HRV:Monitoring Parameters in the ICU

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Monitoring Parameters in the ICU

Overview of Respiratory Monitoring in the ICU

Respiratory monitoring is vital in the current management of patients suffering from acute respiratory failure. Clinicians understanding of disease processes and effects of clinical interventions can be improved through the appropriate use of various available monitoring techniques and correct interpretation of data. A table summarizing the current modes of respiratory monitoring and their potential usefulness in the clinical setting is available here. These techniques are essentially categorized into timing (continuous/intermittent), specific situations to be used in, potential usefulness, and their limitations.

Overview of key parameters that can and should be monitored in the critically ill patient with respiratory failure:

Category	Parameters
Gas Exchange	SpO2 PaO2 SaO2 Transcutaneous PCO2 PetCO2 PaCO2 PEEP (Positive end-expiratory pressure) PaO2/FiO2 ratio EVLW (Extravascular Lung Water)
Respiratory Mechanics	Resistive pressure (difference between maximal pressure, Pmax and plateau pressure, Pplat) Elastic pressure (difference between Pplat and PEEP) Airflow Pressure/Volume curves (P/V) Diaphragmatic electrical activity Work of Breathing (WOB) Airway pressures (esophageal pressure and transpulmonary pressure) Occlusion pressure Pressure-time product IAP (Intra-abdominal pressure) Shunt fraction (Qs/Qt)
Lung Volumes	FRC (Functional Residual Capacity) EELV (End-expiratory Lung Volume) Alveolar reaeration Tidal Volume

Table 3.1a: Key respiratory parameters divided into sub-categories.

Overview of Hemodynamic Monitoring in the ICU

Hemodynamic monitoring is crucial in critically ill patients as they are often hemodynamically unstable due to hypovolemia, cardiac dysfunction and or alterations of vasomotor function which all leads to organ failure as there is a mismatch between oxygen delivery and demand. Monitoring of the cardiovascular system allows clinicians to guide their medical management to prevent or treat organ failure. A summary table of currently available monitoring techniques and its advantages and disadvantages, categorized into the degree of invasiveness of the techniques as well as whether or not they are calibrated can be found here.

The basic vital parameters that can and should be monitored for patients who hemodynamically unstable:

• Heart rate
• Blood pressure
• CVP
• Peripheral and central venous oxygen saturation
• Respiratory variables
• Urine output
  

When the basic parameters fall out of a safe range, there is an increased for hemodynamic monitoring of:

• CO
• PAOP or wedge pressure
• PAP
• SvO₂
• SVV
• Extravascular water

Overview of various cardiac output monitoring techniques and other key parameters that need to be monitored for patients who are suffering from organ failure or are at risk:

Degree of Invasiveness	Monitoring Technique	Parameters
Invasive	Pulmonary Artery Catheter (PAC)	CVP PAP PAOP or wedge pressure SvO2 RVEF CEDV CO (average value of the last 5 minutes and not a true continuous monitoring)
Less invasive	Transpulmonary thermodilution	CO (Intermittent and continuous measurement) Stroke volume SVV PPV GEDV ITBV EVLW
	Transpulmonary dye dilution	All the parameters measured by PAC PPV SVV CO (Real time, beat-to-beat variations)
	Ultrasound flow dilution	CO (continuous measurement through arterial waveform) TEDV CBV ACV
	Respiratory derived cardiac output monitoring	CO (using CO2 as an indicator)
	Pulse contour and pulse pressure analysis	CO (only an estimation)
	Transesophageal echocardiography	Real time images of cardiac structures Blood flow
	Esophageal Doppler	Stroke volume CO Preload estimate (via corrected flow time)
Non-invasive	Transthoracic echocardiography Non-invasive pulse contour systems Bioimpedance	CO
	Estimated continuous cardiac output	Heart rate Peripheral oxygen saturation Non-invasive blood pressure CO (only an estimation)
	Ultrasonic cardiac output monitoring	Flow velocity in the aortic and pulmonary outflow tracts CO (only an estimation)

Table 3.2a: Summary of current hemodynamic monitoring techniques and the respective parameters measured categorized into degree of invasiveness.

Heart Rate

Heart rate (HR) is one of the most basic cardiovascular parameters. The average resting heart rate (HR) is 72 beats per minute (bpm) but varies depending on age, physical activity and diet. HR can be measured by manually counting the number of pulses by placing your fingers on your wrist or neck. In clinical settings, HR is measured using HR monitors or ECG. In the last few years, many wearable devices such as sports watches can measure HR to track fitness during exercise and rest.

An abnormal heart rhythm can lead to more serious complications. Arryhthmia is the condition in which the patient's heart rate is irregular and has many different forms:

Tachycardia

Tachycardia is the condition for a resting HR that is too quick, typically above 100 bpm.
Types of tachycardia:

• Supraventricular tachycardia: occurs in the atria
• Ventricular tachycardia: occurs in the ventricles
• Sinus tachycardia: increase in HR during excitement or illness, can return to normal

Bradycardia

When the resting HR is too slow (below 60 bpm), the patient is said to have bradycardia. Bradycardia occurs when there is disruption of the electrical signals as they travel from the atria to the ventricles. Most athletes will have slower heart rates due to their exercise or training regimes, though this is not a clinical condition.

Atrial Fibrillation

Atrial fibrillation is one form of arrhythmia where the atria contract randomly and too quickly.

Cardiac Output

Cardiac Output (CO) is the amount of blood in litres that the heart pumps per minute and is calculated by equation (2) below:
CO = HR x SV — (2)
where CO is the cardiac output, HR is the heart rate and SV is the stroke volume.

A high cardiac output could indicate sepsis while a low cardiac output could result from heart failure.

Blood Pressure (BP)

Blood pressure is made of the systolic and diastolic pressures, which are the pressure during the contraction and relaxation of the heart respectively. Normal BP is in the range of 90/60 mmHg to 120/80 mmHg. High BP or hypertension can lead to heart attacks and strokes.

Figure 3.5: Blood Pressure Chart Levels

Mean Arterial Pressure (MAP)

Septic shock is caused by a low BP. It is more common to track MAP in a patient for the potential onset of septic shock, however.

MAP is more efficiently measured through invasive monitoring by providing up-to-date measurements. Below is a procedure outlined by the Association of Anaesthesists:

1. The ulnar artery is located by taking Allen's test, which simply involves clenching the fist. An arterial cannula (needle) is then inserted into the appropriate site where the arterial lumen should be located.
2. The cannula is connected to a saline column enabling electrical conduction to an adjacent transducer. The transducer contains a diaphragm that alters electrical output depending on BP levels.
3. The outputs of the transducer are measured visually on a connected display.

The invasive system also features a "flushing" system consisting of a bag of saline pressurised at 300mmHg. This flushing can be done manually or automatically, and allows the ejection of blood samples and air at certain intervals.

Such a method brings very accurate output measurements and graphical charts conveniently tracking the progress of MAP over time. There is a risk of several complications arising (listed below), but this should be very low if the flushing system is adequate and the initial procedure of cannula insertion is done carefully.

• Thrombosis (blood clot) can develop within the artery
• Ischaemia (restricted blood supply to tissues) upon delivery of external drugs to the blood

If measurements are taken non-invasively, MAP is calculated through the formula:
MAP=DP+¹⁄₃(SP-DP) or MAP=DP+¹⁄₃(PP)
Where DP is the diastolic blood pressure, SP is the systolic blood pressure, and PP is the pulse pressure. It is straightforward to calculate this as all the variables can be tracked in real time.

Invasive monitoring procedure

Figures 3.5.1a-b: set-up of the invasive system measuring MAP

Non-invasive BP monitors on the market are very diverse. Prices can start from £30 (Figures 3.5.1c-d) for monitors that are hassle-free to set up, however they do not calculate MAP automatically. This is problematic if this is exact parameter needs to be tracked over time. On the other hand, monitors priced from £1200 (Figure 3.5.1e) features integration of EMRs from partner companies (TPP, EMIS, etc.) and technical support.

Non-invasive monitors

Figures 3.5.1c-e (left to right): Boots Pharmaceuticals Automatic Blood Pressure Monitor - Wrist, Amazon LOVIA BP Machine with Large Cuff, IEM Mobil-O-Graph 24 Hour ABPM.

All BP monitors score in terms of portability, regardless of price. The major difference is the more expensive monitors have more reliable and efficient methods for recording measurements.

By comparing invasive and non-invasive monitors, it is evident that there is a trade-off between comfort and efficiency of obtaining measurements. With non-invasive monitors, patients do not need to worry about procedures involving the insertion of needles into their skin. However, time has to be spent manually calculating MAP. This type of monitor can be improved by having the calculations done automatically, but it should be noted that 3 variables (1 of SP and 2 of DP) are involved in this calculation. Without invasive measuring, each variable would introduce a margin of error, and this margin would be even larger upon calculating MAP. The advantages of invasive monitors seem to outweigh those of non-invasive monitors.

Oxygen Saturation

SpO₂ is normally between 95% - 100% and, if it is <90%, the patient requires immediate treatment. The Oxygen-haemoglobin dissocation curve (Figure 3.6a) shows the relationship between SpO₂ and the partial pressure of Oxygen (P_O2). The S-shape of the curve is a safety factor, as a large decrease in P_O2 results in a relatively small decrease in SpO₂ (for higher levels of P_O2).The Bohr effect explains how several factors such as pH, temperature and CO shifts the curve to the left or right.

Oxygen-haemoglobin dissociation curve

Figure 3.6a: Oxygen-haemoglobin dissociation curve

Temperature

The normal body temperature is between 36 to 37 degrees Celsius. High body temperatures are an indication of an infection or sepsis.

Challenges of Monitoring in the ICU

In the ICU, nurses are responsible for the primary monitoring of patients which includes checking monitors. While doctors oversee patients by assessing their progress, it is nurses who predominantly look at monitors. Nurses identify early changes in a patient’s conditions and alert the relevant physicians. Nurses in ICUs normally only have 1 or 2 patients at a time, in order to closely follow their progress. In ICU, the interface for health care providers to monitors the various physiological parameters is a large monitor which displays either graphically or numerically the different variables. These monitors have various functions which can include alarm systems to warn nurses when a particular parameter has gone out of the ‘healthy’ range. Nurses in ICUs have reported problems they experience when using these monitors :

• False Alarms- there is a high frequency of alarms of which are high proportion are false, a suggestion has been made that alarm thresholds should be adjustable depending on the individual. This has been implemented in a number of monitors; however nurses cite the complexity of menu structure of monitors meant they were not able to change alarm parameters.
• Integration of Information- one such example is that oxygen saturation falls when body temperature lowers, this in itself is not a reason for concern. However, alarms sound for the decreased oxygen saturation and nurses then have to check other parameters to see whether it has fallen due to a decline in health or just do to fluctuations in other parameters.
• Lack of standardisation in display- different monitors have different displays which require training in order to use to the best of its functionality. Nurses lack the time needed in order to properly understand all the different menu pathways which leads in inefficient use of the devices.