Comprehensive Clinical Monitoring in Anesthesiology

Responsibilities and the Monitoring Process

One of the primary responsibilities of an anesthesiologist is to act as a guardian for the anesthetized patient. This guardianship is constant and extends through three distinct phases: before the surgery, during the surgery, and in the postoperative period.

Vigilance is a complex process constituted by several sequential stages. It begins with the generation of signals followed by data acquisition. Once data is acquired, it must undergo transmission and subsequent processing. Finally, the processed data is displayed through exhibition. It is the specific duty of the anesthesiologist to analyze these exhibited data points to make informed clinical decisions.

Definition and Utility of Clinical Monitoring

Monitoring is defined as the quantification of different physiological variables in a manner that is rapid, frequent, and repeated. These quantifications serve several vital functions in clinical practice. They are used to know the current state of the patient and to take immediate measures at a given moment. Furthermore, monitoring allows for the realization of diagnoses, the orientation of clinical management, and the determination of patient prognosis. A primary goal of these practices is to decrease anesthetic morbidity and mortality.

Classification of Monitoring Systems

Monitoring can be bifurcated into two main categories: basic and advanced.

Basic monitoring requires the constant presence of qualified anesthesia personnel and the continuous vigilance of four key areas: oxygenation, ventilation, circulation, and temperature.

Advanced monitoring implies the execution of various procedures, often invasive in nature, to monitor the cardiovascular, respiratory, and nervous systems, among others.

Areas of Specialized Monitoring

Respiratory monitoring focuses on arterial oxygen pressure, arterial oxygen saturation, respiratory mechanics, and the analysis of inspired and expired gases.

Cardiovascular monitoring involves the use of the electrocardiogram, arterial pressure measurement, central venous pressure monitoring, palpation of the pulse, and auscultation of heart sounds.

Neurological monitoring includes several specific techniques: intracerebral pressure measurement, transcranial Doppler, electroencephalogram, sensory and motor evoked potentials, jugular venous saturation, and cerebral venous saturation.

Specific Monitoring Data and Ischemia Detection

A practical example of monitoring data for a patient in Bed 3 includes at 10:11:17 on 02/02/2011:

• ECG/Heart Rate ($FC$): $64-65\,bpm$
• Non-Invasive Blood Pressure ($PANI$): $115/84\,mmHg$ (mean of $94\,mmHg$)
• Oxygen Saturation ($SpO_2$): $98\%$
• Respiratory Rate ($FR$): $10\,rpm$
• End-tidal $CO_2$ ($etCO_2$): $32.4\,mmHg$
• Temperature ($T1$): $32.4^\circ C$

For the detection of ischemia, a five-lead system is utilized. This involves simultaneous monitoring of $D-II$ and $V-5$. The $D-II$ lead is specifically used to detect ischemia of the right coronary artery, while the $V-5$ lead is used to detect ischemia of the anterior descending artery.

Non-Invasive Arterial Pressure (NIBP)

Non-invasive monitoring involves the occlusion of an artery using a pressurized cuff. It measures either the oscillations of the pressure within the cuff or the pressure at which arterial flow reappears while the cuff is being deflated. Automatic techniques include oscillometry, while manual techniques rely on Korotkoff sounds and the palpation of the radial pulse.

Errors in non-invasive arterial pressure measurement can lead to overestimation or underestimation. Overestimation of arterial pressure occurs when using cuffs that are too narrow (the ideal width should be $30\text{-}40\%$ of the arm circumference), using a loosely adjusted cuff, determining pressure in an extremity located below the level of the heart, or in patients with tissues that are minimally distensible (such as a shivering patient) or minimally compressible (as seen in obese patients or those with severe arteriosclerosis).

Underestimation of arterial pressure occurs in patients with decreased blood flow (such as cardiogenic shock or the use of vasoconstrictors), the use of cuffs that are too wide, determining pressure in a limb located above the heart level, deflating the cuff too rapidly, unequal compression of the artery (e.g., the humeral artery), or inadequate adjustment of the cuff to the arm circumference.

Invasive Arterial and Central Venous Pressure (CVP)

Invasive arterial pressure requires the use of a pressure transducer. Central venous pressure measurement is achieved through the puncture of the internal jugular vein. Specific methods of access include the Jernigan route, the Daily route, and the Mostert route. The procedure involves the placement of a cannula with an internal needle, penetrating the vein, extracting the needle, and leaving the cannula in place.

Indications for central venous pressure monitoring include procedures in cardiac patients with actual or expected ventricular dysfunction and monitoring in serious clinical situations. Other indications include the administration of irritant drugs, parenteral nutrition, rapid intravenous perfusion of large volumes of fluids, patients with poor peripheral venous access, or providing access for the placement of an endocavitary pacemaker or a pulmonary artery catheter. Risks associated with this procedure include infection, thrombosis, and others such as pneumothorax.

Interpretation of central venous pressure (CVP) reflects the patient's volemic status. A decrease in CVP can indicate an increase in cardiac performance, a greater impedance to venous return, or a decrease in mean systemic pressure (volemia). An increase in CVP indicates a decrease in cardiac performance, a decrease in impedance to venous return, or an increase in mean systemic pressure (volume).

Respiratory Function Monitoring and Pulsioximetry

Arterial oxygen pressure ($PaO_2$) can be monitored through intermittent invasive methods (arterial blood gas) or continuous invasive methods (intra-arterial catheter).

Pulsioximetry is the non-invasive measurement of oxygen transported by hemoglobin inside the blood vessels. The device emits light at two specific wavelengths: $660\,nm$ (red light) and $940\,nm$ (infrared light). These wavelengths characterize reduced hemoglobin ($Hb$) and oxyhemoglobin ($O_2Hb$), respectively.

Erroneous readings in pulsioximetry can be caused by severe anemia (hemoglobin must be lower than $5\,mg/dl$ [should be $g/dl$ per standard clinical notation but as stated in notes] to cause false readings), interference with other electrical devices, movement of the transducer, and intravenous contrasts that absorb light at wavelengths similar to hemoglobin. External factors include intense ambient light (xenon, infrared, fluorescent), poor peripheral perfusion due to cold, decreased body temperature, hypotension, or vasoconstriction. Obstacles to light absorption like painted nails and dishemoglobinemias (such as carboxyhemoglobin in carbon monoxide poisoning or methemoglobin) also result in errors because they absorb wavelengths similar to oxyhemoglobin.

Respiratory Mechanics and Gas Analysis

Monitoring respiratory mechanics includes measuring air pressure ($P_{peak}$, $P_{plateau}$), Tidal Volume ($V_t$), Respiratory Frequency ($F_r$), Minute Volume ($V_m$), airway resistance, and distensibility or compliance.

Analysis of inspired and expired gases covers $O_2$, $CO_2$ (specifically end-tidal $CO_2$ or $ETCO_2$), and anesthetic gases including $N_2O$ and halogenated agents such as Halothane, Isoflurane, Desflurane, and Sevoflurane.

Capnography and CO2 Variations

Capnography provides a visual representation of $CO_2$ across the respiratory cycle, divided into four phases:

• Phase I: Represents the start of expiration; the gas is free of $CO_2$ as it comes from the anatomical dead space and the anesthetic circuit.
• Phase II: A rapid rise in the tracing due to the increasing concentration of $CO_2$ as gas from the anatomical dead space mixes with alveolar gas.
• Phase III: The alveolar plateau phase; the gas is rich in $CO_2$ coming entirely from the alveoli. The concentration reaches its maximum at the end of expiration (end-tidal $CO_2$). A rising slope indicates "slow" alveoli.
• Phase 0: Represents the start of the next inspiration where the capnogram falls rapidly back to the baseline.

Hypercapnia (increased $CO_2$) is caused by inadequate ventilation (insufficient $V_m$), increased production of $CO_2$, malignant hyperthermia, intravenous administration of large quantities of bicarbonate, or the injection of $CO_2$ into the peritoneum during laparoscopy.

Hypocapnia (decreased $CO_2$) is caused by hyperventilation, an increase in dead space (for example, following a massive pulmonary thromboembolism), or a decrease in the production of $CO_2$. Total absence of $CO_2$ on the capnogram indicates apnea.

Neurological and Temperature Monitoring

Advanced neurological monitoring techniques include the Bispectral Index (BIS), intracerebral pressure via ventriculostomy catheter, Transcranial Doppler (DTC), and processed electroencephalograms (BIS, Entropy) which range from $0$ to $100\%$. The EEG signal is processed based on frequency bands:

• Beta: $13\text{-}32\,Hz$
• Alpha: $9\text{-}13\,Hz$
• Theta: $5\text{-}9\,Hz$
• Delta: $1\text{-}4\,Hz$

Temperature monitoring can be conducted at multiple anatomical sites, including the axilla, rectum, esophagus, nasopharynx, tympanic membrane, bladder, and directly in the blood.