Bhavani Shankar Kodali* and Ahalya Kodali**
* Vice Chairman, Department of Anesthesiology, Brigham and Women’s Hospital, 75 Francis street, Boston, MA 02090 and Associate Professor, Harvard Medical School, Boston, MA, 02115, USA. (t: +1 617 525 8449; e:email@example.com)
** Department of Anesthesiology, Boston Medical Center, Boston, MA 02115, USA
Capnography has existed in clinical practice for over three decades. It has resulted in improvements outside the operating room that enhance patient safety during sedation and has become the standard of care during anaesthesia in some – though not many – countries. There is a trend worldwide towards incorporating capnography during anaesthesia and sedation, but the cost of the equipment is a barrier. The use of capnography in intensive care units (ICUs), in comparison with operating rooms and sedation areas, has been variable and is by no means routine; a survey conducted in the UK indicated that although a majority of ICUs have the requisite equipment, use of capnography is limited . The limited use of capnography is partly due to the lack of clinicians and nurses who appreciate its value in the ICU . However, this view is changing, following the publication of studies in the UK demonstrating that capnography is able to identify many conditions that contribute to morbidity and mortality in the ICU setting . There are now clear arguments for the routine use of capnography in the ICU setting; frequent use will also enable clinicians to apply the device effectively in a crisis situation .
The fundamental premise in capnography is that it not only monitors the ventilation and integrity of the airway, but is also an indirect measure of cardiac output for a given ventilation in acute settings. Figure 1 shows a basic capnogram with its components [4−6]. The expiratory segment is divided into three phases. The first, Phase I, represents free carbon dioxide (CO2) gas being expelled from the conducting airways to the lungs. There is no exchange of gases in the airways (anatomical dead space) and the CO2 concentration is initially zero at the beginning of expiration. Phase II consists of a rapid increase in CO2 concentration, wherein there is a mixing of expired air from the anatomical dead space and the alveoli to produce the CO2 increase. Phase III comprises a plateau, with a slightly positive slope that represents CO2-rich gas from the alveoli. There is an abrupt decrease in CO2 when the inspiration begins at the conclusion of expiration (Phase 0 of the inspiratory segment). Inspiration yields no CO2 as long as there is no rebreathing. The alpha angle is the angle between Phase II and Phase III, and it is affected by changes in the slope of Phases II and III; the clinical relevance of this angle is that it is an indirect indication of ventilation/perfusion (V/Q) ratio mismatch in the lungs. The beta angle is the angle between Phase III and the descending segment of the capnogram, or the beginning of Phase 0, and is usually 90°. The maximum concentration of CO2 at the end of the breath is designated as end-tidal CO2 (ETCO2) and, if given in mmHg, is designated as PETCO2. It is typically approximately 35–36 mmHg and lower than arterial CO2 (PaCO2) by about 4–5 mmHg. The difference between PETCO2 and PaCO2 represents the alveolar dead space (i.e. PaCO2 – PETCO2). The height of Phase III (PETCO2) is dependent on ventilation, perfusion and, more importantly, V/Q ratios in the components of the lung; hence, PETCO2 is an indirect measure of cardiac output [7, 8].
Figure 1: The basic components of a capnogram (see text for explanation)
Case presentation 1: Capnography used as an indirect measure of cardiac output
A 60-year-old patient (height, 1.83 m; weight, 86 kg), presented to the emergency department with shortness of breath on exertion, with three-pillow orthopnoea. The ejection fraction was 15%. An electrocardiogram showed wide complex ventricular tachycardia and an echocardiogram further showed severe dilatation of the left ventricle, depressed function, and severe hypokinesis of all walls. The right ventricle was also enlarged and depressed. There was reduced pulsatility of the inferior vena cava, indicating increased right ventricular pressure. The patient was receiving infusions of calcium chloride, dopamine, milrinone, and phenylephrine hydrochloride (Neo-Synephrine®, Neofrin).
Management and outcome
Since there was no successful cardioversion with multiple defibrillation shocks, the patient underwent ablation of the right ventricular ectopic foci close to the right ventricular outflow tract. The outflow tract of the right ventricle was very thin. After what was assumed to be a successful procedure, 6 hours later the capnogram showed dramatically abrupt decreases in PETCO2 from 40 mmHg to zero. The femoral arterial pressure also decreased. The abrupt decrease in PETCO2 indicated a no-cardiac-output status and was conveyed to the cardiologists. Immediate echocardiography and imaging showed pericardial tamponade, which was most likely due to rupture of the right ventricular outflow tract. Cardiopulmonary resuscitation (CPR) was immediately initiated, with opening of the pericardium via a substernal approach. CPR was continued and the patient was transported from the operating theatre to a separate facility for an emergency cardiopulmonary bypass and repair of the ventricular tear. A biventricular assist device was placed to assist intrinsic ventricular function.
The patient subsequently had a heart transplant, with a successful recovery. The CPR during the transport was monitored with a portable capnography system. Blips in the capnography waveform indicated the adequacy of perfusion generated by the chest compressions. Continuous waveform capnography ensured successful CPR during the patient’s transportation from the cardiac catheterisation laboratory to the operating room. PETCO2 values of 20 mmHg ensured adequate perfusion during CPR, as values less than 10 mmHg are associated with an undesirable outcome. Moreover, the presence of CO2 confirms the integrity of the airway and correct placement of the endotracheal tube. Since this case, we have been using an adapted system for portable capnography that can be transported when required for all intubations and cardiac arrests throughout our institution, to which we have added a video laryngoscope to facilitate intubation in locations within our institution where cardiac arrest might occur (Figure 2).
Figure 2: Portable capnogram system
Figure 3 below shows the blips of CO2 that occurred during CPR and the abrupt increase in PETCO2, indicating the spontaneous return of circulation.
Figure 3: Capnogram during CPR
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