ischemia. Myocardial ischemia leads to T-wave changes (flattening, inversion) followed by ST-segment changes (depression, elevation) and lastly by Q waves.

Heart rate and blood pressure must be checked at least every 5 minutes. In most instances the heart rate is determined using the pulse oximetry waveform or less commonly by direct palpation of the pulse. Noninvasive blood pressure can be detected by palpation, auscultation of Korotkoff sounds, and oscillotonometers. The oscillometric method is used most frequently in automated blood pressure readings; the cuff is inflated until oscillations are no longer seen (systolic blood pressure) and then deflated until maximal oscillations (mean arterial pressure). The diastolic blood pressure is actually a calculated value derived from the mean and systolic pressures. The upper arm is the most commonly used extremity for BP monitoring; regardless where the BP is monitored, it is important to ensure that the appropriate cuff size is applied. The cuff’s width should be approximately 40% of the extremity’s circumference and the length should be 60% of the circumference. A cuff that is too small can result in falsely high blood pressure measurements while a cuff that is too large can lead to falsely low measurements.

Temperature

Heat loss is common in surgery due to the exposed state of skin, operating room environment, and both general and regional anesthesia inhibiting thermoregulatory control. Temperature monitoring is required to allow the anesthetist to detect changes in body temperature while the patient is anesthetized. In most instances, temperature is measured by using electrical probes that change their electrical resistance depending on temperature changes.

Urine Output

Although not required as part of the American Society of Anesthesiologists standards for monitoring, urine output is measured in most anesthetics with a duration of more than 2 hours or where significant fluid shifts are anticipated. In some ways a “poor man’s CVP,” an adequate urine output of 1/2 mL/kg suggests an adequate volume resuscitation and cardiac output. Measuring and recording urine output hourly, if not half hourly, can provide quite useful information in the absence of the monitoring of central venous pressure. 

Invasive Monitoring

Depending on the type of surgery and comorbidities of the patient, more intense monitoring may be required to adequately care for the patient. The decision to use a particular invasive monitor involves balancing the benefits with the risks of insertion.

Arterial Line

In certain instances intermittent BP monitoring is not adequate and continuous invasive arterial blood pressure monitoring is indicated. For example, if wide swings in blood pressure are expected such as during a carotid endarterectomy or aortic aneurysm repair, or if tight control of BP is necessary during an intracranial or cardiac case. At times an arterial line is needed to closely monitor arterial blood gases and laboratory studies. The blood pressure is monitored by transducing a small catheter placed directly into a peripheral artery. The most common site is the radial artery, but the brachial, axillary, femoral, and dorsalis pedis arteries can also be cannulated. The blood pressure transducer attached to the catheter converts the transmitted arterial pulsatile force into a change in voltage. The arterial BP trace is continuously monitored.

Invasive arterial pressure monitoring is dependent upon an appropriate zeroing of the transducer, and that the transducer is positioned properly at the level of the right atrium. Mechanical pitfalls include a potential damping of the signal (such as from bubbles entrained in tubing), or an under damping or amplification of the signal (observed, eg, when an excessive length of tubing is used). Complications of the arterial cannulation include hematoma, thrombosis, embolism, infection, aneurysm formation, nerve damage, and distal ischemia.

Figure 7–2. Normal CVP tracing where a wave-right atrial contraction, c wavebulging of tricuspid valve in right atrium, x descent-atrial relaxation, v wave-rise in right atrial pressure before tricuspid valve opens, and y descent-right atrial emptying into right ventricle.

Central Venous Line

Most often central venous pressure (CVP) monitoring is used to estimate volume status by measuring the right atrial pressure, which provides an estimate of right ventricular preload. The CVP tracing (see Figure 7–2) depends on many factors: heart rate, arrhythmias, tricuspid valve function, and right ventricular compliance. Assuming no cardiopulmonary disease, the CVP is often used as a marker of left ventricular preload. The most common site of insertion is the right internal jugular vein, but the left internal jugular vein, subclavian veins, and external jugular veins can also be utilized. Complications of placement include arterial puncture, nerve injury, pneumothorax, air embolism, sepsis from catheter infection, thrombophlebitis, and venous thrombosis.

Pulmonary Artery Monitoring

A pulmonary artery (PA) catheter is a 110-cm plastic tube; the tip is placed in the pulmonary artery by “floating” the balloon through a large vein (internal jugular, subclavian, external jugular) →right atrium →tricuspid valve →right ventricle → pulmonary artery. Catheter location is indicated by the pressure waveform from the distal end of the catheter (see the following discussion). The PA catheter can measure cardiac output, mixed venous oxygen saturation, PA, and right atrial pressures as well as calculating systemic vascular resistance, pulmonary vascular resistance, and stroke index. The pulmonary artery occlusive pressure (PAOP) is measured when the distal balloon is inflated in the pulmonary artery; this is used as an estimate for left ventricular end-diastolic pressure. There is a static column of fluid between the tip of the catheter and left atrium.

The indications for the use of PA monitoring include poor left ventricular function, valvular heart disease, recent myocardial infarction, adult respiratory distress syndrome, massive trauma, and major vascular surgery. Complications of PA monitoring include those listed earlier for CVP monitor as well as dysrhythmias (ventricular tachycardia or fibrillation, right bundle branch block, complete heart block), pulmonary artery rupture, pulmonary infarction, and valvular or endocardial vegetations.

Echocardiography

Echocardiography uses a piezoelectric crystal to emit ultrasound waves; these ultrasound waves penetrate tissue and then bounce back to the crystal giving information about velocity, distance, and density. Echocardiography has revolutionized perioperative cardiac monitoring by enabling the clinician to determine stroke volume, cardiac valve function, presence of intracardiac air, ventricular preload, and possible myocardial ischemia through the detection of wall motion abnormalities.

Category 1 indications (supported by the strongest evidence or expert opinion) include heart valve repair, congenital heart surgery, hypertrophic obstructive cardiomyopathy repair, thoracic aortic aneurysm/dissection repair, pericardial window procedures, unstable patients with unexplained hemodynamic disturbances, suspected valvular disease, or thromboembolic disease. Complications of TEE (transesophageal echocardiogram) use include esophageal trauma, dysrhythmias, hemodynamic instability, lip/dental injuries, hoarseness, and dysphagia.

Monitoring of Neurological Function

Awareness is an uncommon but dreaded complication of general anesthesia; it has been estimated to occur in 1 to 2 out of 1,000 general anesthetics. The typical signs of “light” anesthesia rely on signs of sympathetic stimulation such as an increase in heart rate and blood pressure, presumably in response to the sensation of surgical pain. Despite the availability of monitors to determine these hemodynamic parameters, it is difficult to accurately quantify a patient’s anesthetic depth consistently. The bispectral index or BIS monitor has been developed to monitor the patient’s level of consciousness.

The bispectral index, also known as the BIS monitor, is one measure of anesthetic depth which is derived from a reduction of the EEG. Selected EEG signals are reduced to a number, between 1 and 100 (the algorithm is currently under patent). A reading of 100 represents the awake state and numbers between 45 to 60 are considered optimal for general anesthesia. While certainly a useful monitor, there is still controversy as to what level is required to ensure amnesia, and the BIS is not perfect. Although controversial, this monitor has not been added to the list of required monitors by the American Society of Anesthesiologists.

Certain surgeries may place important neural structures at risk (spine, brainstem, intra-cerebral surgery.) These neural pathways can be stimulated with evoked potentials (small electrical signals) in order to assess functionality. Somatosensory evoked potentials (SSEPs) monitor the integrity of the dorsal columns of the spinal cord by placing stimulating electrodes near the median or ulnar nerves of the arms and posterior tibial nerves of the legs. The electrical signal is applied to the peripheral nerve and travels to the dorsal root ganglia→posterior columns of the spinal cord →dorsal column nuclei →medial lemniscus → opposite thalamus → frontoparietal sensorimotor cortex. Anesthetic agents such as volatile anesthetics, nitrous oxide, and barbiturates can decrease the amplitude and increase latency of SSEPs similarly to ischemia’s effects making it difficult to determine a clear etiology. Motor evoked potentials (MEPs) monitor the functionality of the corticospinal tracts; a stimulating electrode is placed on the scalp and the recording electrode is placed on the contracting muscle. MEPs are more sensitive to anesthetic agents and more difficult to obtain limiting their accuracy and clinical use.

CONCLUSION

Continuous monitoring of the patient is a key part of the anesthesiologist’s responsibility during surgery. Basic monitoring requirements include the minimal acceptable standards for patients. The complexity of additional monitoring is determined by the type of surgery and the patients; however there is no monitor that can replace the required vigilance of the anesthesiologist.

Comprehension Questions

7.1. Which of the following is the most important skill of an anesthesiologist?

    A. Caring for patients through the acquisition of knowledge

    B. Technical facility with invasive lines

    C. Vigilance

    D. An understanding that mastering anesthesiology is not only a science, but also an art

7.2. For any anesthetics, the basic standards of monitoring include a continuous evaluation of all but which one of the following?

    A. Oxygenation

    B. Ventilation

    C. Circulation

    D. Hematocrit

    E. Temperature

ANSWERS

7.1. C. While each of the answers is true for the anesthesiologist, the most important attribute of an anesthesiologist is vigilance.

7.2. D. The basic monitoring standards of the American Society of Anesthesiologists states that “the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated.” D, continuous monitoring of the hematocrit, is not necessary and even during cases associated with a massive blood loss, is rarely accomplished.

Clinical Pearls

➤ Simply adhering to the basic monitoring standards adopted by the American Society of Anesthesiologists has been found to reduce anesthetic mortality.

➤ CO2 can also be contained in a gastric “bubble.” Hence, it is important to confirm endotracheal intubation by observing the end-tidal CO2 for three breaths, and auscultating the chest.

➤ An appropriately sized blood pressure cuff is necessary to accurately measure blood pressure using a noninvasive technique.

References

American Society of Anesthesiologists. (last amended Oct. 25, 2005). Standards for Basic Monitoring, 1986. 

American Society of Anesthesiologists. (last amended Oct. 18, 2006). Statement on Transesophageal Echocardiography, 2001. 

Barash P, Cullen B, Stoelting R. Clinical Anesthesia. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005. 

Miller R, Stoelting R. Basics of Anesthesia. 5th ed. New York, NY: Churchill Livingstone; 2006. 

Murray M, Morgan E, Mikhail M. Clinical Anesthesiology. 4th ed. New York, NY: McGraw Hill Medical; 2005. 

Pickering T, et al. American Heart Association’s recommendations for blood pressure monitoring in humans and experimental animals. Hypertension. 2005;45:142-161.

HomeCase FilesIntroduction to Patient Monitoring Case File