Monitoring the Critically Ill Patient Part I
Harold Davis Jr., BA, RVT, VTS (Emergency and Critical Care)
The hallmark of critical care is monitoring, reevaluation, and assessment. Once you have completed these three steps you must then act on your findings. It is not acceptable to ignore any negative findings in hopes that they will go away. It is also helpful to monitor as many parameters as possible in regards to a particular body system. By monitoring several parameters you will be able to get a better handle on the respective system. The monitoring of the cardiovascular system will be discussed.
It is important to monitor the trends of parameters. One particular measurement is more meaningful when compared to previous measurements. The trends should tell you if a patient is improving or deteriorating. An important part of critical care is the recording of trends thus methods of charting these trends will also be discussed.
Heart rate is a nonspecific parameter. It is usually measured by auscultation of the heart and palpation of an artery, automatically taken from an ECG or arterial pulse pressure wave.
Increase in heart rate (tachycardia) may be caused by hypovolemia (the tachycardia is a compensatory mechanism), fever, excitement, exercise and pain. Tachycardia is generally defined as a heart rate greater than 160 beats per minutes (bpm). Decrease in heart rate (bradycardia) may be caused by high vagal tone, severe electrolyte disturbances and atrioventricular conduction blocks. Bradycardia is generally defined as a heart rate less than 60 bpm.
When irregularities in heart sounds are heard, the heart rate should be compared to pulse rate and the difference in rates are called pulse deficits. Pulse deficits are indicative of arrhythmias. Hypoxia, myocardial contusions and metabolic or acid base imbalance may cause arrhythmias. Some examples of cardiac arrhythmias include: premature atrial contraction (PAC), atrial fibrillation, premature ventricular contraction (PVC) and ventricular tachycardia. All pulse abnormalities should be confirmed by a electrocardiogram (ECG).
The ECG reflects the electrical activity of the heart. The presence of a normal ECG does not insure effective mechanical activity of the heart.
When assessing an ECG you should check the following:
1) What is the heart rate?
2) Is the rhythm regular or irregular?
3) Is there a P wave for every QRS complex and is there a QRS complex for every P wave?
4) Is the P - R interval prolonged?
5) Is the QRS complex form normal in size and shape?
6) Are the T waves abnormally large?
7) Is there S-T segment depression or slurring?
An arrhythmia is defined as an irregular heart rhythm. The heart rate defines the rhythm as a tachyrhythmia or a bradyarrhythmia. Wide bizarre QRS complexes may be ventricular premature contractions (VPC's), right ventricular hypertrophy or right bundle branch block (BBB). VPC's are often not preceded by a P wave (the other two are). VPC's and right BBB are associated with wide QRS waveforms. Small normal appearing QRS complexes may be due to pericardial effusion. Abnormally tall-tented T waves may be due to hyperkalemia. S-T segment slurring or depression may be due to myocardial hypoxia. For a more detailed discussion on ECG interpretation you should refer to a book in veterinary electrocardiography.
It's not necessary that a Veterinary Technician is able to identify all arrhythmias but they should recognize the normals and alert the doctor to the abnormals.
Mucous Membrane Color And Capillary Refill Time
The normal mucous membrane color is pink (mm color). In diseased states the mm color may be yellow, pale, white, brick red or blue (table 1).
Capillary refill time (CRT) is an indication of peripheral perfusion and should not be thought of as an indicator of blood pressure. CRT is the rate at which blood returns to the capillary bed after it has been compressed digitally. To measure CRT lift up the lip and compress the gum with your finger until it blanches out, when you release the pressure the gums should return to their original color within 1-2 Sec. Prolonged CRT is due to vasoconstriction. Vasoconstriction may be caused by hypovolemia, excitement, fear and pain.
Table 1 Mucous membrane color and it's possible significance
Central Venous Pressure
Central venous pressure (CVP) is a measurement of right atrial pressure. It evaluates three things: 1) The heart's ability to function as a pump; 2) blood volume in relation to volume capacity; and 3) vasomotor tone (indirectly). The normal CVP range is 0-10 Cm H2O.
A CVP less than 0 may be due to vasodilation (increased volume capacitance) or hypovolemia. A CVP in a normal range but in the face of signs consistent with vasoconstriction may be due to hypovolemia. A CVP greater than 10 may be due to the heart's inability to function as a pump or fluid over- load, vasoconstriction (decreased volume capacitance), pericardial effusion and positive pressure ventilation.
A catheter placed in the anterior vena cava via the jugular vein is required for measurement of the CVP. To assure proper placement of the catheter, a fluctuation in the fluid meniscus within the manometer synchronous with the heart beat or chest excursions should be seen. A water manometer is placed in the fluid line via a three-way stopcock. The stopcock is turned off toward the patient filling the manometer. The manometer is filled approximately three quarters full. When the manometer is filled the stopcock is turned off toward the IV fluids, this opens the pathway to the patient and the fluid level drops in the manometer. When the fluid stops dropping, note the reading and the zero point is then determined. To determine the zero point a horizontal line is drawn between the manometer and the top of the manubrium. The point where the horizontal line intersects the manometer is the zero point. The difference between the initial reading and the zero point is the CVP measurement. For example the initial reading is 15 Cm H2O, the zero point is 10 Cm H2O the CVP is 5 Cm H2O. CVP measurement is indicated in renal failure, heart failure, shock, and during rapid fluid administration.
Arterial Blood Pressure
Blood pressure measurement is a valuable monitoring tool when evaluated with other cardiovascular parameters. The trends tend to be more informative than a single value.
Blood pressure is a product of cardiac output, vascular capacity and blood volume. These three components are in a careful balance; impairment of one of the components is usually compensated for by the other two so as to maintain adequate blood pressure.
The systolic pressure is the maximal pressure obtained with each cardiac ejection. The diastolic pressure is the minimal pressure prior to the next ejection cycle. Mean blood pressure is the average driving pressure, which determines cerebral and coronary perfusion. Mean blood pressure can be estimated by the following formula:
Systolic BP + 2(Diastolic BP)
The normal ranges are:
Systolic 100-160 mmHg
Diastolic 50-100 mmHg
Mean 70-120 mmHg
We should become concerned when the systolic blood pressure is less than 80 mmHg or the mean blood pressure is less than 60 mmHg. Some causes for hypotension include vasodilation, hypovolemia, arrhythmias, and anesthetic drugs.
There are two methods of blood pressure measurement, direct and indirect. Direct measurement requires the placement of an arterial catheter and the use of a transducer and oscilloscope. This method produces the most accurate results and provides continuous systolic, diastolic, and mean blood pressure measurement.
There are two methods of measuring blood pressure indirectly. The ultrasonic doppler measures systolic blood pressure. A doppler crystal is placed over a peripheral artery, the crystal sends out ultrasound energy into the underlying tissue, the flow of the blood reflects back the ultrasound energy and is picked up by the crystal. The reflected energy is converted to an audible signal. A cuff is placed snugly around a leg. If the cuff is too tight you will get an erroneously low reading, likewise, if the cuff is placed too loose you will get an erroneously high reading. The cuff is inflated until you no longer hear the flow of blood, then you slowly deflate the cuff, the first time you hear the flow of blood the systolic pressure is noted.
The Dinamap a a blood pressure unit utilizes the oscillometric technique. A cuff is applied to a limb and automatically inflated, the cuff detects oscillations in the underlying artery. As the cuff is gradually deflated oscillations are detected. The first detected pulsation is the systolic pressure; the point of maximum pulsation is the mean blood pressure; and the point were oscillations disappear is the diastolic pressure.
Toe Web-Rectal Temperature
It has been shown that skin temperature correlates well with peripheral perfusion and cardiac output. An excellent and non-invasive technique for monitoring peripheral perfusion is the toe web - rectal temperature.
Measurements are made with an electronic thermometer, thermistor probe or a mercury thermometer that ranges from 70 - 110oF. A thermometer is placed between the toes of the rear paw; the temperature is compared to the rectal temperature. The toe web temperature is usually 2 - 9oF less than that of the rectum. The greater the difference in toe web and rectal temperature, the greater the reduction in peripheral perfusion and cardiac output. As the patient responds to treatment the temperature differential should return to normal.