Critically ill patients are often accompanied by circulatory dysfunction or even circulatory failure caused by various reasons, which could result in adverse outcomes. Circulatory function monitoring could detect and identify the circulatory status of patients, and timely prompt physicians to intervene, further improving the outcome and prognosis of critically ill patients [1]. Therefore, circulatory function monitoring is of great significance during the process of diagnosis and treatment in critically ill patients. Circulatory function monitoring includes non-invasive monitoring and invasive monitoring. The former includes non-invasive blood pressure, electrocardiogram, transthoracic echocardiography, while the latter mainly includes invasive arterial pressure, central venous pressure, continuous cardiac output monitoring with pulse indication, pulmonary artery catheter, and transesophageal echocardiography (TEE) [2,3]. The main disadvantages of noninvasive monitoring are discontinuity, which may miss sharp changes in circulation [4]. While in critically ill patients, invasive operations may cause certain adverse reactions, and the complexity of the disease also brings certain challenges to the implementation of invasive monitoring [5].

Carbon dioxide (CO2) measurement was first developed in the early 1900s. However, its complexity limited its widespread use. Advances in technology have made end-tidal carbon dioxide (ETCO2) a basic and commonly used monitoring method in many clinical applications. CO2 monitoring is performed using mainstream and side stream detectors. Mainstream detectors utilize infrared sensors to directly locate the path of gas flow and generate real-time CO2 waveform maps. These units tend to be more expensive and are mainly used in intubated patients. Side stream detectors are located outside the main path of the airway circuit and are mainly applied in the nasal monitoring of ETCO2. The numerical and graphical information provided by ETCO2 shows that it can be used as a noninvasive indicator of alveolar ventilation [6]. Several ETCO2-related parameters, such as information regarding ventilation/gas exchange function, cardiac output, ventilation/perfusion ratio (V/Q), and metabolism, can be derived from the calculation combining the characteristics of concentration index and waveform [7]. ETCO2 monitoring is increasingly used in anesthesia, emergency medicine, pre-hospital emergency medicine, critical care, and other fields [[8], [9], [10], [11]]. The significance of ETCO2 monitoring which concluded according to above contents is shown in Fig. 1. Continuous ETCO2 monitoring in patients with tracheal intubation can effectively assess the intubation and prevent the adverse effects of abnormal mechanical ventilation on clinical outcomes [12]. The common ETCO2 waveforms (normal or abnormal) during perioperative period are summarized in Fig. 2 [[13], [14], [15]]. When the patient is undergoing mechanical ventilation, the ventilator setting parameters can be adjusted according to the ETCO2 value and waveform changes, so as to effectively avoid the occurrence of hypopnea or hyperventilation [16,17]. The above that ETCO2 reflects respiratory state of patients has been widely used and well known [18].

For critically ill patients, circulatory failure caused by various reasons often leads to adverse outcomes. When excluding patients with unstable respiratory function and metabolic abnormalities (such as malignant hyperthermia), ETCO2 could early identify the circulatory status of patients, so as to prompt anesthetists to take timely intervention measures. Its clinical application value has been demonstrated, especially in critical ill patients, which has drawn attention from clinicians [19]. Therefore, this narrative review mainly discusses the new perspective assessment of ETCO2 in circulatory function of patients with cardiac arrest, shock, pre-hospital emergency patients, which is expected to draw attention by the anesthetists who could learn from the relevant experience, so as to better apply ETCO2 monitoring to the rapid circulatory assessment and guide treatment of critically ill patients during perioperative period.